Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Hi Bob, your example is correct. However I was not talking about OCXO specifically, but about crystal oscillators in general. And the effect I have mentioned is not limited to wide-pull VCXO, but may occur at normal VCXO also. I named the modulation audio for sake of simplicity of expression - it was certainly not accurate enough. If you are modulating data (FSK) then such interferences have a risk to occur even at moderate data rates. I do not talk about theorectical can be's but about practical experience. Best regards Bernd -Ursprüngliche Nachricht- Von: time-nuts [mailto:time-nuts-boun...@febo.com] Im Auftrag von Bob Camp Gesendet: Donnerstag, 11. September 2014 00:18 An: Discussion of precise time and frequency measurement Betreff: Re: [time-nuts] OCXO Voltage Input? (Bob Camp) Hi If you are modulating a normal OCXO EFC with audio, and the output frequency is not being multiplied up, the modulation index will be very low. Low modulation index means that the higher order FM sidebands will be quite far down. If you take “audio” to be 10 KHz, and a VHF OCXO to be 100 MHz: With a 10 ppm EFC range, you get 1.0 KHz of deviation. The modulation index is 1 a decade below your upper modulation frequency. That’s already a pretty wide swing OCXO and a fairly high modulation frequency for an EFC line. If you have a spur that is in the 50 to 150 KHz range, you are talking about the 5th to 15th sideband off of 10 KHz or the 50th to 150th sideband off of 1 KHz. At 50 sidebands out and an index of 1, you are in the “forget about it” region. Even at 10 KHz, the sideband is not likely to create much of an issue. The distortion from the non-linear EFC slope will be more of a problem in a practical sense. —— Since the modulation is single sideband, yes it converts PM - AM. It also will be impacted by any limiters in the system and will not multiply the same way as a pure PM modulation. The phase of the sideband will change as you go through the resonance, further messing up the multiplication / limiter math. Bob On Sep 10, 2014, at 12:50 PM, Magnus Danielson mag...@rubidium.dyndns.org wrote: Bernd, Brilliant point. Easy to miss if one has a to simple model of the oscillator at hand. Since it is a single-side-band mode, it will show up both as AM and PM with the same amplitude. Cheers, Magnus On 09/10/2014 03:27 PM, Bernd Neubig wrote: Hi Bob, your description oft he spurious coming from higher overtone of low-frequency modes is correct. I want to add, that all thickness-shear mode crystals (such as AT, BT and SC-cut) have so-called an-harmonic spurious modes, which is a whole ensemble of spurs located slightly above above the main mode (fundamental or overtone mode). slightly means starting at about 50 kHz to 200 kHz above for fundamental mode and about 30 ... 50 kHz above for overtone modes. These an-harmonic modes are relaled to the length and width of the active area (electrode). These spurious modes do not come only into play for wide-pull VCXO, but also in the case that the EFC input is used for modulation with signals in the audio frequency range. Remember that a frequency modulated signal has side-lines which are N* the audio frequency apart from the carrier. The amplitude of these side lines follows the so-called Bessel functions and varies with the modulation index. If it happens that such a Bessel-line for a particular modulation frequency coincides with such a spur, it comes to an interference, This means the modulation frequency response becomes a discontinuity (dip) at a sharp frequency. Such band breaks do even occur if the spurious is so weak that it can barely be seen on a network analyzer. Regards Bernd DK1AG AXTAL GmbH Co. KG www.axtal.com -Ursprüngliche Nachricht- Von: time-nuts [mailto:time-nuts-boun...@febo.com] Im Auftrag von Bob Camp Gesendet: Sonntag, 7. September 2014 04:21 An: Discussion of precise time and frequency measurement Betreff: Re: [time-nuts] OCXO Voltage Input? (Bob Camp) Hi Simple answer = crystals are never perfect. Longer winded, but very incomplete answer = A spurious response in a crystal normally refers to a mode that is not one of the “identified” modes of the crystal. An AT has a set of identified modes, an SC has a more complex set of modes. In the case of the AT it would be the fundamental and the odd overtones. In the case of the SC you have the A, B, C modes and their odd overtones. None of those are considered spurious. A spur can come from a lot of different places. One common one is higher order vibrations in a longer dimension face of the resonator. The 183rd overtone of the width of the blank is still a legitimate resonant mode. Another source are modes other than shear (like flex). Deriving a full catalog of all the modes of an arbitrary blank design is a major project. There are only a handful
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Hi I understand that we are talking about a couple of different things. Since we started out talking about OCXO’s I figured it was worth it to bring it back to where we started. Bob On Sep 11, 2014, at 4:32 AM, Bernd Neubig bneu...@t-online.de wrote: Hi Bob, your example is correct. However I was not talking about OCXO specifically, but about crystal oscillators in general. And the effect I have mentioned is not limited to wide-pull VCXO, but may occur at normal VCXO also. I named the modulation audio for sake of simplicity of expression - it was certainly not accurate enough. If you are modulating data (FSK) then such interferences have a risk to occur even at moderate data rates. I do not talk about theorectical can be's but about practical experience. Best regards Bernd -Ursprüngliche Nachricht- Von: time-nuts [mailto:time-nuts-boun...@febo.com] Im Auftrag von Bob Camp Gesendet: Donnerstag, 11. September 2014 00:18 An: Discussion of precise time and frequency measurement Betreff: Re: [time-nuts] OCXO Voltage Input? (Bob Camp) Hi If you are modulating a normal OCXO EFC with audio, and the output frequency is not being multiplied up, the modulation index will be very low. Low modulation index means that the higher order FM sidebands will be quite far down. If you take “audio” to be 10 KHz, and a VHF OCXO to be 100 MHz: With a 10 ppm EFC range, you get 1.0 KHz of deviation. The modulation index is 1 a decade below your upper modulation frequency. That’s already a pretty wide swing OCXO and a fairly high modulation frequency for an EFC line. If you have a spur that is in the 50 to 150 KHz range, you are talking about the 5th to 15th sideband off of 10 KHz or the 50th to 150th sideband off of 1 KHz. At 50 sidebands out and an index of 1, you are in the “forget about it” region. Even at 10 KHz, the sideband is not likely to create much of an issue. The distortion from the non-linear EFC slope will be more of a problem in a practical sense. —— Since the modulation is single sideband, yes it converts PM - AM. It also will be impacted by any limiters in the system and will not multiply the same way as a pure PM modulation. The phase of the sideband will change as you go through the resonance, further messing up the multiplication / limiter math. Bob On Sep 10, 2014, at 12:50 PM, Magnus Danielson mag...@rubidium.dyndns.org wrote: Bernd, Brilliant point. Easy to miss if one has a to simple model of the oscillator at hand. Since it is a single-side-band mode, it will show up both as AM and PM with the same amplitude. Cheers, Magnus On 09/10/2014 03:27 PM, Bernd Neubig wrote: Hi Bob, your description oft he spurious coming from higher overtone of low-frequency modes is correct. I want to add, that all thickness-shear mode crystals (such as AT, BT and SC-cut) have so-called an-harmonic spurious modes, which is a whole ensemble of spurs located slightly above above the main mode (fundamental or overtone mode). slightly means starting at about 50 kHz to 200 kHz above for fundamental mode and about 30 ... 50 kHz above for overtone modes. These an-harmonic modes are relaled to the length and width of the active area (electrode). These spurious modes do not come only into play for wide-pull VCXO, but also in the case that the EFC input is used for modulation with signals in the audio frequency range. Remember that a frequency modulated signal has side-lines which are N* the audio frequency apart from the carrier. The amplitude of these side lines follows the so-called Bessel functions and varies with the modulation index. If it happens that such a Bessel-line for a particular modulation frequency coincides with such a spur, it comes to an interference, This means the modulation frequency response becomes a discontinuity (dip) at a sharp frequency. Such band breaks do even occur if the spurious is so weak that it can barely be seen on a network analyzer. Regards Bernd DK1AG AXTAL GmbH Co. KG www.axtal.com -Ursprüngliche Nachricht- Von: time-nuts [mailto:time-nuts-boun...@febo.com] Im Auftrag von Bob Camp Gesendet: Sonntag, 7. September 2014 04:21 An: Discussion of precise time and frequency measurement Betreff: Re: [time-nuts] OCXO Voltage Input? (Bob Camp) Hi Simple answer = crystals are never perfect. Longer winded, but very incomplete answer = A spurious response in a crystal normally refers to a mode that is not one of the “identified” modes of the crystal. An AT has a set of identified modes, an SC has a more complex set of modes. In the case of the AT it would be the fundamental and the odd overtones. In the case of the SC you have the A, B, C modes and their odd overtones. None of those are considered spurious. A spur can come from a lot of different places. One common one is higher order vibrations
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
I have on purpose stayed out of this discussions but I think it is time to mention some benchmarks. Last year we did a work over of the Shera controller when we where allowed to release assembly and hex code. In order to make sure that the code was solid we ran extensive tests using a 1 pps from a tbolt the original AD1861 which by no means is an ideal DAC and a Morion MV89. The DAC resolution is 1.7 E-13 per bit and after a couple of days warmup the DAC did not change a single bit for 0.2 hrs and 1 bit over 0.4 hrs. Over an 80 hr. period the total change was 120 bits mostly aging but you can also see ambient temperature change. Data and plots are available but can not be attached because of data limitation Bert Kehren In a message dated 9/11/2014 7:37:03 A.M. Eastern Daylight Time, kb...@n1k.org writes: Hi I understand that we are talking about a couple of different things. Since we started out talking about OCXO’s I figured it was worth it to bring it back to where we started. Bob On Sep 11, 2014, at 4:32 AM, Bernd Neubig bneu...@t-online.de wrote: Hi Bob, your example is correct. However I was not talking about OCXO specifically, but about crystal oscillators in general. And the effect I have mentioned is not limited to wide-pull VCXO, but may occur at normal VCXO also. I named the modulation audio for sake of simplicity of expression - it was certainly not accurate enough. If you are modulating data (FSK) then such interferences have a risk to occur even at moderate data rates. I do not talk about theorectical can be's but about practical experience. Best regards Bernd -Ursprüngliche Nachricht- Von: time-nuts [mailto:time-nuts-boun...@febo.com] Im Auftrag von Bob Camp Gesendet: Donnerstag, 11. September 2014 00:18 An: Discussion of precise time and frequency measurement Betreff: Re: [time-nuts] OCXO Voltage Input? (Bob Camp) Hi If you are modulating a normal OCXO EFC with audio, and the output frequency is not being multiplied up, the modulation index will be very low. Low modulation index means that the higher order FM sidebands will be quite far down. If you take “audio” to be 10 KHz, and a VHF OCXO to be 100 MHz: With a 10 ppm EFC range, you get 1.0 KHz of deviation. The modulation index is 1 a decade below your upper modulation frequency. That’s already a pretty wide swing OCXO and a fairly high modulation frequency for an EFC line. If you have a spur that is in the 50 to 150 KHz range, you are talking about the 5th to 15th sideband off of 10 KHz or the 50th to 150th sideband off of 1 KHz. At 50 sidebands out and an index of 1, you are in the “forget about it” region. Even at 10 KHz, the sideband is not likely to create much of an issue. The distortion from the non-linear EFC slope will be more of a problem in a practical sense. —— Since the modulation is single sideband, yes it converts PM - AM. It also will be impacted by any limiters in the system and will not multiply the same way as a pure PM modulation. The phase of the sideband will change as you go through the resonance, further messing up the multiplication / limiter math. Bob On Sep 10, 2014, at 12:50 PM, Magnus Danielson mag...@rubidium.dyndns.org wrote: Bernd, Brilliant point. Easy to miss if one has a to simple model of the oscillator at hand. Since it is a single-side-band mode, it will show up both as AM and PM with the same amplitude. Cheers, Magnus On 09/10/2014 03:27 PM, Bernd Neubig wrote: Hi Bob, your description oft he spurious coming from higher overtone of low-frequency modes is correct. I want to add, that all thickness-shear mode crystals (such as AT, BT and SC-cut) have so-called an-harmonic spurious modes, which is a whole ensemble of spurs located slightly above above the main mode (fundamental or overtone mode). slightly means starting at about 50 kHz to 200 kHz above for fundamental mode and about 30 ... 50 kHz above for overtone modes. These an-harmonic modes are relaled to the length and width of the active area (electrode). These spurious modes do not come only into play for wide-pull VCXO, but also in the case that the EFC input is used for modulation with signals in the audio frequency range. Remember that a frequency modulated signal has side-lines which are N* the audio frequency apart from the carrier. The amplitude of these side lines follows the so-called Bessel functions and varies with the modulation index. If it happens that such a Bessel-line for a particular modulation frequency coincides with such a spur, it comes to an interference, This means the modulation frequency response becomes a discontinuity (dip) at a sharp frequency. Such band breaks do even occur if the spurious is so weak that it can barely be seen on a network analyzer. Regards Bernd DK1AG AXTAL GmbH Co. KG
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Hi Bob, your description oft he spurious coming from higher overtone of low-frequency modes is correct. I want to add, that all thickness-shear mode crystals (such as AT, BT and SC-cut) have so-called an-harmonic spurious modes, which is a whole ensemble of spurs located slightly above above the main mode (fundamental or overtone mode). slightly means starting at about 50 kHz to 200 kHz above for fundamental mode and about 30 ... 50 kHz above for overtone modes. These an-harmonic modes are relaled to the length and width of the active area (electrode). These spurious modes do not come only into play for wide-pull VCXO, but also in the case that the EFC input is used for modulation with signals in the audio frequency range. Remember that a frequency modulated signal has side-lines which are N* the audio frequency apart from the carrier. The amplitude of these side lines follows the so-called Bessel functions and varies with the modulation index. If it happens that such a Bessel-line for a particular modulation frequency coincides with such a spur, it comes to an interference, This means the modulation frequency response becomes a discontinuity (dip) at a sharp frequency. Such band breaks do even occur if the spurious is so weak that it can barely be seen on a network analyzer. Regards Bernd DK1AG AXTAL GmbH Co. KG www.axtal.com -Ursprüngliche Nachricht- Von: time-nuts [mailto:time-nuts-boun...@febo.com] Im Auftrag von Bob Camp Gesendet: Sonntag, 7. September 2014 04:21 An: Discussion of precise time and frequency measurement Betreff: Re: [time-nuts] OCXO Voltage Input? (Bob Camp) Hi Simple answer = crystals are never perfect. Longer winded, but very incomplete answer = A spurious response in a crystal normally refers to a mode that is not one of the “identified” modes of the crystal. An AT has a set of identified modes, an SC has a more complex set of modes. In the case of the AT it would be the fundamental and the odd overtones. In the case of the SC you have the A, B, C modes and their odd overtones. None of those are considered spurious. A spur can come from a lot of different places. One common one is higher order vibrations in a longer dimension face of the resonator. The 183rd overtone of the width of the blank is still a legitimate resonant mode. Another source are modes other than shear (like flex). Deriving a full catalog of all the modes of an arbitrary blank design is a major project. There are only a handful of people out there who are into that sort of thing (as opposed to simply cranking through some formulas). Practical answer = Don’t worry about it. Unless you are building a wide pull VCXO or a wide deviation VCXO (often the same thing) you will never notice them. Bob On Sep 6, 2014, at 9:13 PM, Hal Murray hmur...@megapathdsl.net wrote: kb...@n1k.org said: The biggest problem comes from crystal spurs rather than crystal Q. What's the mechanism for making spurs with a crystal? -- These are my opinions. I hate spam. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Bernd, Brilliant point. Easy to miss if one has a to simple model of the oscillator at hand. Since it is a single-side-band mode, it will show up both as AM and PM with the same amplitude. Cheers, Magnus On 09/10/2014 03:27 PM, Bernd Neubig wrote: Hi Bob, your description oft he spurious coming from higher overtone of low-frequency modes is correct. I want to add, that all thickness-shear mode crystals (such as AT, BT and SC-cut) have so-called an-harmonic spurious modes, which is a whole ensemble of spurs located slightly above above the main mode (fundamental or overtone mode). slightly means starting at about 50 kHz to 200 kHz above for fundamental mode and about 30 ... 50 kHz above for overtone modes. These an-harmonic modes are relaled to the length and width of the active area (electrode). These spurious modes do not come only into play for wide-pull VCXO, but also in the case that the EFC input is used for modulation with signals in the audio frequency range. Remember that a frequency modulated signal has side-lines which are N* the audio frequency apart from the carrier. The amplitude of these side lines follows the so-called Bessel functions and varies with the modulation index. If it happens that such a Bessel-line for a particular modulation frequency coincides with such a spur, it comes to an interference, This means the modulation frequency response becomes a discontinuity (dip) at a sharp frequency. Such band breaks do even occur if the spurious is so weak that it can barely be seen on a network analyzer. Regards Bernd DK1AG AXTAL GmbH Co. KG www.axtal.com -Ursprüngliche Nachricht- Von: time-nuts [mailto:time-nuts-boun...@febo.com] Im Auftrag von Bob Camp Gesendet: Sonntag, 7. September 2014 04:21 An: Discussion of precise time and frequency measurement Betreff: Re: [time-nuts] OCXO Voltage Input? (Bob Camp) Hi Simple answer = crystals are never perfect. Longer winded, but very incomplete answer = A spurious response in a crystal normally refers to a mode that is not one of the “identified” modes of the crystal. An AT has a set of identified modes, an SC has a more complex set of modes. In the case of the AT it would be the fundamental and the odd overtones. In the case of the SC you have the A, B, C modes and their odd overtones. None of those are considered spurious. A spur can come from a lot of different places. One common one is higher order vibrations in a longer dimension face of the resonator. The 183rd overtone of the width of the blank is still a legitimate resonant mode. Another source are modes other than shear (like flex). Deriving a full catalog of all the modes of an arbitrary blank design is a major project. There are only a handful of people out there who are into that sort of thing (as opposed to simply cranking through some formulas). Practical answer = Don’t worry about it. Unless you are building a wide pull VCXO or a wide deviation VCXO (often the same thing) you will never notice them. Bob On Sep 6, 2014, at 9:13 PM, Hal Murray hmur...@megapathdsl.net wrote: kb...@n1k.org said: The biggest problem comes from crystal spurs rather than crystal Q. What's the mechanism for making spurs with a crystal? -- These are my opinions. I hate spam. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Hi If you are modulating a normal OCXO EFC with audio, and the output frequency is not being multiplied up, the modulation index will be very low. Low modulation index means that the higher order FM sidebands will be quite far down. If you take “audio” to be 10 KHz, and a VHF OCXO to be 100 MHz: With a 10 ppm EFC range, you get 1.0 KHz of deviation. The modulation index is 1 a decade below your upper modulation frequency. That’s already a pretty wide swing OCXO and a fairly high modulation frequency for an EFC line. If you have a spur that is in the 50 to 150 KHz range, you are talking about the 5th to 15th sideband off of 10 KHz or the 50th to 150th sideband off of 1 KHz. At 50 sidebands out and an index of 1, you are in the “forget about it” region. Even at 10 KHz, the sideband is not likely to create much of an issue. The distortion from the non-linear EFC slope will be more of a problem in a practical sense. —— Since the modulation is single sideband, yes it converts PM - AM. It also will be impacted by any limiters in the system and will not multiply the same way as a pure PM modulation. The phase of the sideband will change as you go through the resonance, further messing up the multiplication / limiter math. Bob On Sep 10, 2014, at 12:50 PM, Magnus Danielson mag...@rubidium.dyndns.org wrote: Bernd, Brilliant point. Easy to miss if one has a to simple model of the oscillator at hand. Since it is a single-side-band mode, it will show up both as AM and PM with the same amplitude. Cheers, Magnus On 09/10/2014 03:27 PM, Bernd Neubig wrote: Hi Bob, your description oft he spurious coming from higher overtone of low-frequency modes is correct. I want to add, that all thickness-shear mode crystals (such as AT, BT and SC-cut) have so-called an-harmonic spurious modes, which is a whole ensemble of spurs located slightly above above the main mode (fundamental or overtone mode). slightly means starting at about 50 kHz to 200 kHz above for fundamental mode and about 30 ... 50 kHz above for overtone modes. These an-harmonic modes are relaled to the length and width of the active area (electrode). These spurious modes do not come only into play for wide-pull VCXO, but also in the case that the EFC input is used for modulation with signals in the audio frequency range. Remember that a frequency modulated signal has side-lines which are N* the audio frequency apart from the carrier. The amplitude of these side lines follows the so-called Bessel functions and varies with the modulation index. If it happens that such a Bessel-line for a particular modulation frequency coincides with such a spur, it comes to an interference, This means the modulation frequency response becomes a discontinuity (dip) at a sharp frequency. Such band breaks do even occur if the spurious is so weak that it can barely be seen on a network analyzer. Regards Bernd DK1AG AXTAL GmbH Co. KG www.axtal.com -Ursprüngliche Nachricht- Von: time-nuts [mailto:time-nuts-boun...@febo.com] Im Auftrag von Bob Camp Gesendet: Sonntag, 7. September 2014 04:21 An: Discussion of precise time and frequency measurement Betreff: Re: [time-nuts] OCXO Voltage Input? (Bob Camp) Hi Simple answer = crystals are never perfect. Longer winded, but very incomplete answer = A spurious response in a crystal normally refers to a mode that is not one of the “identified” modes of the crystal. An AT has a set of identified modes, an SC has a more complex set of modes. In the case of the AT it would be the fundamental and the odd overtones. In the case of the SC you have the A, B, C modes and their odd overtones. None of those are considered spurious. A spur can come from a lot of different places. One common one is higher order vibrations in a longer dimension face of the resonator. The 183rd overtone of the width of the blank is still a legitimate resonant mode. Another source are modes other than shear (like flex). Deriving a full catalog of all the modes of an arbitrary blank design is a major project. There are only a handful of people out there who are into that sort of thing (as opposed to simply cranking through some formulas). Practical answer = Don’t worry about it. Unless you are building a wide pull VCXO or a wide deviation VCXO (often the same thing) you will never notice them. Bob On Sep 6, 2014, at 9:13 PM, Hal Murray hmur...@megapathdsl.net wrote: kb...@n1k.org said: The biggest problem comes from crystal spurs rather than crystal Q. What's the mechanism for making spurs with a crystal? -- These are my opinions. I hate spam. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Bob, On 09/06/2014 03:00 AM, Bob Camp wrote: Hi Oddly enough (and yes it is odd) you can modulate an oscillator well outside the crystal’s bandwidth. The bigger issue is that the EFC does not pull the crystal very far on a normal OCXO. The FM modulation index drops to very small numbers pretty fast as you go up in modulation frequency. You typically only worry about modulation sidebands that are above the phase noise floor. Since phase modulation sidebands go down as 1/Fmod on an FM modulator (for small modulation index) they get pretty low pretty fast. If your OCXO has an EFC range of 0.1 ppm at 10 MHz, it will swing 1 Hz p-p (+/- 0.5 Hz) for the full EFC voltage. At 5 Hz, you have a modulation index of 0.1. Of course if you are multiplying to 10 GHz, the index could be quite large. This gets back to the “this all depends on what you are doing”. If your EFC is 5V, a reasonably quiet signal would have noise below 0.5 mV. That’s already 80 db down. A very quiet supply should be in the 5 nV / sqrt(Hz) range. That would put the noise down 180 db. It’s unlikely that your OCXO has a phase noise spec of -180 dbc / Hz at 10 Hz. We may already be done … To bring all the numbers together: At 1 Hz the modulation will do a sideband X db down at your desired frequency. You will drop 20 db by the time you get to 10 Hz simply due to the 1/F FM-PM. Since the oscillator integrate frequency into phase, you have a 1/(2*pi*f) factor. The typical LaPlace model for an oscillator is Ko/s, where Ko is the input sensitivity of the oscillator. A more complete model needs to include the Q of the crystal, naturally, unless you are in-band of that Q where it has less drastic properties. Bottom line - it’s not all that hard to get a quiet enough EFC voltage. Agreed. I've found that thinking about systematic noises of low frequency (i.e. comparator frequency and overtones) as well as loop dynamics is what one should think about. Lack of DAC resolution hurts. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Hi Yes indeed, as you go below 1 Hz (or 1 radian/sec) all the things that “help” you roll off wise now hurt you. If you are worried about sidebands inside 1 Hz, you need to change a sign here and there. The only thing that saves you is that the noise floor is now coming up pretty fast. If you modulate a crystal oscillator, the loaded frequency of the crystal is changed to accomplish the modulation. When your FM swings 100 Hz high, your crystal is tuned 100 Hz high. When your modulation swings 100 Hz low, your crystal is tuned 100 Hz low. The Q has no impact in this case. No I did not believe it worked that way until I did it …. Since then I’ve built a *lot* of VCXO’s with modulation bandwidths than their crystal Q bandwidths. The biggest problem comes from crystal spurs rather than crystal Q. Bob On Sep 6, 2014, at 6:09 AM, Magnus Danielson mag...@rubidium.dyndns.org wrote: Bob, On 09/06/2014 03:00 AM, Bob Camp wrote: Hi Oddly enough (and yes it is odd) you can modulate an oscillator well outside the crystal’s bandwidth. The bigger issue is that the EFC does not pull the crystal very far on a normal OCXO. The FM modulation index drops to very small numbers pretty fast as you go up in modulation frequency. You typically only worry about modulation sidebands that are above the phase noise floor. Since phase modulation sidebands go down as 1/Fmod on an FM modulator (for small modulation index) they get pretty low pretty fast. If your OCXO has an EFC range of 0.1 ppm at 10 MHz, it will swing 1 Hz p-p (+/- 0.5 Hz) for the full EFC voltage. At 5 Hz, you have a modulation index of 0.1. Of course if you are multiplying to 10 GHz, the index could be quite large. This gets back to the “this all depends on what you are doing”. If your EFC is 5V, a reasonably quiet signal would have noise below 0.5 mV. That’s already 80 db down. A very quiet supply should be in the 5 nV / sqrt(Hz) range. That would put the noise down 180 db. It’s unlikely that your OCXO has a phase noise spec of -180 dbc / Hz at 10 Hz. We may already be done … To bring all the numbers together: At 1 Hz the modulation will do a sideband X db down at your desired frequency. You will drop 20 db by the time you get to 10 Hz simply due to the 1/F FM-PM. Since the oscillator integrate frequency into phase, you have a 1/(2*pi*f) factor. The typical LaPlace model for an oscillator is Ko/s, where Ko is the input sensitivity of the oscillator. A more complete model needs to include the Q of the crystal, naturally, unless you are in-band of that Q where it has less drastic properties. Bottom line - it’s not all that hard to get a quiet enough EFC voltage. Agreed. I've found that thinking about systematic noises of low frequency (i.e. comparator frequency and overtones) as well as loop dynamics is what one should think about. Lack of DAC resolution hurts. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Hi Bob, Agreed. I often find that modulations eats your margin out. PWM is interesting in this regard. PWM has the property that the lowest frequency has the highest amplitude and the overtones then decay with 1/f from that. For a given clock rate, as you add a bit of PWM precision, you half the PWM repetition rate and thus move the frequency down... where we are more sensitive to the modulation it causes, and the 1/f slope of the oscillator does not help. I designed a PWM-like signal that has reversed PWM spectrum so that the highest frequency has the strongest amplitude. The 1/f of the oscillator integration makes the modulation flat among the different bits and much easier to handle phase-noise wide. Another approach is sigma-delta style modulation, which noises out the amplitude. Higher-degree sigma-delta needs to avoid idle-tones for optimum result. Thus, paying attention to these details pays of with simplifying the effort to achieve good phase-noise properties. There is more dangers that can occur in PWM-space, but this should be enough of a starting-point. Cheers, Magnus On 09/06/2014 01:39 PM, Bob Camp wrote: Hi Yes indeed, as you go below 1 Hz (or 1 radian/sec) all the things that “help” you roll off wise now hurt you. If you are worried about sidebands inside 1 Hz, you need to change a sign here and there. The only thing that saves you is that the noise floor is now coming up pretty fast. If you modulate a crystal oscillator, the loaded frequency of the crystal is changed to accomplish the modulation. When your FM swings 100 Hz high, your crystal is tuned 100 Hz high. When your modulation swings 100 Hz low, your crystal is tuned 100 Hz low. The Q has no impact in this case. No I did not believe it worked that way until I did it …. Since then I’ve built a *lot* of VCXO’s with modulation bandwidths than their crystal Q bandwidths. The biggest problem comes from crystal spurs rather than crystal Q. Bob On Sep 6, 2014, at 6:09 AM, Magnus Danielson mag...@rubidium.dyndns.org wrote: Bob, On 09/06/2014 03:00 AM, Bob Camp wrote: Hi Oddly enough (and yes it is odd) you can modulate an oscillator well outside the crystal’s bandwidth. The bigger issue is that the EFC does not pull the crystal very far on a normal OCXO. The FM modulation index drops to very small numbers pretty fast as you go up in modulation frequency. You typically only worry about modulation sidebands that are above the phase noise floor. Since phase modulation sidebands go down as 1/Fmod on an FM modulator (for small modulation index) they get pretty low pretty fast. If your OCXO has an EFC range of 0.1 ppm at 10 MHz, it will swing 1 Hz p-p (+/- 0.5 Hz) for the full EFC voltage. At 5 Hz, you have a modulation index of 0.1. Of course if you are multiplying to 10 GHz, the index could be quite large. This gets back to the “this all depends on what you are doing”. If your EFC is 5V, a reasonably quiet signal would have noise below 0.5 mV. That’s already 80 db down. A very quiet supply should be in the 5 nV / sqrt(Hz) range. That would put the noise down 180 db. It’s unlikely that your OCXO has a phase noise spec of -180 dbc / Hz at 10 Hz. We may already be done … To bring all the numbers together: At 1 Hz the modulation will do a sideband X db down at your desired frequency. You will drop 20 db by the time you get to 10 Hz simply due to the 1/F FM-PM. Since the oscillator integrate frequency into phase, you have a 1/(2*pi*f) factor. The typical LaPlace model for an oscillator is Ko/s, where Ko is the input sensitivity of the oscillator. A more complete model needs to include the Q of the crystal, naturally, unless you are in-band of that Q where it has less drastic properties. Bottom line - it’s not all that hard to get a quiet enough EFC voltage. Agreed. I've found that thinking about systematic noises of low frequency (i.e. comparator frequency and overtones) as well as loop dynamics is what one should think about. Lack of DAC resolution hurts. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Hi One of the easy things to do with PWM is to dither the LSB. That gives you one more bit of precision. It still keeps the main tone at the same place. Your worst case tone happens at 50% duty cycle (perfect square wave). If you do your 50/50 as a square wave at Fmax(not Fmin), your fundamental “worst tone” is at your highest frequency rather than the lowest. Not easy with MCU PWM’s, pretty simple with an FPGA. By far the best thing to do is to clock your PWM at a nice high frequency (like a couple hundred MHz). That way you get lots of bits and your fundamental tone is still pretty high. Again, nice for 400 MHz clock FPGA’s, not so much for $0.50 MCU’s. Bob On Sep 6, 2014, at 12:52 PM, Magnus Danielson mag...@rubidium.dyndns.org wrote: Hi Bob, Agreed. I often find that modulations eats your margin out. PWM is interesting in this regard. PWM has the property that the lowest frequency has the highest amplitude and the overtones then decay with 1/f from that. For a given clock rate, as you add a bit of PWM precision, you half the PWM repetition rate and thus move the frequency down... where we are more sensitive to the modulation it causes, and the 1/f slope of the oscillator does not help. I designed a PWM-like signal that has reversed PWM spectrum so that the highest frequency has the strongest amplitude. The 1/f of the oscillator integration makes the modulation flat among the different bits and much easier to handle phase-noise wide. Another approach is sigma-delta style modulation, which noises out the amplitude. Higher-degree sigma-delta needs to avoid idle-tones for optimum result. Thus, paying attention to these details pays of with simplifying the effort to achieve good phase-noise properties. There is more dangers that can occur in PWM-space, but this should be enough of a starting-point. Cheers, Magnus On 09/06/2014 01:39 PM, Bob Camp wrote: Hi Yes indeed, as you go below 1 Hz (or 1 radian/sec) all the things that “help” you roll off wise now hurt you. If you are worried about sidebands inside 1 Hz, you need to change a sign here and there. The only thing that saves you is that the noise floor is now coming up pretty fast. If you modulate a crystal oscillator, the loaded frequency of the crystal is changed to accomplish the modulation. When your FM swings 100 Hz high, your crystal is tuned 100 Hz high. When your modulation swings 100 Hz low, your crystal is tuned 100 Hz low. The Q has no impact in this case. No I did not believe it worked that way until I did it …. Since then I’ve built a *lot* of VCXO’s with modulation bandwidths than their crystal Q bandwidths. The biggest problem comes from crystal spurs rather than crystal Q. Bob On Sep 6, 2014, at 6:09 AM, Magnus Danielson mag...@rubidium.dyndns.org wrote: Bob, On 09/06/2014 03:00 AM, Bob Camp wrote: Hi Oddly enough (and yes it is odd) you can modulate an oscillator well outside the crystal’s bandwidth. The bigger issue is that the EFC does not pull the crystal very far on a normal OCXO. The FM modulation index drops to very small numbers pretty fast as you go up in modulation frequency. You typically only worry about modulation sidebands that are above the phase noise floor. Since phase modulation sidebands go down as 1/Fmod on an FM modulator (for small modulation index) they get pretty low pretty fast. If your OCXO has an EFC range of 0.1 ppm at 10 MHz, it will swing 1 Hz p-p (+/- 0.5 Hz) for the full EFC voltage. At 5 Hz, you have a modulation index of 0.1. Of course if you are multiplying to 10 GHz, the index could be quite large. This gets back to the “this all depends on what you are doing”. If your EFC is 5V, a reasonably quiet signal would have noise below 0.5 mV. That’s already 80 db down. A very quiet supply should be in the 5 nV / sqrt(Hz) range. That would put the noise down 180 db. It’s unlikely that your OCXO has a phase noise spec of -180 dbc / Hz at 10 Hz. We may already be done … To bring all the numbers together: At 1 Hz the modulation will do a sideband X db down at your desired frequency. You will drop 20 db by the time you get to 10 Hz simply due to the 1/F FM-PM. Since the oscillator integrate frequency into phase, you have a 1/(2*pi*f) factor. The typical LaPlace model for an oscillator is Ko/s, where Ko is the input sensitivity of the oscillator. A more complete model needs to include the Q of the crystal, naturally, unless you are in-band of that Q where it has less drastic properties. Bottom line - it’s not all that hard to get a quiet enough EFC voltage. Agreed. I've found that thinking about systematic noises of low frequency (i.e. comparator frequency and overtones) as well as loop dynamics is what one should think about. Lack of DAC resolution hurts. Cheers, Magnus ___ time-nuts
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Hi Bob, Indeed. The way to keep the MCU PWM doing reasonable stuff is to use a higher rate, and then update the PWM value in sync with the wrap-around, and then alter the value (dither or whatever) so that the average has higher precision. First degree sigma-delta is actually not a bad strategy and fairly simple to do. With FPGA you can do more funky stuff, which is what I do. It is even better if you can use a linear DAC of sufficient rate and resolution. If you do hold-over functionality with static steering value, that is you stop updating the EFC-steering (this is what most folks do), then the resolution of the full steering can dominate the initial frequency offset. So, one should think about that too. As you go into hold-over, all of a sudden you can run into idle-tones in a way that normal dynamics would dither out. Cheers, Magnus On 09/06/2014 07:01 PM, Bob Camp wrote: Hi One of the easy things to do with PWM is to dither the LSB. That gives you one more bit of precision. It still keeps the main tone at the same place. Your worst case tone happens at 50% duty cycle (perfect square wave). If you do your 50/50 as a square wave at Fmax(not Fmin), your fundamental “worst tone” is at your highest frequency rather than the lowest. Not easy with MCU PWM’s, pretty simple with an FPGA. By far the best thing to do is to clock your PWM at a nice high frequency (like a couple hundred MHz). That way you get lots of bits and your fundamental tone is still pretty high. Again, nice for 400 MHz clock FPGA’s, not so much for $0.50 MCU’s. Bob On Sep 6, 2014, at 12:52 PM, Magnus Danielson mag...@rubidium.dyndns.org wrote: Hi Bob, Agreed. I often find that modulations eats your margin out. PWM is interesting in this regard. PWM has the property that the lowest frequency has the highest amplitude and the overtones then decay with 1/f from that. For a given clock rate, as you add a bit of PWM precision, you half the PWM repetition rate and thus move the frequency down... where we are more sensitive to the modulation it causes, and the 1/f slope of the oscillator does not help. I designed a PWM-like signal that has reversed PWM spectrum so that the highest frequency has the strongest amplitude. The 1/f of the oscillator integration makes the modulation flat among the different bits and much easier to handle phase-noise wide. Another approach is sigma-delta style modulation, which noises out the amplitude. Higher-degree sigma-delta needs to avoid idle-tones for optimum result. Thus, paying attention to these details pays of with simplifying the effort to achieve good phase-noise properties. There is more dangers that can occur in PWM-space, but this should be enough of a starting-point. Cheers, Magnus On 09/06/2014 01:39 PM, Bob Camp wrote: Hi Yes indeed, as you go below 1 Hz (or 1 radian/sec) all the things that “help” you roll off wise now hurt you. If you are worried about sidebands inside 1 Hz, you need to change a sign here and there. The only thing that saves you is that the noise floor is now coming up pretty fast. If you modulate a crystal oscillator, the loaded frequency of the crystal is changed to accomplish the modulation. When your FM swings 100 Hz high, your crystal is tuned 100 Hz high. When your modulation swings 100 Hz low, your crystal is tuned 100 Hz low. The Q has no impact in this case. No I did not believe it worked that way until I did it …. Since then I’ve built a *lot* of VCXO’s with modulation bandwidths than their crystal Q bandwidths. The biggest problem comes from crystal spurs rather than crystal Q. Bob On Sep 6, 2014, at 6:09 AM, Magnus Danielson mag...@rubidium.dyndns.org wrote: Bob, On 09/06/2014 03:00 AM, Bob Camp wrote: Hi Oddly enough (and yes it is odd) you can modulate an oscillator well outside the crystal’s bandwidth. The bigger issue is that the EFC does not pull the crystal very far on a normal OCXO. The FM modulation index drops to very small numbers pretty fast as you go up in modulation frequency. You typically only worry about modulation sidebands that are above the phase noise floor. Since phase modulation sidebands go down as 1/Fmod on an FM modulator (for small modulation index) they get pretty low pretty fast. If your OCXO has an EFC range of 0.1 ppm at 10 MHz, it will swing 1 Hz p-p (+/- 0.5 Hz) for the full EFC voltage. At 5 Hz, you have a modulation index of 0.1. Of course if you are multiplying to 10 GHz, the index could be quite large. This gets back to the “this all depends on what you are doing”. If your EFC is 5V, a reasonably quiet signal would have noise below 0.5 mV. That’s already 80 db down. A very quiet supply should be in the 5 nV / sqrt(Hz) range. That would put the noise down 180 db. It’s unlikely that your OCXO has a phase noise spec of -180 dbc / Hz at 10 Hz. We may already be done … To bring all the numbers together: At 1 Hz the modulation will do a sideband
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Hi If you are counting on your loop noise to spread your tones out - indeed not a good idea. There are several ways you can “go quiet” in your loop…. Bob On Sep 6, 2014, at 2:10 PM, Magnus Danielson mag...@rubidium.dyndns.org wrote: Hi Bob, Indeed. The way to keep the MCU PWM doing reasonable stuff is to use a higher rate, and then update the PWM value in sync with the wrap-around, and then alter the value (dither or whatever) so that the average has higher precision. First degree sigma-delta is actually not a bad strategy and fairly simple to do. With FPGA you can do more funky stuff, which is what I do. It is even better if you can use a linear DAC of sufficient rate and resolution. If you do hold-over functionality with static steering value, that is you stop updating the EFC-steering (this is what most folks do), then the resolution of the full steering can dominate the initial frequency offset. So, one should think about that too. As you go into hold-over, all of a sudden you can run into idle-tones in a way that normal dynamics would dither out. Cheers, Magnus On 09/06/2014 07:01 PM, Bob Camp wrote: Hi One of the easy things to do with PWM is to dither the LSB. That gives you one more bit of precision. It still keeps the main tone at the same place. Your worst case tone happens at 50% duty cycle (perfect square wave). If you do your 50/50 as a square wave at Fmax(not Fmin), your fundamental “worst tone” is at your highest frequency rather than the lowest. Not easy with MCU PWM’s, pretty simple with an FPGA. By far the best thing to do is to clock your PWM at a nice high frequency (like a couple hundred MHz). That way you get lots of bits and your fundamental tone is still pretty high. Again, nice for 400 MHz clock FPGA’s, not so much for $0.50 MCU’s. Bob On Sep 6, 2014, at 12:52 PM, Magnus Danielson mag...@rubidium.dyndns.org wrote: Hi Bob, Agreed. I often find that modulations eats your margin out. PWM is interesting in this regard. PWM has the property that the lowest frequency has the highest amplitude and the overtones then decay with 1/f from that. For a given clock rate, as you add a bit of PWM precision, you half the PWM repetition rate and thus move the frequency down... where we are more sensitive to the modulation it causes, and the 1/f slope of the oscillator does not help. I designed a PWM-like signal that has reversed PWM spectrum so that the highest frequency has the strongest amplitude. The 1/f of the oscillator integration makes the modulation flat among the different bits and much easier to handle phase-noise wide. Another approach is sigma-delta style modulation, which noises out the amplitude. Higher-degree sigma-delta needs to avoid idle-tones for optimum result. Thus, paying attention to these details pays of with simplifying the effort to achieve good phase-noise properties. There is more dangers that can occur in PWM-space, but this should be enough of a starting-point. Cheers, Magnus On 09/06/2014 01:39 PM, Bob Camp wrote: Hi Yes indeed, as you go below 1 Hz (or 1 radian/sec) all the things that “help” you roll off wise now hurt you. If you are worried about sidebands inside 1 Hz, you need to change a sign here and there. The only thing that saves you is that the noise floor is now coming up pretty fast. If you modulate a crystal oscillator, the loaded frequency of the crystal is changed to accomplish the modulation. When your FM swings 100 Hz high, your crystal is tuned 100 Hz high. When your modulation swings 100 Hz low, your crystal is tuned 100 Hz low. The Q has no impact in this case. No I did not believe it worked that way until I did it …. Since then I’ve built a *lot* of VCXO’s with modulation bandwidths than their crystal Q bandwidths. The biggest problem comes from crystal spurs rather than crystal Q. Bob On Sep 6, 2014, at 6:09 AM, Magnus Danielson mag...@rubidium.dyndns.org wrote: Bob, On 09/06/2014 03:00 AM, Bob Camp wrote: Hi Oddly enough (and yes it is odd) you can modulate an oscillator well outside the crystal’s bandwidth. The bigger issue is that the EFC does not pull the crystal very far on a normal OCXO. The FM modulation index drops to very small numbers pretty fast as you go up in modulation frequency. You typically only worry about modulation sidebands that are above the phase noise floor. Since phase modulation sidebands go down as 1/Fmod on an FM modulator (for small modulation index) they get pretty low pretty fast. If your OCXO has an EFC range of 0.1 ppm at 10 MHz, it will swing 1 Hz p-p (+/- 0.5 Hz) for the full EFC voltage. At 5 Hz, you have a modulation index of 0.1. Of course if you are multiplying to 10 GHz, the index could be quite large. This gets back to the “this all depends on what you are doing”. If your EFC is 5V, a reasonably quiet
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Bob, Agreed. I'm just saying that it goes static if you have PWM or something similar. As you see, there are many little details out there. Cheers, Magnus On 09/06/2014 08:30 PM, Bob Camp wrote: Hi If you are counting on your loop noise to spread your tones out - indeed not a good idea. There are several ways you can “go quiet” in your loop…. Bob On Sep 6, 2014, at 2:10 PM, Magnus Danielson mag...@rubidium.dyndns.org wrote: Hi Bob, Indeed. The way to keep the MCU PWM doing reasonable stuff is to use a higher rate, and then update the PWM value in sync with the wrap-around, and then alter the value (dither or whatever) so that the average has higher precision. First degree sigma-delta is actually not a bad strategy and fairly simple to do. With FPGA you can do more funky stuff, which is what I do. It is even better if you can use a linear DAC of sufficient rate and resolution. If you do hold-over functionality with static steering value, that is you stop updating the EFC-steering (this is what most folks do), then the resolution of the full steering can dominate the initial frequency offset. So, one should think about that too. As you go into hold-over, all of a sudden you can run into idle-tones in a way that normal dynamics would dither out. Cheers, Magnus On 09/06/2014 07:01 PM, Bob Camp wrote: Hi One of the easy things to do with PWM is to dither the LSB. That gives you one more bit of precision. It still keeps the main tone at the same place. Your worst case tone happens at 50% duty cycle (perfect square wave). If you do your 50/50 as a square wave at Fmax(not Fmin), your fundamental “worst tone” is at your highest frequency rather than the lowest. Not easy with MCU PWM’s, pretty simple with an FPGA. By far the best thing to do is to clock your PWM at a nice high frequency (like a couple hundred MHz). That way you get lots of bits and your fundamental tone is still pretty high. Again, nice for 400 MHz clock FPGA’s, not so much for $0.50 MCU’s. Bob On Sep 6, 2014, at 12:52 PM, Magnus Danielson mag...@rubidium.dyndns.org wrote: Hi Bob, Agreed. I often find that modulations eats your margin out. PWM is interesting in this regard. PWM has the property that the lowest frequency has the highest amplitude and the overtones then decay with 1/f from that. For a given clock rate, as you add a bit of PWM precision, you half the PWM repetition rate and thus move the frequency down... where we are more sensitive to the modulation it causes, and the 1/f slope of the oscillator does not help. I designed a PWM-like signal that has reversed PWM spectrum so that the highest frequency has the strongest amplitude. The 1/f of the oscillator integration makes the modulation flat among the different bits and much easier to handle phase-noise wide. Another approach is sigma-delta style modulation, which noises out the amplitude. Higher-degree sigma-delta needs to avoid idle-tones for optimum result. Thus, paying attention to these details pays of with simplifying the effort to achieve good phase-noise properties. There is more dangers that can occur in PWM-space, but this should be enough of a starting-point. Cheers, Magnus On 09/06/2014 01:39 PM, Bob Camp wrote: Hi Yes indeed, as you go below 1 Hz (or 1 radian/sec) all the things that “help” you roll off wise now hurt you. If you are worried about sidebands inside 1 Hz, you need to change a sign here and there. The only thing that saves you is that the noise floor is now coming up pretty fast. If you modulate a crystal oscillator, the loaded frequency of the crystal is changed to accomplish the modulation. When your FM swings 100 Hz high, your crystal is tuned 100 Hz high. When your modulation swings 100 Hz low, your crystal is tuned 100 Hz low. The Q has no impact in this case. No I did not believe it worked that way until I did it …. Since then I’ve built a *lot* of VCXO’s with modulation bandwidths than their crystal Q bandwidths. The biggest problem comes from crystal spurs rather than crystal Q. Bob On Sep 6, 2014, at 6:09 AM, Magnus Danielson mag...@rubidium.dyndns.org wrote: Bob, On 09/06/2014 03:00 AM, Bob Camp wrote: Hi Oddly enough (and yes it is odd) you can modulate an oscillator well outside the crystal’s bandwidth. The bigger issue is that the EFC does not pull the crystal very far on a normal OCXO. The FM modulation index drops to very small numbers pretty fast as you go up in modulation frequency. You typically only worry about modulation sidebands that are above the phase noise floor. Since phase modulation sidebands go down as 1/Fmod on an FM modulator (for small modulation index) they get pretty low pretty fast. If your OCXO has an EFC range of 0.1 ppm at 10 MHz, it will swing 1 Hz p-p (+/- 0.5 Hz) for the full EFC voltage. At 5 Hz, you have a modulation index of 0.1. Of course if you are multiplying to 10 GHz, the index could be quite large. This gets back to
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
kb...@n1k.org said: The biggest problem comes from crystal spurs rather than crystal Q. What's the mechanism for making spurs with a crystal? -- These are my opinions. I hate spam. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
On Sat, Sep 6, 2014 at 9:13 PM, Hal Murray hmur...@megapathdsl.net wrote: What's the mechanism for making spurs with a crystal? Get the corners nice and pointy and strap it to a boot. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Hi Simple answer = crystals are never perfect. Longer winded, but very incomplete answer = A spurious response in a crystal normally refers to a mode that is not one of the “identified” modes of the crystal. An AT has a set of identified modes, an SC has a more complex set of modes. In the case of the AT it would be the fundamental and the odd overtones. In the case of the SC you have the A, B, C modes and their odd overtones. None of those are considered spurious. A spur can come from a lot of different places. One common one is higher order vibrations in a longer dimension face of the resonator. The 183rd overtone of the width of the blank is still a legitimate resonant mode. Another source are modes other than shear (like flex). Deriving a full catalog of all the modes of an arbitrary blank design is a major project. There are only a handful of people out there who are into that sort of thing (as opposed to simply cranking through some formulas). Practical answer = Don’t worry about it. Unless you are building a wide pull VCXO or a wide deviation VCXO (often the same thing) you will never notice them. Bob On Sep 6, 2014, at 9:13 PM, Hal Murray hmur...@megapathdsl.net wrote: kb...@n1k.org said: The biggest problem comes from crystal spurs rather than crystal Q. What's the mechanism for making spurs with a crystal? -- These are my opinions. I hate spam. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Hi Bob, Being relatively new to this 'high end' time stuff, there's lots to learn... So, how much bandwidth might a typical OCXO have on the EFC pin? My assumption is that it is very low, but I have nothing to back that up. If I had 10Mhz or some other high frequency on the EFC line, would a typical OCXO respond to that? The concern is that any HF noise on the power pins of the driving op-amp might make it onto the EFC line. Of course most amps have good PSRR at low frequencies. If the EFC pin only has low frequency response, there shouldn't be an issue. Are there any OCXO schematics out on the web, that one could study? Dan On 9/5/2014 7:02 AM, time-nuts-requ...@febo.com wrote: Hi The EFC pin *might* have a bypass cap on it. If you drive it with an op-amp an isolating resistor might be needed. If so, a couple hundred ohms is likely enough to stabilize the op-amp. In a closed loop / control loop setting, the noise on the EFC will be whatever the loop generates. As long as you stay with good quality op-amps and low impedances in your filters, things should be plenty quiet. Things like EFC range, output frequency, phase noise, and intended use / circuit would be needed to come up with more specific information. Bob ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Some OCXO schematics: http://leapsecond.com/museum/10544/ http://leapsecond.com/museum/10811a/ /tvb (i5s) On Sep 5, 2014, at 5:50 AM, Dan Kemppainen d...@irtelemetrics.com wrote: Hi Bob, Being relatively new to this 'high end' time stuff, there's lots to learn... So, how much bandwidth might a typical OCXO have on the EFC pin? My assumption is that it is very low, but I have nothing to back that up. If I had 10Mhz or some other high frequency on the EFC line, would a typical OCXO respond to that? The concern is that any HF noise on the power pins of the driving op-amp might make it onto the EFC line. Of course most amps have good PSRR at low frequencies. If the EFC pin only has low frequency response, there shouldn't be an issue. Are there any OCXO schematics out on the web, that one could study? Dan On 9/5/2014 7:02 AM, time-nuts-requ...@febo.com wrote: Hi The EFC pin *might* have a bypass cap on it. If you drive it with an op-amp an isolating resistor might be needed. If so, a couple hundred ohms is likely enough to stabilize the op-amp. In a closed loop / control loop setting, the noise on the EFC will be whatever the loop generates. As long as you stay with good quality op-amps and low impedances in your filters, things should be plenty quiet. Things like EFC range, output frequency, phase noise, and intended use / circuit would be needed to come up with more specific information. Bob ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Oh, while we are at it, how about the 10543? Cheers, Magnus On 09/05/2014 03:49 PM, Tom Van Baak (lab) wrote: Some OCXO schematics: http://leapsecond.com/museum/10544/ http://leapsecond.com/museum/10811a/ /tvb (i5s) On Sep 5, 2014, at 5:50 AM, Dan Kemppainen d...@irtelemetrics.com wrote: Hi Bob, Being relatively new to this 'high end' time stuff, there's lots to learn... So, how much bandwidth might a typical OCXO have on the EFC pin? My assumption is that it is very low, but I have nothing to back that up. If I had 10Mhz or some other high frequency on the EFC line, would a typical OCXO respond to that? The concern is that any HF noise on the power pins of the driving op-amp might make it onto the EFC line. Of course most amps have good PSRR at low frequencies. If the EFC pin only has low frequency response, there shouldn't be an issue. Are there any OCXO schematics out on the web, that one could study? Dan On 9/5/2014 7:02 AM, time-nuts-requ...@febo.com wrote: Hi The EFC pin *might* have a bypass cap on it. If you drive it with an op-amp an isolating resistor might be needed. If so, a couple hundred ohms is likely enough to stabilize the op-amp. In a closed loop / control loop setting, the noise on the EFC will be whatever the loop generates. As long as you stay with good quality op-amps and low impedances in your filters, things should be plenty quiet. Things like EFC range, output frequency, phase noise, and intended use / circuit would be needed to come up with more specific information. Bob ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Tom, Awesome! Thanks! Section 2-14. Since noise on the EFC line affects the oscillator's stability (noise appears as FM on the output) care must be taken to ensure that a relatively noise free EFC... I was thinking the Varactor had be tied to the crystal, which only makes sense. So, the bottom line is the EFC inputs are probably susceptible to HF noise... The bottom line is, it's worth while ensuring the EFC amp and EFC signal are as clean as possible. Good stuff to know! Thanks! Dan On 9/5/2014 12:00 PM, time-nuts-requ...@febo.com wrote: Some OCXO schematics: http://leapsecond.com/museum/10544/ http://leapsecond.com/museum/10811a/ /tvb (i5s) ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
no that is not so bad, there --inside of the box is always a small RC which takes care the RF can't get into the oscillator, just look the oscillator circuirs 73 Alex On 9/5/2014 10:18 AM, Dan Kemppainen wrote: Tom, Awesome! Thanks! Section 2-14. Since noise on the EFC line affects the oscillator's stability (noise appears as FM on the output) care must be taken to ensure that a relatively noise free EFC... I was thinking the Varactor had be tied to the crystal, which only makes sense. So, the bottom line is the EFC inputs are probably susceptible to HF noise... The bottom line is, it's worth while ensuring the EFC amp and EFC signal are as clean as possible. Good stuff to know! Thanks! Dan On 9/5/2014 12:00 PM, time-nuts-requ...@febo.com wrote: Some OCXO schematics: http://leapsecond.com/museum/10544/ http://leapsecond.com/museum/10811a/ /tvb (i5s) ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
d...@irtelemetrics.com said: If I had 10Mhz or some other high frequency on the EFC line, would a typical OCXO respond to that? Some VCXOs actually specify their bandwidth. High audio is sometimes useful. I haven't seen anything beyond that, but I'm just listening to discussions like this one. There could well be applications that use a higher frequency. One application is correcting for mechanical vibrations. This is interesting in radar used on helicopters. (They do Doppler filtering to remove clutter. The lower speed of objects that can get through the filter depends on the clock stability.) PCs often FM modulate their clocks. It's a hack to get past the FCC EMI requirements. It spreads a spike in the frequency domain into a blob with a lower peak. I think 30 KHz is typical. The PCI specs were tweaked to allow this so they probably say something about the legal frequency limit. PCs probably don't use expensive OCXOs, but that technology might get used in other applications. How do FM modulators work? -- These are my opinions. I hate spam. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
it is not so easy to FM modulate a crystal oscillator, since the crystal has a high Q therefore the modulation bandwidth of a crystal oscillator is very narrow example: Q = F/dF - df = F/Q if F = 10MHz, Q = 60,000 dF = 166Hz 73 KJ6UHN Alex On 9/5/2014 1:10 PM, Hal Murray wrote: d...@irtelemetrics.com said: If I had 10Mhz or some other high frequency on the EFC line, would a typical OCXO respond to that? Some VCXOs actually specify their bandwidth. High audio is sometimes useful. I haven't seen anything beyond that, but I'm just listening to discussions like this one. There could well be applications that use a higher frequency. One application is correcting for mechanical vibrations. This is interesting in radar used on helicopters. (They do Doppler filtering to remove clutter. The lower speed of objects that can get through the filter depends on the clock stability.) PCs often FM modulate their clocks. It's a hack to get past the FCC EMI requirements. It spreads a spike in the frequency domain into a blob with a lower peak. I think 30 KHz is typical. The PCI specs were tweaked to allow this so they probably say something about the legal frequency limit. PCs probably don't use expensive OCXOs, but that technology might get used in other applications. How do FM modulators work? ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OCXO Voltage Input? (Bob Camp)
Hi Oddly enough (and yes it is odd) you can modulate an oscillator well outside the crystal’s bandwidth. The bigger issue is that the EFC does not pull the crystal very far on a normal OCXO. The FM modulation index drops to very small numbers pretty fast as you go up in modulation frequency. You typically only worry about modulation sidebands that are above the phase noise floor. Since phase modulation sidebands go down as 1/Fmod on an FM modulator (for small modulation index) they get pretty low pretty fast. If your OCXO has an EFC range of 0.1 ppm at 10 MHz, it will swing 1 Hz p-p (+/- 0.5 Hz) for the full EFC voltage. At 5 Hz, you have a modulation index of 0.1. Of course if you are multiplying to 10 GHz, the index could be quite large. This gets back to the “this all depends on what you are doing”. If your EFC is 5V, a reasonably quiet signal would have noise below 0.5 mV. That’s already 80 db down. A very quiet supply should be in the 5 nV / sqrt(Hz) range. That would put the noise down 180 db. It’s unlikely that your OCXO has a phase noise spec of -180 dbc / Hz at 10 Hz. We may already be done … To bring all the numbers together: At 1 Hz the modulation will do a sideband X db down at your desired frequency. You will drop 20 db by the time you get to 10 Hz simply due to the 1/F FM-PM. You are 80 or 180 db down (depending on your EFC) beyond that. So you are at X - 20 - 80 or 100 db below your 1Hz sideband. (noisy EFC) Chances are (unless you are at microwaves), you are well below the phase noise floor already. You are X - 20 - 180 or 200 db below the 1 Hz sideband (quiet EFC). Even at microwaves, you are below the phase noise floor. Most likely you are below it by 80 db. Bottom line - it’s not all that hard to get a quiet enough EFC voltage. Bob On Sep 5, 2014, at 7:32 PM, Alex Pummer a...@pcscons.com wrote: it is not so easy to FM modulate a crystal oscillator, since the crystal has a high Q therefore the modulation bandwidth of a crystal oscillator is very narrow example: Q = F/dF - df = F/Q if F = 10MHz, Q = 60,000 dF = 166Hz 73 KJ6UHN Alex On 9/5/2014 1:10 PM, Hal Murray wrote: d...@irtelemetrics.com said: If I had 10Mhz or some other high frequency on the EFC line, would a typical OCXO respond to that? Some VCXOs actually specify their bandwidth. High audio is sometimes useful. I haven't seen anything beyond that, but I'm just listening to discussions like this one. There could well be applications that use a higher frequency. One application is correcting for mechanical vibrations. This is interesting in radar used on helicopters. (They do Doppler filtering to remove clutter. The lower speed of objects that can get through the filter depends on the clock stability.) PCs often FM modulate their clocks. It's a hack to get past the FCC EMI requirements. It spreads a spike in the frequency domain into a blob with a lower peak. I think 30 KHz is typical. The PCI specs were tweaked to allow this so they probably say something about the legal frequency limit. PCs probably don't use expensive OCXOs, but that technology might get used in other applications. How do FM modulators work? ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
