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 <[email protected]> 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:[email protected]] 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 <[email protected]> > 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:[email protected]] 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 <[email protected]> wrote: >>> >>>> >>>> [email protected] 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 -- [email protected] To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >> _______________________________________________ >> time-nuts mailing list -- [email protected] To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
