Re: [time-nuts] AVAR Femtoseconds
Hi The use of femtoseconds come from the AVAR it's self. It was originally defined by time domain people. It's delineated by a Tau dimensioned in seconds. The time domain noise that's 1x10^-12 or 1x10^-15 down at one second does indeed have units of 1x10^-12 or 1x10^-15 seconds. As with any real world system one has to be very careful about the difference between resolution and accuracy. Resolution generally is easy, accuracy is more difficult. Any of the commonly used measurement techniques used to drive the AVAR are capable of enormous resolution. The problem is that past a certain point the added digits are simply internal noise and do not represent the DUT or the reference. One very simple example: A heterodyne system beats two 10 MHz oscillators down to a 1 Hz note. That gives you a 1x10^7 expansion. Drive the note directly into any of the common 11 digit per second counter (no limiters, no amps, straight in). You now have a LSD that has dimensions of 1x10^-18. You could claim that you have a system with a resolution of 1 atto-seocnd. A quick look at the output of the counter would show you that a lot of those digits were simply random numbers. You could do equally well by taking a 6 digit / second counter and a simple PIC program to make up another 5 digits of data. Bob On Jun 20, 2010, at 10:46 AM, Robert Benward wrote: Steve, I am a professional engineer, but in this arena I am an amateur. That is why I'm asking the questions, not to put down, but to understand some of the claims made. And as I said in one of my previous emails, I've seen amateurs run circles around the professionals, and those professional admitting utter astonishment at those amateur accomplishments (this is in the area of amateur astrophotography). What I have heard throughout this thread is a lot of bashing of those asking the questions, surfacing as derogatory and berating comments on other's understanding. I have also heard much claims to a certain procedure without one iota of numerical mumbo-jumbo to back it up. The issue here is an inability to describe a simple claim. Pete has attempted to put things in simple numbers, and I see where he is going, and I concur with some of his calculations. If one can not describe what appears to be a simple procedure, then I must question the basic understanding behind the explanation. If you make a wild claim, and then you can't even get the bullet on the paper, then I must question the shooter's understanding. I guess I am not comfortable with the use of femtoseconds to describe frequency accuracy. Technically, a locked PLL is at the exact frequency as the reference, as measured in the long term. The phase between the two may not be at zero, that depends on the type of phase detector and the DC offsets in the system. On the short term, phase noise of the reference will cause the loop to generate error terms which will change the phase of the DUT. Oscillators are also specified using phase noise, e.g. 135dB down @ 100Hz. That specifies how much energy is not in the bandwidth of the carrier. It also implies the phase is constantly changing! If the phase is changing, the error term is changing, and so forth and so on.Your measurement can only be as good as your reference oscillator. A DVM can only average this error, it can't give you the instantaneous value of the peak deviation of the error signal, which is what you would need to claim fs cycle to cycle timing. Fs units are appropriate for cycle to cycle variation, not long term or multicycle assements. Even the best HP DVM is only good to 3ppm on the 100mV scale and the shortest reading is 167us. That's 10 orders of magnitude greater that the deviation you are trying to measure. If you average the mixer output, you can no longer claim fs timing. What you can claim is a long term frequency stability in ppm. This is my simple understanding of phase detectors and mixers. You might get there by dividing down a bunch of numbers but I don't think the method supports the claim (of fs timing). Bob - Original Message - From: Steve Rooke To: Discussion of precise time and frequency measurement Sent: Sunday, June 20, 2010 2:00 AM Subject: Re: [time-nuts] Advantages Disadvantages of the TPLL Method Bob, Can I answer this one. On 20 June 2010 04:36, Robert Benward rbenw...@verizon.net wrote: Warren, I was responding to ke5fx comment using a 12-bit, 480-Hz serial DAQ in place of the voltage-to-frequency converter in the diagram above. A DAQ is a multifaceted data acquisition system, where as in your annotated diagram you showed an ADC. The DAQ that Warren is referring to to has a 12bit ADC input capable of performing up to 480 samples per second. I understand it's analog, but you said: Say you have a nice logic gate with 1 ns delay . So back to the analog
Re: [time-nuts] AVAR Femtoseconds
Bob Boy, you guys are really making me read a lot. I'm digesting Wiki right now. I see tau, but does identifying a tau of 1E-14 allow you to say you are locked to 10fs? The smallest tau I've seen in my E1938 collection is 1E-1. Bob - Original Message - From: Bob Camp To: Discussion of precise time and frequency measurement Sent: Sunday, June 20, 2010 11:32 AM Subject: Re: [time-nuts] AVAR Femtoseconds Hi The use of femtoseconds come from the AVAR it's self. It was originally defined by time domain people. It's delineated by a Tau dimensioned in seconds. The time domain noise that's 1x10^-12 or 1x10^-15 down at one second does indeed have units of 1x10^-12 or 1x10^-15 seconds. As with any real world system one has to be very careful about the difference between resolution and accuracy. Resolution generally is easy, accuracy is more difficult. Any of the commonly used measurement techniques used to drive the AVAR are capable of enormous resolution. The problem is that past a certain point the added digits are simply internal noise and do not represent the DUT or the reference. One very simple example: A heterodyne system beats two 10 MHz oscillators down to a 1 Hz note. That gives you a 1x10^7 expansion. Drive the note directly into any of the common 11 digit per second counter (no limiters, no amps, straight in). You now have a LSD that has dimensions of 1x10^-18. You could claim that you have a system with a resolution of 1 atto-seocnd. A quick look at the output of the counter would show you that a lot of those digits were simply random numbers. You could do equally well by taking a 6 digit / second counter and a simple PIC program to make up another 5 digits of data. Bob On Jun 20, 2010, at 10:46 AM, Robert Benward wrote: Steve, I am a professional engineer, but in this arena I am an amateur. That is why I'm asking the questions, not to put down, but to understand some of the claims made. And as I said in one of my previous emails, I've seen amateurs run circles around the professionals, and those professional admitting utter astonishment at those amateur accomplishments (this is in the area of amateur astrophotography). What I have heard throughout this thread is a lot of bashing of those asking the questions, surfacing as derogatory and berating comments on other's understanding. I have also heard much claims to a certain procedure without one iota of numerical mumbo-jumbo to back it up. The issue here is an inability to describe a simple claim. Pete has attempted to put things in simple numbers, and I see where he is going, and I concur with some of his calculations. If one can not describe what appears to be a simple procedure, then I must question the basic understanding behind the explanation. If you make a wild claim, and then you can't even get the bullet on the paper, then I must question the shooter's understanding. I guess I am not comfortable with the use of femtoseconds to describe frequency accuracy. Technically, a locked PLL is at the exact frequency as the reference, as measured in the long term. The phase between the two may not be at zero, that depends on the type of phase detector and the DC offsets in the system. On the short term, phase noise of the reference will cause the loop to generate error terms which will change the phase of the DUT. Oscillators are also specified using phase noise, e.g. 135dB down @ 100Hz. That specifies how much energy is not in the bandwidth of the carrier. It also implies the phase is constantly changing! If the phase is changing, the error term is changing, and so forth and so on.Your measurement can only be as good as your reference oscillator. A DVM can only average this error, it can't give you the instantaneous value of the peak deviation of the error signal, which is what you would need to claim fs cycle to cycle timing. Fs units are appropriate for cycle to cycle variation, not long term or multicycle assements. Even the best HP DVM is only good to 3ppm on the 100mV scale and the shortest reading is 167us. That's 10 orders of magnitude greater that the deviation you are trying to measure. If you average the mixer output, you can no longer claim fs timing. What you can claim is a long term frequency stability in ppm. This is my simple understanding of phase detectors and mixers. You might get there by dividing down a bunch of numbers but I don't think the method supports the claim (of fs timing). Bob - Original Message - From: Steve Rooke To: Discussion of precise time and frequency measurement Sent: Sunday, June 20, 2010 2:00 AM Subject: Re: [time-nuts] Advantages Disadvantages of the TPLL Method Bob, Can I answer this one. On 20 June 2010 04
Re: [time-nuts] AVAR Femtoseconds
Robert Benward wrote: Bob Boy, you guys are really making me read a lot. I'm digesting Wiki right now. I see tau, but does identifying a tau of 1E-14 allow you to say you are locked to 10fs? The smallest tau I've seen in my E1938 collection is 1E-1. Bob tau is the time over which the measurement is made, typically 1 second or greater. loosely speaking, the 1e-14 is the average fractional deviation of frequency over that time period. ___ 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] AVAR Femtoseconds
On 06/20/2010 11:53 PM, jimlux wrote: Robert Benward wrote: Bob Boy, you guys are really making me read a lot. I'm digesting Wiki right now. I see tau, but does identifying a tau of 1E-14 allow you to say you are locked to 10fs? The smallest tau I've seen in my E1938 collection is 1E-1. Bob tau is the time over which the measurement is made, typically 1 second or greater. loosely speaking, the 1e-14 is the average fractional deviation of frequency over that time period. It is a RMS type (much like statisticians standard deviation) of frequency stability over the observation interval of tau (little greek letter looking similar but not quite like a little t, which is the real reason for using it). Since it is a RMS type of measure, it indicates the effective power of noise, but not what the actual deviation in frequency will be, it's just a statistical measure. You may form a confidence interval such as that for 99,7 % or something which forms a scale-factor, quite similar to the use of the error function for the Gaussian distribution. An Allan deviation measure of 1E-14 is however not quite the same as 10 fs. Besides the units being wrong (Allan deviation is a relative and unit-less measure, essentially Hz/Hz) the Allan variance (and hence the Allan deviation) is a frequency stability measure, indicating the stability of normalized frequency rather than stability of normalized phase. The time deviation represents the stability of phase over some observation time. Assuming the nominal frequency and linear effects removed, then this would indicate the time error noise of the phase, here use of seconds could be used, but it would be to stretch things a bit. The time and frequency world has it's own qualities of noise... 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] AVAR Femtoseconds
Hi While the units are more properly femto seconds/ second you do indeed see 1s AVAR plots labeled in fS. Bob On Jun 20, 2010, at 6:25 PM, Magnus Danielson wrote: On 06/20/2010 11:53 PM, jimlux wrote: Robert Benward wrote: Bob Boy, you guys are really making me read a lot. I'm digesting Wiki right now. I see tau, but does identifying a tau of 1E-14 allow you to say you are locked to 10fs? The smallest tau I've seen in my E1938 collection is 1E-1. Bob tau is the time over which the measurement is made, typically 1 second or greater. loosely speaking, the 1e-14 is the average fractional deviation of frequency over that time period. It is a RMS type (much like statisticians standard deviation) of frequency stability over the observation interval of tau (little greek letter looking similar but not quite like a little t, which is the real reason for using it). Since it is a RMS type of measure, it indicates the effective power of noise, but not what the actual deviation in frequency will be, it's just a statistical measure. You may form a confidence interval such as that for 99,7 % or something which forms a scale-factor, quite similar to the use of the error function for the Gaussian distribution. An Allan deviation measure of 1E-14 is however not quite the same as 10 fs. Besides the units being wrong (Allan deviation is a relative and unit-less measure, essentially Hz/Hz) the Allan variance (and hence the Allan deviation) is a frequency stability measure, indicating the stability of normalized frequency rather than stability of normalized phase. The time deviation represents the stability of phase over some observation time. Assuming the nominal frequency and linear effects removed, then this would indicate the time error noise of the phase, here use of seconds could be used, but it would be to stretch things a bit. The time and frequency world has it's own qualities of noise... 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.
