Re: [time-nuts] Measuring gpsdo vs itself
On 11/05/2012 06:30 AM, Tom Van Baak wrote: Ah, I see what you mean now. Yes, that setup can give you a rough estimate of the counter's noise floor. I can't give you specific numbers but one danger with this sort of test is that the input and the timebase are artificially locked together (i.e. fixed phase relationship) through the common reference. Your measurements may thus show artificially less noise than a real-life case of independent input(s) and reference. This can happen if your sub-ns counter is based on interpolators. Because the input and the timebase are locked in phase, the counter lands near the same point of the interpolator scale on every single measurement, rather than experiencing the noise (and non-linearity) of the entire scale. It's a little more complex than interpolator non-linearities alone. You also need to include cross-talk between the signals. This cross-talk is usually higher between A and B inputs than from reference, but never the less. You would need to sweep the trigger input delays to illustrate these non-linearities. From a single measurement you can get both a better or worse number compared to the average which is what you would expect to see for free-running signals. So, you can get a rough idea about the baseline, but it is not a sufficient method. See the SR620 manual for a plot of non-linearities. 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] Measuring gpsdo vs itself
Thanks guys, Like usual more complicated than I thought. I was hoping that this would cancel any stability issues common to both the reference and the signal thus giving me best case ability. I seem to be getting numbers too good to be true so there must be a hitch. I get an ADEV 5x10-13 at 1 s mostly linear to 7x10-16 at 10,000 s with a small hump at 20s-80s. Figured there was some kind of gotcha. Doc On Mon, Nov 5, 2012 at 2:26 AM, Magnus Danielson mag...@rubidium.dyndns.org wrote: On 11/05/2012 06:30 AM, Tom Van Baak wrote: Ah, I see what you mean now. Yes, that setup can give you a rough estimate of the counter's noise floor. I can't give you specific numbers but one danger with this sort of test is that the input and the timebase are artificially locked together (i.e. fixed phase relationship) through the common reference. Your measurements may thus show artificially less noise than a real-life case of independent input(s) and reference. This can happen if your sub-ns counter is based on interpolators. Because the input and the timebase are locked in phase, the counter lands near the same point of the interpolator scale on every single measurement, rather than experiencing the noise (and non-linearity) of the entire scale. It's a little more complex than interpolator non-linearities alone. You also need to include cross-talk between the signals. This cross-talk is usually higher between A and B inputs than from reference, but never the less. You would need to sweep the trigger input delays to illustrate these non-linearities. From a single measurement you can get both a better or worse number compared to the average which is what you would expect to see for free-running signals. So, you can get a rough idea about the baseline, but it is not a sufficient method. See the SR620 manual for a plot of non-linearities. Cheers, Magnus __**_ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/** mailman/listinfo/time-nutshttps://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. -- Doc Bill Dailey KXØO ___ 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] Measuring gpsdo vs itself
Try this setup: feed the GPSDO into A and B inputs but not to the reference. That is, use the counter internal reference to time the difference so that you have an uncorrelated source that can span all the interpolator's nonlinearities. On Mon, Nov 5, 2012 at 11:42 AM, Bill Dailey docdai...@gmail.com wrote: Thanks guys, Like usual more complicated than I thought. I was hoping that this would cancel any stability issues common to both the reference and the signal thus giving me best case ability. I seem to be getting numbers too good to be true so there must be a hitch. I get an ADEV 5x10-13 at 1 s mostly linear to 7x10-16 at 10,000 s with a small hump at 20s-80s. Figured there was some kind of gotcha. Doc On Mon, Nov 5, 2012 at 2:26 AM, Magnus Danielson mag...@rubidium.dyndns.org wrote: On 11/05/2012 06:30 AM, Tom Van Baak wrote: Ah, I see what you mean now. Yes, that setup can give you a rough estimate of the counter's noise floor. I can't give you specific numbers but one danger with this sort of test is that the input and the timebase are artificially locked together (i.e. fixed phase relationship) through the common reference. Your measurements may thus show artificially less noise than a real-life case of independent input(s) and reference. This can happen if your sub-ns counter is based on interpolators. Because the input and the timebase are locked in phase, the counter lands near the same point of the interpolator scale on every single measurement, rather than experiencing the noise (and non-linearity) of the entire scale. It's a little more complex than interpolator non-linearities alone. You also need to include cross-talk between the signals. This cross-talk is usually higher between A and B inputs than from reference, but never the less. You would need to sweep the trigger input delays to illustrate these non-linearities. From a single measurement you can get both a better or worse number compared to the average which is what you would expect to see for free-running signals. So, you can get a rough idea about the baseline, but it is not a sufficient method. See the SR620 manual for a plot of non-linearities. Cheers, Magnus __**_ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/** mailman/listinfo/time-nuts https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. -- Doc Bill Dailey KXØO ___ 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] Measuring gpsdo vs itself
Hi As a practical example - a SR620 will look much better reading it's own reference than it will looking at almost anything else. That said, it's still a good idea to make sure the counter looks good reading it's own reference. If it doesn't look good, then you need to fix something. Bob On Nov 5, 2012, at 5:42 AM, Bill Dailey docdai...@gmail.com wrote: Thanks guys, Like usual more complicated than I thought. I was hoping that this would cancel any stability issues common to both the reference and the signal thus giving me best case ability. I seem to be getting numbers too good to be true so there must be a hitch. I get an ADEV 5x10-13 at 1 s mostly linear to 7x10-16 at 10,000 s with a small hump at 20s-80s. Figured there was some kind of gotcha. Doc On Mon, Nov 5, 2012 at 2:26 AM, Magnus Danielson mag...@rubidium.dyndns.org wrote: On 11/05/2012 06:30 AM, Tom Van Baak wrote: Ah, I see what you mean now. Yes, that setup can give you a rough estimate of the counter's noise floor. I can't give you specific numbers but one danger with this sort of test is that the input and the timebase are artificially locked together (i.e. fixed phase relationship) through the common reference. Your measurements may thus show artificially less noise than a real-life case of independent input(s) and reference. This can happen if your sub-ns counter is based on interpolators. Because the input and the timebase are locked in phase, the counter lands near the same point of the interpolator scale on every single measurement, rather than experiencing the noise (and non-linearity) of the entire scale. It's a little more complex than interpolator non-linearities alone. You also need to include cross-talk between the signals. This cross-talk is usually higher between A and B inputs than from reference, but never the less. You would need to sweep the trigger input delays to illustrate these non-linearities. From a single measurement you can get both a better or worse number compared to the average which is what you would expect to see for free-running signals. So, you can get a rough idea about the baseline, but it is not a sufficient method. See the SR620 manual for a plot of non-linearities. Cheers, Magnus __**_ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/** mailman/listinfo/time-nutshttps://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. -- Doc Bill Dailey KXØO ___ 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] Measuring gpsdo vs itself
Yes when I was measuring the noise floors of my 5370B's, the noise floor appeared to be noticeably better when the same oscillator was used as both the time base for the 5370B and the signal source for the test. Sorry I'm on the road right now (in Mexico City) and can't post a plot showing this. (For the original poster the list archives also contain some good information about the importance of matching signal levels to the counters in question and other related information which may also affect the results of these tests.) Best Regards Mark Spencer Message: 3 Date: Sun, 4 Nov 2012 21:30:41 -0800 From: Tom Van Baak t...@leapsecond.com To: Discussion of precise time and frequency measurement time-nuts@febo.com Subject: Re: [time-nuts] Measuring gpsdo vs itself Message-ID: C5FCD557A7C9416E8AA5E36FBF30AB2A@pc52 Content-Type: text/plain; charset=iso-8859-1 Ah, I see what you mean now. Yes, that setup can give you a rough estimate of the counter's noise floor. I can't give you specific numbers but one danger with this sort of test is that the input and the timebase are artificially locked together (i.e. fixed phase relationship) through the common reference. Your measurements may thus show artificially less noise than a real-life case of independent input(s) and reference. This can happen if your sub-ns counter is based on interpolators. Because the input and the timebase are locked in phase, the counter lands near the same point of the interpolator scale on every single measurement, rather than experiencing the noise (and non-linearity) of the entire scale. /tvb ___ 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] Measuring gpsdo vs itself
Hi Bob, On 11/05/2012 01:30 PM, Bob Camp wrote: Hi As a practical example - a SR620 will look much better reading it's own reference than it will looking at almost anything else. That said, it's still a good idea to make sure the counter looks good reading it's own reference. If it doesn't look good, then you need to fix something. I agree that this is a good self-test strategy, it usually gives a good clue if something is really bad or not, but as I pointed out, it doesn't give a fair idea of the measurement noise floor. 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] Measuring gpsdo vs itself
I would measure the counter both ways to characterize it, and you may learn a little more about how it reacts. Use its internal oscillator for the counter, apply the same 1PPS signal to both inputs. Use an external time base for the counter, apply the same 1 PPS to both inputs - make sure the timebase is not related to the 1 PPS. Then repeat, and use a common oscillator for all. You may also introduce delay into the stop channel and see what that causes and any differences, etc. Log your results in your time-nuts log book, you may have a reason to look at it again, down the road. Brian KD4FM ___ 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] Measuring gpsdo vs itself
On Tue, 06 Nov 2012 02:14:04 +0100, Magnus Danielson mag...@rubidium.dyndns.org wrote: Hi Bob, On 11/05/2012 01:30 PM, Bob Camp wrote: Hi As a practical example - a SR620 will look much better reading it's own reference than it will looking at almost anything else. That said, it's still a good idea to make sure the counter looks good reading it's own reference. If it doesn't look good, then you need to fix something. I agree that this is a good self-test strategy, it usually gives a good clue if something is really bad or not, but as I pointed out, it doesn't give a fair idea of the measurement noise floor. Cheers, Magnus A lot of counters support a self check mode which counts the internal reference. I tracked down an interesting problem in a Tektronix DC505 by wiring the internal channel A input to measure the internal reference. Noise from the display multiplexing was getting into the level shifter located before the last 4 integrated counter stages and causing spurious counts. Based on the documentation and how it did not match the actual circuit, I suspect Tektronix fiddled with the values in production until it seemed to work but never figured out the real problem or back annotated the documentation. ___ 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] Measuring gpsdo vs itself
Bill, This is usually a good idea, since the counter then has both a good short- and long-term stable/accurate timebase, inherited from the GPSDO. It means the internal timebase of the counter is no longer a factor in measurement stability or accuracy. There are exceptions to this, but I'll guess your setup is not one of them. This configuration is especially good for 1PPS TI measurements since it means a short 100 ns TI measurement is just as accurate as a long 0.99900 s measurement. I'm not sure I'd call this a reference independent system; it's simply using a GPSDO as the reference instead of the internal XO timebase of the counter. /tvb - Original Message - From: Bill Dailey docdai...@gmail.com To: Time Nuts time-nuts@febo.com Sent: Sunday, November 04, 2012 7:19 PM Subject: [time-nuts] Measuring gpsdo vs itself If I use a gpsdo as my reference and feed the same 10MHz into a counter does that yield the reference independent noise floor of the measuring system? Seems to me it would look like an ideal reference with respect to the measuring system. Thanks, Doc KX0O ___ 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] Measuring gpsdo vs itself
I guess what I am saying is if I discipline the counter with 10MHz and then measure the same 10MHz. Just making sure we are on the same page. Doc Sent from my iPad On Nov 4, 2012, at 10:22 PM, Tom Van Baak t...@leapsecond.com wrote: Bill, This is usually a good idea, since the counter then has both a good short- and long-term stable/accurate timebase, inherited from the GPSDO. It means the internal timebase of the counter is no longer a factor in measurement stability or accuracy. There are exceptions to this, but I'll guess your setup is not one of them. This configuration is especially good for 1PPS TI measurements since it means a short 100 ns TI measurement is just as accurate as a long 0.99900 s measurement. I'm not sure I'd call this a reference independent system; it's simply using a GPSDO as the reference instead of the internal XO timebase of the counter. /tvb - Original Message - From: Bill Dailey docdai...@gmail.com To: Time Nuts time-nuts@febo.com Sent: Sunday, November 04, 2012 7:19 PM Subject: [time-nuts] Measuring gpsdo vs itself If I use a gpsdo as my reference and feed the same 10MHz into a counter does that yield the reference independent noise floor of the measuring system? Seems to me it would look like an ideal reference with respect to the measuring system. Thanks, Doc KX0O ___ 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] Measuring gpsdo vs itself
docdai...@gmail.com said: I guess what I am saying is if I discipline the counter with 10MHz and then measure the same 10MHz. Just making sure we are on the same page. The input signal will be at a fixed offset from the reference clock. That offset will depend on cable lengths. If that setup gives a bad result, you know something doesn't work well. If it gives a good result, I don't think you can assume it will work well with other input signals. You might try using the GPSDO as the reference and measuring a typical crystal oscillator package. They are usually quite stable short term. Long term they track temperature. Beware, some low cost oscillator packages have internal DDS. Check the specs for jitter. [I think the idea is to ship unprogrammed packages to distributors where they can be programmed to order and shipped with low turn-around-time.] -- 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] Measuring gpsdo vs itself
Hi Tom, I think you missed the point. He is trying to measure the noise floor of the counter itself. So what he wants to know is if using the same signal for the time base and input, would that cancel out the signals contribution to the noise measurement. BillWB6BNQ Tom Van Baak wrote: Bill, This is usually a good idea, since the counter then has both a good short- and long-term stable/accurate timebase, inherited from the GPSDO. It means the internal timebase of the counter is no longer a factor in measurement stability or accuracy. There are exceptions to this, but I'll guess your setup is not one of them. This configuration is especially good for 1PPS TI measurements since it means a short 100 ns TI measurement is just as accurate as a long 0.99900 s measurement. I'm not sure I'd call this a reference independent system; it's simply using a GPSDO as the reference instead of the internal XO timebase of the counter. /tvb - Original Message - From: Bill Dailey docdai...@gmail.com To: Time Nuts time-nuts@febo.com Sent: Sunday, November 04, 2012 7:19 PM Subject: [time-nuts] Measuring gpsdo vs itself If I use a gpsdo as my reference and feed the same 10MHz into a counter does that yield the reference independent noise floor of the measuring system? Seems to me it would look like an ideal reference with respect to the measuring system. Thanks, Doc KX0O ___ 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] Measuring gpsdo vs itself
Ah, I see what you mean now. Yes, that setup can give you a rough estimate of the counter's noise floor. I can't give you specific numbers but one danger with this sort of test is that the input and the timebase are artificially locked together (i.e. fixed phase relationship) through the common reference. Your measurements may thus show artificially less noise than a real-life case of independent input(s) and reference. This can happen if your sub-ns counter is based on interpolators. Because the input and the timebase are locked in phase, the counter lands near the same point of the interpolator scale on every single measurement, rather than experiencing the noise (and non-linearity) of the entire scale. /tvb - Original Message - From: Bill Dailey docdai...@gmail.com To: Tom Van Baak t...@leapsecond.com; Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Sunday, November 04, 2012 8:33 PM Subject: Re: [time-nuts] Measuring gpsdo vs itself I guess what I am saying is if I discipline the counter with 10MHz and then measure the same 10MHz. Just making sure we are on the same page. Doc Sent from my iPad On Nov 4, 2012, at 10:22 PM, Tom Van Baak t...@leapsecond.com wrote: Bill, This is usually a good idea, since the counter then has both a good short- and long-term stable/accurate timebase, inherited from the GPSDO. It means the internal timebase of the counter is no longer a factor in measurement stability or accuracy. There are exceptions to this, but I'll guess your setup is not one of them. This configuration is especially good for 1PPS TI measurements since it means a short 100 ns TI measurement is just as accurate as a long 0.99900 s measurement. I'm not sure I'd call this a reference independent system; it's simply using a GPSDO as the reference instead of the internal XO timebase of the counter. /tvb - Original Message - From: Bill Dailey docdai...@gmail.com To: Time Nuts time-nuts@febo.com Sent: Sunday, November 04, 2012 7:19 PM Subject: [time-nuts] Measuring gpsdo vs itself If I use a gpsdo as my reference and feed the same 10MHz into a counter does that yield the reference independent noise floor of the measuring system? Seems to me it would look like an ideal reference with respect to the measuring system. Thanks, Doc KX0O ___ 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.
