Re: [time-nuts] SRS SR620 External Source Issue -- Help Request
1. Same test with a Fluke 6680 yeilds 10MHz mean exactly. 2.Tried different length cables and swapping the cables. Same result. 3. Autocal was done (a few times) - Original Message - From: Tom Van Baak [EMAIL PROTECTED] To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Thursday, February 22, 2007 00:12 Subject: Re: [time-nuts] SRS SR620 External Source Issue -- Help Request I have an SR620 counter that I set up with a Z3801A as an external reference. If I put a bnc T connector at the output of the Z3801A and use two equal length bnc cables, one to the ext. ref input on the back and the other to channel A then do a frequency measurement, I get a mean that is about .0015 Hz below 10,000,000.Hz. Does any know why this might be happening? I would expect it to read 10,000,000. exactly give or take a couple on the last digit. Three comments that may help. (1) 0.0015 Hz out of 10 MHz is 1.5e-10 which seems a little high but not too bad. But do not expect exactly 10 MHz with this sort of test. What you are giving to the channel A input is the most highly phase correlated signal you can imagine relative to the internal clock and the interpolators. This won't happen in real life with real input frequencies. (2) Try different lengths of the channel A cable and see if the number changes. My prediction is it will. (3) Run the setup menu self-calibration if you haven't in a while. /tvb ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request
I read the same spec on pg 63 but I read it as a fluctuation not an offset. I'm curious if you try the test Bruce suggested and set the gate time to 0.1s and then 0.01s and see if the offset increase by a factor of 10 for each change? I'm finding it hard to believe that this top of the line instrument can't handle this rudimentary task when my old Fluke 6680 with 500ps one shot precision (25 times slower than the SR620) handles it perfectly. I'm still not buying this offset is correct. - Original Message - From: Robert Crawford [EMAIL PROTECTED] To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Wednesday, February 21, 2007 23:13 Subject: Re: [time-nuts] SRS SR620 External Source Issue -- Help Request I ran into this same problem with my first SR620. I thought it was defective, so I sent it to SRS for a cal and refurbishment. It got better, but still had an offset of 0.0004 Hz. Later I bought a second one, and it, too, had an offset similar to yours. A careful reading of the manual, page 63, Performance Tests, Accuracy (where the 10 MHz output from the back panel is measured at the A input) reveals that a +/- 0.0035 Hz offset on a 10 MHz input is acceptable and within spec. I think this also applies to your test configuration. I have a photograph of my two SR620s, both using an external clock (PRS-10/GPS from an FS710 Distribution Amp), and both measuring the same signal on their A inputs. One reads 10,000,000.00096 Hz and the other reads 9.999,999.99827 Hz. I used a gate time of 1 second averaged for 100 readings. Both SR620s had fresh factory calibrations. Bob Crawford [EMAIL PROTECTED] wrote: I have an SR620 counter that I set up with a Z3801A as an external reference. If I put a bnc T connector at the output of the Z3801A and use two equal length bnc cables, one to the ext. ref input on the back and the other to channel A then do a frequency measurement, I get a mean that is about .0015 Hz below 10,000,000.Hz. Does any know why this might be happening? I would expect it to read 10,000,000. exactly give or take a couple on the last digit. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request
Dear ???, I encountered pretty much the same effect when I bought my second hand SR620. As you, I have been extremly surprised by the fact that the counter did not display exactly 10.0 MHz under the conditions that you describe. However, the answer is in the software: The built in time base has a coarse gain adjustment screw (electronic adjustment) and the fine gain adjustment is done by calibration byte number 4 in the software. Please note that this calibration byte (in fact it is 12 bits) is NOT influenced by the autocal procedure so no wonder that autocal brought you no changes. It seems, that even with an external time base connected this calibration byte stays active and that you have to set it by hand to its most correct value as explained under point 5 of Calibration Procedure / Clock Oscillator Calibration on page 77 of the handbook. If you don't have the handbook available: You may download it (but missing the schematics) from the SRS web site. Best regards Ulrich Bangert, DF6JB -Ursprüngliche Nachricht- Von: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Im Auftrag von [EMAIL PROTECTED] Gesendet: Mittwoch, 21. Februar 2007 23:57 An: Discussion of precise time and frequency measurement Betreff: [time-nuts] SRS SR620 External Source Issue -- Help Request I have an SR620 counter that I set up with a Z3801A as an external reference. If I put a bnc T connector at the output of the Z3801A and use two equal length bnc cables, one to the ext. ref input on the back and the other to channel A then do a frequency measurement, I get a mean that is about .0015 Hz below 10,000,000.Hz. Does any know why this might be happening? I would expect it to read 10,000,000. exactly give or take a couple on the last digit. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi- bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request
Flavio, Your experience exactly parallels my surprise and disappointment when I bought my first SR620. In fact, that's why I bought my second one: I just couldn't believe the results, even though it had been freshly calibrated by the factory. However, the Specifications on page vii of the manual tells the story. The Frequency Error is listed as +/- ((100 ps typ) [350 ps max])/Gate + Timebase Error) x Frequency. If you make timebase error zero and use a 1 second gate, for a 10 MHz signal you get +/- 0.001 Hz, or +/- 0.0035 Hz max. For a 0.1 second gate, the typical error goes up to 0.01 Hz, which is exactly what you are seeing. I haven't played with the CALBYTE 4 value since I sent both of mine to SRS for calibration. If I remember right you need to move a jumper inside the unit to enable adjustment of this value, and I didn't want to break the factory calibration stickers. I thought this was simply the internal time base frequency adjustment. Bob Crawford [EMAIL PROTECTED] wrote: I read the same spec on pg 63 but I read it as a fluctuation not an offset. I'm curious if you try the test Bruce suggested and set the gate time to 0.1s and then 0.01s and see if the offset increase by a factor of 10 for each change? I'm finding it hard to believe that this top of the line instrument can't handle this rudimentary task when my old Fluke 6680 with 500ps one shot precision (25 times slower than the SR620) handles it perfectly. I'm still not buying this offset is correct. - Original Message - From: Robert Crawford [EMAIL PROTECTED] To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Wednesday, February 21, 2007 23:13 Subject: Re: [time-nuts] SRS SR620 External Source Issue -- Help Request I ran into this same problem with my first SR620. I thought it was defective, so I sent it to SRS for a cal and refurbishment. It got better, but still had an offset of 0.0004 Hz. Later I bought a second one, and it, too, had an offset similar to yours. A careful reading of the manual, page 63, Performance Tests, Accuracy (where the 10 MHz output from the back panel is measured at the A input) reveals that a +/- 0.0035 Hz offset on a 10 MHz input is acceptable and within spec. I think this also applies to your test configuration. I have a photograph of my two SR620s, both using an external clock (PRS-10/GPS from an FS710 Distribution Amp), and both measuring the same signal on their A inputs. One reads 10,000,000.00096 Hz and the other reads 9.999,999.99827 Hz. I used a gate time of 1 second averaged for 100 readings. Both SR620s had fresh factory calibrations. Bob Crawford [EMAIL PROTECTED] wrote: I have an SR620 counter that I set up with a Z3801A as an external reference. If I put a bnc T connector at the output of the Z3801A and use two equal length bnc cables, one to the ext. ref input on the back and the other to channel A then do a frequency measurement, I get a mean that is about .0015 Hz below 10,000,000.Hz. Does any know why this might be happening? I would expect it to read 10,000,000. exactly give or take a couple on the last digit. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request
Ulrich wrote that even with an external time base connected this calibration byte stays active and that you have to set it by hand I tried it, but on my unit calbyte 4 does not have an effect when the external timebase is connect, unlike yours. -thank Flavio - Original Message - From: Ulrich Bangert [EMAIL PROTECTED] To: 'Discussion of precise time and frequency measurement' time-nuts@febo.com Sent: Thursday, February 22, 2007 11:24 Subject: Re: [time-nuts] SRS SR620 External Source Issue -- Help Request Dear ???, I encountered pretty much the same effect when I bought my second hand SR620. As you, I have been extremly surprised by the fact that the counter did not display exactly 10.0 MHz under the conditions that you describe. However, the answer is in the software: The built in time base has a coarse gain adjustment screw (electronic adjustment) and the fine gain adjustment is done by calibration byte number 4 in the software. Please note that this calibration byte (in fact it is 12 bits) is NOT influenced by the autocal procedure so no wonder that autocal brought you no changes. It seems, that even with an external time base connected this calibration byte stays active and that you have to set it by hand to its most correct value as explained under point 5 of Calibration Procedure / Clock Oscillator Calibration on page 77 of the handbook. If you don't have the handbook available: You may download it (but missing the schematics) from the SRS web site. Best regards Ulrich Bangert, DF6JB -Ursprüngliche Nachricht- Von: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Im Auftrag von [EMAIL PROTECTED] Gesendet: Mittwoch, 21. Februar 2007 23:57 An: Discussion of precise time and frequency measurement Betreff: [time-nuts] SRS SR620 External Source Issue -- Help Request I have an SR620 counter that I set up with a Z3801A as an external reference. If I put a bnc T connector at the output of the Z3801A and use two equal length bnc cables, one to the ext. ref input on the back and the other to channel A then do a frequency measurement, I get a mean that is about .0015 Hz below 10,000,000.Hz. Does any know why this might be happening? I would expect it to read 10,000,000. exactly give or take a couple on the last digit. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi- bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request
Bob, I see what your saying about the frequency error spec on pg vii, but I'm still reading as a fluctuation not an offset of the mean. In my mind, I don't see why there should be some sort of adjustment for this. I really doesn't make sense that the would make a counter that doesn't display a mean that is accurate. -Flavio - Original Message - From: Robert Crawford [EMAIL PROTECTED] To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Thursday, February 22, 2007 11:58 Subject: Re: [time-nuts] SRS SR620 External Source Issue -- Help Request Flavio, Your experience exactly parallels my surprise and disappointment when I bought my first SR620. In fact, that's why I bought my second one: I just couldn't believe the results, even though it had been freshly calibrated by the factory. However, the Specifications on page vii of the manual tells the story. The Frequency Error is listed as +/- ((100 ps typ) [350 ps max])/Gate + Timebase Error) x Frequency. If you make timebase error zero and use a 1 second gate, for a 10 MHz signal you get +/- 0.001 Hz, or +/- 0.0035 Hz max. For a 0.1 second gate, the typical error goes up to 0.01 Hz, which is exactly what you are seeing. I haven't played with the CALBYTE 4 value since I sent both of mine to SRS for calibration. If I remember right you need to move a jumper inside the unit to enable adjustment of this value, and I didn't want to break the factory calibration stickers. I thought this was simply the internal time base frequency adjustment. Bob Crawford [EMAIL PROTECTED] wrote: I read the same spec on pg 63 but I read it as a fluctuation not an offset. I'm curious if you try the test Bruce suggested and set the gate time to 0.1s and then 0.01s and see if the offset increase by a factor of 10 for each change? I'm finding it hard to believe that this top of the line instrument can't handle this rudimentary task when my old Fluke 6680 with 500ps one shot precision (25 times slower than the SR620) handles it perfectly. I'm still not buying this offset is correct. - Original Message - From: Robert Crawford [EMAIL PROTECTED] To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Wednesday, February 21, 2007 23:13 Subject: Re: [time-nuts] SRS SR620 External Source Issue -- Help Request I ran into this same problem with my first SR620. I thought it was defective, so I sent it to SRS for a cal and refurbishment. It got better, but still had an offset of 0.0004 Hz. Later I bought a second one, and it, too, had an offset similar to yours. A careful reading of the manual, page 63, Performance Tests, Accuracy (where the 10 MHz output from the back panel is measured at the A input) reveals that a +/- 0.0035 Hz offset on a 10 MHz input is acceptable and within spec. I think this also applies to your test configuration. I have a photograph of my two SR620s, both using an external clock (PRS-10/GPS from an FS710 Distribution Amp), and both measuring the same signal on their A inputs. One reads 10,000,000.00096 Hz and the other reads 9.999,999.99827 Hz. I used a gate time of 1 second averaged for 100 readings. Both SR620s had fresh factory calibrations. Bob Crawford [EMAIL PROTECTED] wrote: I have an SR620 counter that I set up with a Z3801A as an external reference. If I put a bnc T connector at the output of the Z3801A and use two equal length bnc cables, one to the ext. ref input on the back and the other to channel A then do a frequency measurement, I get a mean that is about .0015 Hz below 10,000,000.Hz. Does any know why this might be happening? I would expect it to read 10,000,000. exactly give or take a couple on the last digit. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request
Perhaps you're running into some sort of meta-stability problem since your clock and input are synchronous. Try lengthening one of the cables a foot or so and see if it makes any difference. I'd be very suspicious of a metastability problem if the results are even somewhat reproducible. Most of the quirks that get blamed on metastability are just simple setup/hold errors such that an input to a state machine has different values at different FFs. The two obvious causes are a longer delay on one path (more logic, longer routing) or slightly different setup times on the FFs so a signal gets to both at the same time and meets setup one one but doesn't the other one. -- These are my opinions, not necessarily my employer's. I hate spam. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request
[EMAIL PROTECTED] wrote: 1. Same test with a Fluke 6680 yeilds 10MHz mean exactly. 2.Tried different length cables and swapping the cables. Same result. 3. Autocal was done (a few times) - Original Message - From: Tom Van Baak [EMAIL PROTECTED] To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Thursday, February 22, 2007 00:12 Subject: Re: [time-nuts] SRS SR620 External Source Issue -- Help Request I have an SR620 counter that I set up with a Z3801A as an external reference. If I put a bnc T connector at the output of the Z3801A and use two equal length bnc cables, one to the ext. ref input on the back and the other to channel A then do a frequency measurement, I get a mean that is about .0015 Hz below 10,000,000.Hz. Does any know why this might be happening? I would expect it to read 10,000,000. exactly give or take a couple on the last digit. Three comments that may help. (1) 0.0015 Hz out of 10 MHz is 1.5e-10 which seems a little high but not too bad. But do not expect exactly 10 MHz with this sort of test. What you are giving to the channel A input is the most highly phase correlated signal you can imagine relative to the internal clock and the interpolators. This won't happen in real life with real input frequencies. (2) Try different lengths of the channel A cable and see if the number changes. My prediction is it will. (3) Run the setup menu self-calibration if you haven't in a while. /tvb ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts Flavio The effect that Tom described (differential nonlinearity in the interpolators) has a period of about 11.11 ns, so changing the relative delay by around 2.7ns (or small odd multiple thereof) is most likely to uncover the effect. If your relative delay changes is a multiple of 11,11ns or close to it you wont see much of an effect. However the manual indicates that this is relatively small when the time interval being measured is longer than a few microseconds or so. If the differential delay of the 2 synchronisers isn't corrected by the calibration procedure then a value of 150 ps would not be unexpected, the synchroniser flipflops are MC10H131's which have a clock to output delay of up to 1.5ns. As far as I can tell the autocal procedure will not correct such an offset. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
[time-nuts] SRS SR620 External Source Issue -- Help Request
I have an SR620 counter that I set up with a Z3801A as an external reference. If I put a bnc T connector at the output of the Z3801A and use two equal length bnc cables, one to the ext. ref input on the back and the other to channel A then do a frequency measurement, I get a mean that is about .0015 Hz below 10,000,000.Hz. Does any know why this might be happening? I would expect it to read 10,000,000. exactly give or take a couple on the last digit. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request
From: [EMAIL PROTECTED] Subject: Re: [time-nuts] SRS SR620 External Source Issue -- Help Request Date: Wed, 21 Feb 2007 18:13:08 -0500 Message-ID: [EMAIL PROTECTED] I've done the self calibration. Nothing changes. I've let everything warm-up and checked both channels A and B, same thing. I've swapped the cables on the front and the back, same thing. I've tried a T-connector on the ref input with 50ohm termination, same thing. The readings are pretty stable, they are just offset by -0.0015 Hz. Hmm. Interestingly enought I've seen something similar on a HP5372A where the A channel was slightly low on the frequency but not the B channel. I have not investigated that in detail to make any conclusions but that 5372A may be in for a slight trim-up. Since it's interpolators allow for interesting trimming of each interpolator step it would be interesting to see if I could trim it in and out of frequency! Now, if the interpolator trimming is the culprit, I need to wrap my head around on how it turns out as a frequency error. I'll check it again tomorrow, as I use it daily right now. I don't have the manuals (and particular the service manual) for the SR620, does anybody have them in electronics form? Cheers, Magnus ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request
Magnus Danielson wrote: I don't have the manuals (and particular the service manual) for the SR620, does anybody have them in electronics form? Cheers, Magnus ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts Hej Magnus http://thinksrs.com/downloads/PDFs/Manuals/SR620m.pdf Has everything except circuit diagrams - may be able to reconstruct from circuit description. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request
[EMAIL PROTECTED] wrote: I've done the self calibration. Nothing changes. I've let everything warm-up and checked both channels A and B, same thing. I've swapped the cables on the front and the back, same thing. I've tried a T-connector on the ref input with 50ohm termination, same thing. The readings are pretty stable, they are just offset by -0.0015 Hz. - Original Message - From: Brooke Clarke [EMAIL PROTECTED] To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Wednesday, February 21, 2007 18:04 Subject: Re: [time-nuts] SRS SR620 External Source Issue -- Help Request Hi Flavio: Have you tried using averaging? The right most digits are from an interpolator and if you check the manual the displayed resolution is much greater than the accuracy, but if you average say 1,000 readings then those digits get much better. Also have you done the self calibration? Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com [EMAIL PROTECTED] wrote: I have an SR620 counter that I set up with a Z3801A as an external reference. If I put a bnc T connector at the output of the Z3801A and use two equal length bnc cables, one to the ext. ref input on the back and the other to channel A then do a frequency measurement, I get a mean that is about .0015 Hz below 10,000,000.Hz. Does any know why this might be happening? I would expect it to read 10,000,000. exactly give or take a couple on the last digit. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts Flavio Your SR620 is well within its specifications, the tolerance for this test is -0.0035 Hz to + 0.0035 Hz. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request
At 02:57 PM 2/21/2007, you wrote: I have an SR620 counter that I set up with a Z3801A as an external reference. If I put a bnc T connector at the output of the Z3801A and use two equal length bnc cables, one to the ext. ref input on the back and the other to channel A then do a frequency measurement, I get a mean that is about .0015 Hz below 10,000,000.Hz. Does any know why this might be happening? I would expect it to read 10,000,000. exactly give or take a couple on the last digit. Perhaps you're running into some sort of meta-stability problem since your clock and input are synchronous. Try lengthening one of the cables a foot or so and see if it makes any difference. Also, it could be the compound result of noise in both the reference input and signal input amplifiers. -Mike- ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request
At 03:50 PM 2/21/2007, you wrote: At 02:57 PM 2/21/2007, you wrote: I have an SR620 counter that I set up with a Z3801A as an external reference. If I put a bnc T connector at the output of the Z3801A and use two equal length bnc cables, one to the ext. ref input on the back and the other to channel A then do a frequency measurement, I get a mean that is about .0015 Hz below 10,000,000.Hz. Does any know why this might be happening? I would expect it to read 10,000,000. exactly give or take a couple on the last digit. Perhaps you're running into some sort of meta-stability problem since your clock and input are synchronous. Try lengthening one of the cables a foot or so and see if it makes any difference. Also, it could be the compound result of noise in both the reference input and signal input amplifiers. -Mike- Another thing...It sounds like the Z3801A is being asked to drive 25 ohms (two 50 ohm loads in parallel), perhaps the waveform is distorting in such a way as to present a slow risetime edge to the counter, thus agravating the effects of noise. -Mike- ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request
Mike Fahmie wrote: At 02:57 PM 2/21/2007, you wrote: I have an SR620 counter that I set up with a Z3801A as an external reference. If I put a bnc T connector at the output of the Z3801A and use two equal length bnc cables, one to the ext. ref input on the back and the other to channel A then do a frequency measurement, I get a mean that is about .0015 Hz below 10,000,000.Hz. Does any know why this might be happening? I would expect it to read 10,000,000. exactly give or take a couple on the last digit. Perhaps you're running into some sort of meta-stability problem since your clock and input are synchronous. Try lengthening one of the cables a foot or so and see if it makes any difference. Also, it could be the compound result of noise in both the reference input and signal input amplifiers. -Mike- ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts Flavio It could also, depending on the design, be something quite mundane like the difference in propagation delay for an ECL gate for rising and falling edges. Or the difference in propagation delay from clock to output of an ECL fliflop between 0-1 and 1-0 output transitions. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request
Your SR620 is well within its specifications, the tolerance for this test is -0.0035 Hz to + 0.0035 Hz. The readings fluctuate by that amount, but the mean is still off by -0.00150 Hz. Changing cable length... Tried it, no difference. 25Ohm load with two terms Tried all variations of with and without term. No difference. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request
From: Dr Bruce Griffiths [EMAIL PROTECTED] Subject: Re: [time-nuts] SRS SR620 External Source Issue -- Help Request Date: Thu, 22 Feb 2007 12:40:57 +1300 Message-ID: [EMAIL PROTECTED] Hej Bruce, http://thinksrs.com/downloads/PDFs/Manuals/SR620m.pdf Many thanks! Has everything except circuit diagrams - may be able to reconstruct from circuit description. But then there is the patent. :) Schematics would be good too. Now, that the measures may be a little off the mark from measure to measure is not a supprise, but that a frequency measure (of its own timebase) would be statically off the mark is interesting. Cheers, Magnus ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request -- A thought
Here's a thought. Perhaps there is a problem with the rear reference input; A and B channels are fine since they read the same error. The question is Is there a way of adjusting the Reference Input so that it correctly read the 10Mhz input? I tried the same setup with my other counter and it read 10,000,000. or thereabouts correctly. So rule out the GPSDO and the cables. - Original Message - From: Dr Bruce Griffiths [EMAIL PROTECTED] To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Wednesday, February 21, 2007 19:47 Subject: Re: [time-nuts] SRS SR620 External Source Issue -- Help Request Hej Magnus The Frequency measurement logic (before the interpolators) appears to use a pair of 2 bit synchonisers, one for the rising edge and one for the falling edge of the internally generated frequency gate. The synchronisers synchronise the respective transitions of the frequency gate signal to the external frequency being measured. One synchroniser uses U508A + U509A the other uses U508B + U509B, so that any mismatch in propagation delays between U509A And U509B will result in a small error in the time interval. Even for ECL flipflops a propagation delay difference between U50(A and U509B of 150ps wouldn't be unusual. As far as I can tell the calibration procedures do not address this issue. The uncalibrated differential delay through the interpolation circuit source selection multiplexer will also contribute an offset. Perhaps a better assignment of propagation delay differences may be U509A and U509B 50ps, multiplexer and wiring delay mismatches 100ps. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request -- A thought
[EMAIL PROTECTED] wrote: Here's a thought. Perhaps there is a problem with the rear reference input; A and B channels are fine since they read the same error. The question is Is there a way of adjusting the Reference Input so that it correctly read the 10Mhz input? I tried the same setup with my other counter and it read 10,000,000. or thereabouts correctly. So rule out the GPSDO and the cables. - Original Message - From: Dr Bruce Griffiths [EMAIL PROTECTED] To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Wednesday, February 21, 2007 19:47 Subject: Re: [time-nuts] SRS SR620 External Source Issue -- Help Request Hej Magnus The Frequency measurement logic (before the interpolators) appears to use a pair of 2 bit synchonisers, one for the rising edge and one for the falling edge of the internally generated frequency gate. The synchronisers synchronise the respective transitions of the frequency gate signal to the external frequency being measured. One synchroniser uses U508A + U509A the other uses U508B + U509B, so that any mismatch in propagation delays between U509A And U509B will result in a small error in the time interval. Even for ECL flipflops a propagation delay difference between U50(A and U509B of 150ps wouldn't be unusual. As far as I can tell the calibration procedures do not address this issue. The uncalibrated differential delay through the interpolation circuit source selection multiplexer will also contribute an offset. Perhaps a better assignment of propagation delay differences may be U509A and U509B 50ps, multiplexer and wiring delay mismatches 100ps. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts Flavio Not easily. However if you run a test with say a 0.1 sec gate time and average several readings, then if my model is correct the apparent offset in the mean will be increased by a factor of 10 over that for a a 1 second gate time. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request -- A thought
...if my model is correct the apparent offset in the mean will be increased by a factor of 10 over that for a a 1 second gate time. Bruce Looks like you're right Bruce. I did both 1, 0.1 and 0.01 gate times and the error increased by a factor of 10. And it doesn't sound like this is a fixable in any easy sort of manner, right? ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request -- A thought
[EMAIL PROTECTED] wrote: ...if my model is correct the apparent offset in the mean will be increased by a factor of 10 over that for a a 1 second gate time. Bruce Looks like you're right Bruce. I did both 1, 0.1 and 0.01 gate times and the error increased by a factor of 10. And it doesn't sound like this is a fixable in any easy sort of manner, right? ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts Flavio If the SR620 can do some simple arithmetic before displaying the result, then this calibration problem is easily corrected. Otherwise you will have to do the corrections on a PC (or hand held calculator). The correction will vary with time and temperature so you will need to measure it periodically. Corrected frequency = measured frequency*(nominal gate time)/(nominal gate time + differential delay). In your case the differential delay appears to be about 150 picoseconds. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request
I ran into this same problem with my first SR620. I thought it was defective, so I sent it to SRS for a cal and refurbishment. It got better, but still had an offset of 0.0004 Hz. Later I bought a second one, and it, too, had an offset similar to yours. A careful reading of the manual, page 63, Performance Tests, Accuracy (where the 10 MHz output from the back panel is measured at the A input) reveals that a +/- 0.0035 Hz offset on a 10 MHz input is acceptable and within spec. I think this also applies to your test configuration. I have a photograph of my two SR620s, both using an external clock (PRS-10/GPS from an FS710 Distribution Amp), and both measuring the same signal on their A inputs. One reads 10,000,000.00096 Hz and the other reads 9.999,999.99827 Hz. I used a gate time of 1 second averaged for 100 readings. Both SR620s had fresh factory calibrations. Bob Crawford [EMAIL PROTECTED] wrote: I have an SR620 counter that I set up with a Z3801A as an external reference. If I put a bnc T connector at the output of the Z3801A and use two equal length bnc cables, one to the ext. ref input on the back and the other to channel A then do a frequency measurement, I get a mean that is about .0015 Hz below 10,000,000.Hz. Does any know why this might be happening? I would expect it to read 10,000,000. exactly give or take a couple on the last digit. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request -- A thought
Dr Bruce Griffiths wrote: [EMAIL PROTECTED] wrote: ...if my model is correct the apparent offset in the mean will be increased by a factor of 10 over that for a a 1 second gate time. Bruce Looks like you're right Bruce. I did both 1, 0.1 and 0.01 gate times and the error increased by a factor of 10. And it doesn't sound like this is a fixable in any easy sort of manner, right? ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts Flavio If the SR620 can do some simple arithmetic before displaying the result, then this calibration problem is easily corrected. Otherwise you will have to do the corrections on a PC (or hand held calculator). The correction will vary with time and temperature so you will need to measure it periodically. Corrected frequency = measured frequency*(nominal gate time)/(nominal gate time + differential delay). In your case the differential delay appears to be about 150 picoseconds. Bruce Bruce/Flavio, Could that be mostly corrected (at constant temperature) by inserting the proper delay at the right place (in the form of a short piece of coax cable)? It seems that the error could be easily reduced by a factor of 10. Didier ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request
I have an SR620 counter that I set up with a Z3801A as an external reference. If I put a bnc T connector at the output of the Z3801A and use two equal length bnc cables, one to the ext. ref input on the back and the other to channel A then do a frequency measurement, I get a mean that is about .0015 Hz below 10,000,000.Hz. Does any know why this might be happening? I would expect it to read 10,000,000. exactly give or take a couple on the last digit. Three comments that may help. (1) 0.0015 Hz out of 10 MHz is 1.5e-10 which seems a little high but not too bad. But do not expect exactly 10 MHz with this sort of test. What you are giving to the channel A input is the most highly phase correlated signal you can imagine relative to the internal clock and the interpolators. This won't happen in real life with real input frequencies. (2) Try different lengths of the channel A cable and see if the number changes. My prediction is it will. (3) Run the setup menu self-calibration if you haven't in a while. /tvb ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] SRS SR620 External Source Issue -- Help Request -- A thought
Didier Juges wrote: Dr Bruce Griffiths wrote: [EMAIL PROTECTED] wrote: ...if my model is correct the apparent offset in the mean will be increased by a factor of 10 over that for a a 1 second gate time. Bruce Looks like you're right Bruce. I did both 1, 0.1 and 0.01 gate times and the error increased by a factor of 10. And it doesn't sound like this is a fixable in any easy sort of manner, right? ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts Flavio If the SR620 can do some simple arithmetic before displaying the result, then this calibration problem is easily corrected. Otherwise you will have to do the corrections on a PC (or hand held calculator). The correction will vary with time and temperature so you will need to measure it periodically. Corrected frequency = measured frequency*(nominal gate time)/(nominal gate time + differential delay). In your case the differential delay appears to be about 150 picoseconds. Bruce Bruce/Flavio, Could that be mostly corrected (at constant temperature) by inserting the proper delay at the right place (in the form of a short piece of coax cable)? It seems that the error could be easily reduced by a factor of 10. Didier ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts Didier No. Unless one inserts about 2cm or so inside the instrument between a synchronisers and the corresponding interpolator. Better not to do this as it will vary with time and temperature. Measuring this error sufficiently often an correcting in software is far better and more flexible. Bruce ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
