Author Topic: Solartron 7071 / 7081 last digit is jumping all over the place  (Read 71727 times)

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Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #100 on: March 02, 2016, 10:21:27 pm »
And some more measurements..

I used the signal from pin 9 of U833 to keep the DG301 out of the measurement. With this signal I triggered my function gen. The function gen creates a 1.5ms puls with a 1.5ms delay after the trigger. This setup is shown in SP201_SP202_vs_force.PNG . The scope is also triggered to the force signal. Yellow is the signal on SP201 and blue on SP202.

I think that is the setup Mickle described.

In this setup the measured voltage is drifting. My first guess was the PLL circuit. Therefore, I measured the force signal frequency simultaneously (drift_vs_frequency.PNG). This means I have to get rid of the PLL again for this measurements.
 

Offline Mickle T.

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #101 on: March 03, 2016, 03:44:53 am »
But how about the noise in the measured voltage?
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #102 on: March 03, 2016, 08:29:40 am »
But how about the noise in the measured voltage?

I couldn't measure it. The drift was much higher between two measurements than the noise.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #103 on: March 03, 2016, 09:41:04 pm »
Today I used my HP 8165A to feed 10.48MHz (no more resolution available on the HP 8165A) into pin 3 of IC835. Afterwards the drift is gone but it is still noisy.

The noise is even a bit higher as with the real ADC? So I don't know how to interpret this. The noise within 200 6.5 digit measurements is now about 51µV RMS and 280µVpp.


Edit:
To get -1V instead of 0V on the Solartron I have to reduce the pulse width by about 200ms. Therefore, a change of 10µV needs a change of 2µs in pulse width. I've measured the pulses from the siglent and the jitter is in the region auf 10ns. So far too small to induce this noise. The 10.48MHz out of the HP gen aren't that stable, but I think that isn't a big problem, cause the HP generates the "master clock" with the force waveform which is then synced with the siglent.


Edit 2:
I saw a ~100ns jitter on the GLUGS signal and the ~5MHz signal at pin 9 of IC205 is also jittering/drifting 1.5ms after the force signal edge. So I decided to get rid of the HP 8165A and use my 10MHz DOCXO. I think 10.48 or 10.0MHz shouldn't matter anything because in the digital part we don't have to reduce any mains frequency related noise.

After some breadboarding to get a TTL signal out of the DOCXO the noise is gone :) (not tested with the real ADC yet!). Also the 200ns jitter in the GLUGS signal is gone.


Edit 3:
I kept the DOCXO for the clock an resoldered SP201 and SP202. The noise is still the same :(

One can see know again these 200ns jitter on the GLUGS signals. Perhaps it may cause more issues than expected?
« Last Edit: March 03, 2016, 11:37:17 pm by e61_phil »
 

Offline acbern

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #104 on: March 04, 2016, 08:33:30 am »
Wow!  I was just thinking that given the level of difficulty in just *repairing* an instrument capable of sub-ppm resolution, I can only conclude that it must have been [and still would be] extremely difficult to *design* such an instrument.  This compels me to have a whole new and higher level of respect for those engineers that designed these high resolution instruments.  It's really quite amazing!

True blue. So there is a good reason why there is no new 3458A on the horizon. And that although the 3458A will no longer be sold in the EU starting 2017 thanks to ROHS regulations (thank you EU for solving a non existing problem for us; not the first one)
 

Online Kleinstein

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #105 on: March 04, 2016, 10:01:32 am »
Hunting down such a small amount of noise is really hard.

I don't think the error is in the digital section, e.g. the counters. A defect there should usually give a signal different from normal noise. Even if some timing is just on the edge this would give more like 1/f noise and dependence on temperature.
There could still be a problem with the PLL part, e.g. a frequency that is not stable and this way gives some noise. I would consider using a 10 MHz crystal clock for a test, even if it is significant off from the 10,48 MHz. This would reduce the 50 Hz / 100 Hz suppression, but residual 50/100 Hz would be different from noise - though it might need a FFT of the data to see it.

Even without any modification a FFT on the noise data might be a good idea, to see if is really white noise or maybe coupling of some frequencies (e.g. the switched mode converter for the 5 V supply).

One point worth checking might be the opto-couplers. They tend to age and than might not give full signal.
 

Offline wiss

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #106 on: March 04, 2016, 11:16:43 am »
Have you established that the clock is stably locked to line-frequency?
Set a scope to line-trig and lock at the forcing waveform or something like that?
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #107 on: March 07, 2016, 10:36:35 pm »
Hunting down such a small amount of noise is really hard.

I don't think the error is in the digital section, e.g. the counters. A defect there should usually give a signal different from normal noise. Even if some timing is just on the edge this would give more like 1/f noise and dependence on temperature.
There could still be a problem with the PLL part, e.g. a frequency that is not stable and this way gives some noise. I would consider using a 10 MHz crystal clock for a test, even if it is significant off from the 10,48 MHz. This would reduce the 50 Hz / 100 Hz suppression, but residual 50/100 Hz would be different from noise - though it might need a FFT of the data to see it.

I tried that already briefly with no success. I will repeat this experiment more careful.


Even without any modification a FFT on the noise data might be a good idea, to see if is really white noise or maybe coupling of some frequencies (e.g. the switched mode converter for the 5 V supply).

On which data should I run the FFT? On the 6.5 digit data from the GPIB interface?  I will try that, too. I will record some data with the PLL circuit and some data with the 10MHz crystal.


One point worth checking might be the opto-couplers. They tend to age and than might not give full signal.

The communication opto-couplers towards the earthed side? I think this error should stay even without the real ADC, but the noise is gone with a good clock and without the real ADC.


Have you established that the clock is stably locked to line-frequency?
Set a scope to line-trig and lock at the forcing waveform or something like that?

Nice experiment :). I will try that.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #108 on: March 07, 2016, 11:06:13 pm »
Have you established that the clock is stably locked to line-frequency?
Set a scope to line-trig and lock at the forcing waveform or something like that?

I did that experiment. The result is shown in the attachment. The blue trace is the 10Hz signal at TP607 and the yellow trace is the 160Hz ADC signal.

It shifts around, but the mains frequency isn't constant here. So, if the mains frequency will shift the PLL filter can't react immediately. For me it seems like a normal PLL operation?
 

Offline wiss

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #109 on: March 08, 2016, 08:28:44 am »
Have you established that the clock is stably locked to line-frequency?
Set a scope to line-trig and lock at the forcing waveform or something like that?

I did that experiment. The result is shown in the attachment. The blue trace is the 10Hz signal at TP607

Isn't the forcing 10 kHz? I'm pretty sure it is that on the 7075 anyway.

Quote
and the yellow trace is the 160Hz ADC signal.

are you on 50 or 60 Hz line?
160 Hz can not be synchronous with either.

Quote
It shifts around, but the mains frequency isn't constant here. So, if the mains frequency will shift the PLL filter can't react immediately. For me it seems like a normal PLL operation?

Yea, at least the 10 Hz stay within a fixed phase-shift.

On my 7075 I can see a static 10 uV shift when I fail to ground properly, a non-synchronous DVM will drift significantly (10s of uV) under the same conditions.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #110 on: March 08, 2016, 08:39:58 am »
Have you established that the clock is stably locked to line-frequency?
Set a scope to line-trig and lock at the forcing waveform or something like that?

I did that experiment. The result is shown in the attachment. The blue trace is the 10Hz signal at TP607

Isn't the forcing 10 kHz? I'm pretty sure it is that on the 7075 anyway.

Quote
and the yellow trace is the 160Hz ADC signal.

are you on 50 or 60 Hz line?
160 Hz can not be synchronous with either.

Quote
It shifts around, but the mains frequency isn't constant here. So, if the mains frequency will shift the PLL filter can't react immediately. For me it seems like a normal PLL operation?

Yea, at least the 10 Hz stay within a fixed phase-shift.

On my 7075 I can see a static 10 uV shift when I fail to ground properly, a non-synchronous DVM will drift significantly (10s of uV) under the same conditions.

I'm on 50Hz mains. The forcing waveform is 160Hz.

The falling edge of the 10Hz is locked to 50Hz mains. The rising edge is shifting.
 

Offline wiss

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #111 on: March 08, 2016, 08:55:41 am »
Could you explain to me how the forcing is clocked?
where does CLK come from?
IC814 divide by 100 and IC815 divide by ?, right?
IC833 make IC815 divide by some other number > 10, right?
 

Offline wiss

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #112 on: March 08, 2016, 09:00:33 am »
It's not that the PLL-feedback-loop is oscillating?
 

Online Kleinstein

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #113 on: March 08, 2016, 03:02:09 pm »
The 10 Hz Signal looks suspicious: the falling slope seems to be locked to the line frequency, but the rising slopes look like having lots of jitter. It looks so bad it might be visible just from the 10 Hz or 160 Hz signal alone.

For me this does not look like normal operation for the PLL. More like lots of line synchronous signal coupled to the PLL.  So I would check/replace C814, C812 and check or test some shielding of the PLL section.

It might be also interesting to see the signal at TP807.
 

Offline wiss

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #114 on: March 08, 2016, 03:18:48 pm »

Now I realize where I've seen you before :)
The 192 is still doing it's work, now mostly 4 terminal resistance measurements when needed.

I've seen ceramic caps decrease in value (~10%), maybe even replace all frequency setting caps in the PLL-oscillator subcrct?
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #115 on: March 08, 2016, 09:28:12 pm »
It might be also interesting to see the signal at TP807.

Sorry, the blue trace shows TP807 not TP607.



Now I realize where I've seen you before :)
The 192 is still doing it's work, now mostly 4 terminal resistance measurements when needed.

That was another meter with jumping last digits :). If I remember correct the problem was a resistor array?


The 10 Hz Signal looks suspicious: the falling slope seems to be locked to the line frequency, but the rising slopes look like having lots of jitter. It looks so bad it might be visible just from the 10 Hz or 160 Hz signal alone.

For me this does not look like normal operation for the PLL. More like lots of line synchronous signal coupled to the PLL.  So I would check/replace C814, C812 and check or test some shielding of the PLL section.

It might be also interesting to see the signal at TP807.

To get rid of the whole PLL circuit I cut (again) pin 5 of IC835 and fed in 5.24288 MHz with a jitter of a few ns. The noise stays.. Did I miss a thing, which will get its frequency not out of the CLK net?

Why did they use these frequencys? The 5.24288 MHz and the 160Hz are not multiples of 50Hz. So, how does the mains rejection work?
 

Offline wiss

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #116 on: March 08, 2016, 09:49:44 pm »
Now I realize where I've seen you before :)
The 192 is still doing it's work, now mostly 4 terminal resistance measurements when needed.

That was another meter with jumping last digits :). If I remember correct the problem was a resistor array?

The divider for the reference was bad, and then one of the switching JFETs was leaking, for the AC-unit so I just amputated that.
Had it been subjected to neutron radiation?  ;D

Quote
The 10 Hz Signal looks suspicious: the falling slope seems to be locked to the line frequency, but the rising slopes look like having lots of jitter. It looks so bad it might be visible just from the 10 Hz or 160 Hz signal alone.

For me this does not look like normal operation for the PLL. More like lots of line synchronous signal coupled to the PLL.  So I would check/replace C814, C812 and check or test some shielding of the PLL section.

It might be also interesting to see the signal at TP807.

To get rid of the whole PLL circuit I cut (again) pin 5 of IC835 and fed in 5.24288 MHz with a jitter of a few ns. The noise stays.. Did I miss a thing, which will get its frequency not out of the CLK net?

Why did they use these frequencys? The 5.24288 MHz and the 160Hz are not multiples of 50Hz. So, how does the mains rejection work?

the 7081 do not use 1 s integration so no need to have multiples of 50, the forcing is 160 or 640 Hz, I think i figured out.
the forcing and the main clock and the line frequency need to add up over one integration cycle, there will be 4 forcing cycles of the 6 ms integration.
If the phase of the 50 Hz drifts relative the integration you will get noise unless you have very good shielding. The 50 Hz is only 4 digit stable so max 80 dB attenuation (?) without phase-lock. (Yes I did almost call up the local power company tom complain about the 49.99 to 50.01 Hz variation :D)
 

Online Kleinstein

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #117 on: March 09, 2016, 06:00:01 pm »
The sampling period (400 ms if I understand it right)  is a multiple of the 20 ms line period (at 50 Hz) and the forcing signal period. So there is an integer number of periods of both signal. It might be intentional that the 160 Hz causes transitions at several positions of the 50 Hz line signal.

I don't know why they choose this strange frequency - it should have worked with other frequencies (e.e. 12 MHz) as well. The main part for the PLL is to get the forcing signal and the sampling period synchronized to the line frequency.

With the 10 Hz signal beeing from TP807 - this signal does not look that bad. The output of the phase comparator shows how much jitter is there between the line signal and the PLL output. So something like 4 µs pulses don't look really bad - though it is somewhat strange to see something that looks like missing pulses and than pulses from 2-4 µs, but no shorter ones and no negative ones. Could be due to the rather slow IC824 = CD4046 ( today I would use the faster 74HC4046).

With the fixed clock frequency one should take a few more data via GPIB and check via FFT if the "noise" is white or contains a dominant low frequency.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #118 on: March 09, 2016, 11:06:23 pm »
Yes, 6.5 digit takes 400ms.

I've attached the FFT. I've collected data from 1600s of 6.5 digit measurements (~4000 data points)

I think there is no significant peak
 

Offline alanambrose

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #119 on: March 10, 2016, 10:33:43 am »
Interesting - btw how did you do the fft? - I want to do something similar.

TIA, Alan
“A foolish consistency is the hobgoblin of little minds"
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #120 on: March 10, 2016, 12:59:14 pm »
Interesting - btw how did you do the fft? - I want to do something similar.

TIA, Alan

I simply used the power density FFT function included in NI LabVIEW.
 

Online Kleinstein

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #121 on: March 10, 2016, 05:30:47 pm »
The FFT looks somewhat unusual: one normally expects to have more noise at lower frequencies. like a white part and and 1/f part. Having much of the noise at the higher frequencies somewhat points to a problem in using a well defined integration time or reaching a stationary signal from the forcing signal. The errors of the single conversions somehow compensate.

So I won't expect a problem with the FET switching (Tr201-Tr204) or the digital counters at the glug signals (of the as errors from that part would add up. Also the integrator seems to work - at least a low frequencies - so IC201 and IC202 should work OK. The comparators or there supply could be still a source of trouble.

It does not look like trouble with insufficient 50 Hz suppression and to much hum at the input, as this should give some signal somewhere below 0.1 Hz. Though coupling from higher line harmonics might shift the signal to higher frequencies. So the fixed clock should not be the problem.

I might be worth looking at the integrator output (using the forcing signal as trigger) - the noise suggests that there is more jitter/fluctuations than it should be.

It could still something in the forcing waveform - but not much left there. Have you checked there is not jitter in the operation of IC307 (e.g. from a not so perfect digital signal) ?

I would keep in mind C201 and get a MKP (or better) capacitor for this. You will likely need it to get good linearity anyway.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #122 on: March 10, 2016, 10:00:49 pm »
The comparators or there supply could be still a source of trouble.

I already replaced the LM311 against new ones. I measured again the supply voltages and it looks ok. Nevertheless, I installed some 1µF MKP directly on the LM311's, but that doesn't change anything

I might be worth looking at the integrator output (using the forcing signal as trigger) - the noise suggests that there is more jitter/fluctuations than it should be.

I had a quick look and it seems to be stable. A jitter is hard to see because of the flat integrator signal.


It could still something in the forcing waveform - but not much left there. Have you checked there is not jitter in the operation of IC307 (e.g. from a not so perfect digital signal) ?

I replaced IC307 with a new one, but I will check the operation again.


I would keep in mind C201 and get a MKP (or better) capacitor for this. You will likely need it to get good linearity anyway.

I put the original 100nF back into the unit




Hmm, I'm beginning to think I'm doing something really stupid..
 

Offline wiss

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #123 on: March 10, 2016, 10:38:52 pm »
Could you measure the pll control voltage and do a fft of that? And compare to the measurement fft.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #124 on: March 11, 2016, 05:59:10 am »
Could you measure the pll control voltage and do a fft of that? And compare to the measurement fft.

During the FFT measurements I didn't use the PLL. I fed in 5.2488MHz from my function generator.


The PLL voltage is shown in https://www.eevblog.com/forum/metrology/solartron-7071-7081-last-digit-is-jumping-all-over-the-place/msg871006/#msg871006  but without a FFT
« Last Edit: March 11, 2016, 06:02:11 am by e61_phil »
 


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