Author Topic: Solartron 7071 / 7081 last digit is jumping all over the place  (Read 71710 times)

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Online Mickle T.

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #150 on: March 24, 2016, 06:36:52 am »
Datron 1271:
 

Online Kleinstein

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #151 on: March 24, 2016, 08:36:31 am »
I just realized that to little forcing should not be the problem: the 5 digit mode use a higher forcing frequency and thus slightly stronger forcing as the capacitor has lower impedance. So not much sense in doing a test with a little more forcing.

For the amplifier, one would need a non inverting version here, if the original comparators are used. I don't think the extra filtering shown in the plan above is needed in this case (rather low forcing frequency), so just an non inverting amplifier with the extra diodes in feedback should be enough.

The extra filter adjustment might be there to improve the step response a little, e.g. need for less delay during autozero mode, which is not that important with the solartron, as the drift is very low. As far as I see, the step response may not be really good for this ADC anyway.
 
One could use the inverting circuit if one also adds the comparators to the extra circuit.
 

Online Mickle T.

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #152 on: March 24, 2016, 08:44:36 am »
The 5 digit mode use the same 160 Hz forcing frequency.
 

Offline wiss

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #153 on: March 24, 2016, 10:41:39 am »
I've measured the 160Hz signal in https://www.eevblog.com/forum/metrology/solartron-7071-7081-last-digit-is-jumping-all-over-the-place/msg885093/#msg885093
The variation was about 1000ppm.

Here you did synchronize to the functiongen and not a real 50 Hz line?
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #154 on: March 24, 2016, 12:38:37 pm »
I've measured the 160Hz signal in https://www.eevblog.com/forum/metrology/solartron-7071-7081-last-digit-is-jumping-all-over-the-place/msg885093/#msg885093
The variation was about 1000ppm.

Here you did synchronize to the functiongen and not a real 50 Hz line?

The 160Hz was generated by the PLL in this measurement. My Siglent function gen might be bad, but not that bad ;)
 

Offline wiss

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #155 on: March 24, 2016, 12:50:41 pm »
The 160Hz was generated by the PLL in this measurement. My Siglent function gen might be bad, but not that bad ;)

Ok :) so then we assume that the +-2.5% variation in integration-time is due to LabView?
(that large line frequency variation will probably ruin larger (and smaller) motors and all kinds of bad stuff might happen...
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #156 on: March 24, 2016, 12:52:21 pm »
The 160Hz was generated by the PLL in this measurement. My Siglent function gen might be bad, but not that bad ;)

Ok :) so then we assume that the +-2.5% variation in integration-time is due to LabView?
(that large line frequency variation will probably ruin larger (and smaller) motors and all kinds of bad stuff might happen...

I don't think so. LabVIEW is normally stable within 1ms in my setup.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #157 on: March 24, 2016, 04:50:46 pm »
Today, I put a thermopile into the meter on top of the floating CPU. I'v measured 54°C at 23°C room temperature. Is that normal? I think it is quite hot.

Thanks for the fold amp schematics. I will think about a fold back amplifier with a gain of max 10 and a maximum output voltage similar to the maximum integrator signal on TP201.
 

Online Mickle T.

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #158 on: March 24, 2016, 06:15:01 pm »
The normal temperature in my 7081 is about 50 C. deg. (in the middle of the analog PCBs).
 

Offline wiss

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #159 on: March 24, 2016, 06:44:57 pm »
The 160Hz was generated by the PLL in this measurement. My Siglent function gen might be bad, but not that bad ;)

Ok :) so then we assume that the +-2.5% variation in integration-time is due to LabView?
(that large line frequency variation will probably ruin larger (and smaller) motors and all kinds of bad stuff might happen...

I don't think so. LabVIEW is normally stable within 1ms in my setup.

But, variations in integration time has to be below 0.25 ms, something has to be wrong here...
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #160 on: March 24, 2016, 06:49:57 pm »
The 160Hz was generated by the PLL in this measurement. My Siglent function gen might be bad, but not that bad ;)

Ok :) so then we assume that the +-2.5% variation in integration-time is due to LabView?
(that large line frequency variation will probably ruin larger (and smaller) motors and all kinds of bad stuff might happen...

I don't think so. LabVIEW is normally stable within 1ms in my setup.

But, variations in integration time has to be below 0.25 ms, something has to be wrong here...

Perhaps it has only todo with another data communication as mentioned earlier

A VCM signal would be really nice now
« Last Edit: March 24, 2016, 06:58:28 pm by e61_phil »
 

Offline wiss

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #161 on: March 24, 2016, 07:24:09 pm »
In the 7075 there is a tp for 50 hz sync, forcing, pll-out etc
Chech the stability on those with your oscope.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #162 on: March 24, 2016, 07:28:58 pm »
I posted already some of these measurements. None would explain these huge variance in the data acquisition time. I think that is due to internal communication between the CPUs 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 #163 on: March 24, 2016, 09:07:24 pm »
Form the PDF linked above, one possible problem might be due to to little forcing signal - this might make the integrator plus PWM feedback part not getting fully stable. One could test this with something like a 10% more forcing (e.g. 390 K in parallel to R221). Other Solartrons even have a pot to adjust the forcing strength to find an good value - so maybe this one is just ad the edge.

390k in parallel to R221 changed nothing
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #164 on: March 24, 2016, 10:46:34 pm »
@e61_phil:

Just curious...  Do you have the latest firmware in your DMM ???

-Ken

I dont't know. But the test with the digital part only looks fine.


I found this https://www.febo.com/pipermail/volt-nuts/2014-July/003430.html
Does anyone know Jean-Louis? It would be interesting if he could fix his problem.


Perhaps it is worth to also change IC201 against a LTC2057? That OP is a slightly slower but has very good noise specs.

On german ebay is an auction with a 7071. Unfortunately, this meter is also noisy. So no chance for cross checks.
 

Offline wiss

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #165 on: March 24, 2016, 10:47:03 pm »
I posted already some of these measurements. None would explain these huge variance in the data acquisition time. I think that is due to internal communication between the CPUs or something like that.

Oscope, trig on forcing, holdoff to whatever is lcd of 50 and 160 Hz, post sync and forcing with persistence.

Or I pick up the multimeter for 50 eur next time I come to de ;)
« Last Edit: March 24, 2016, 10:48:42 pm by wiss »
 

Online Kleinstein

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #166 on: March 25, 2016, 10:36:35 am »
The question on the software version is a very good one. The is a possibility to reduce the observed type of noise by a kind of digital filtering, especially for the 7 and 8 digit modes. As this type of noise could be a principle weakness of the design, newer Version might use this.

IC201 is not expected to contribute much to the noise, and only the more white noise type. So it does not make very much sense to upgrade here. If you really want to, the more direct, upgrade should be a LTC1052.

The more promising candidate for an upgrade would be IC202, as this chip is expected to give a contribution of the right type of noise. For a test one could also just reduce the bandwidth of the existing LM301 by modifying the compensation (e.g. increase value of C204 by a factor of about 5). This is more of a test only as this could have a negative influence on linearity.

More promising would be the extra amplification between the integrator and the comparators - many modern DMMs use this trick to improve on the comparators. A limited bandwidth here would also reduce noise from IC202. This modification is expected to reduce a noise source of the observed type (mainly higher frequency).

A totally different point, might be looking at the supply. "noise" from the supply could influence the comparators, as the decoupling in this area is not very good, but hard to fix / improve. Even if not visible on the scope ripple on supplies could cause trouble and might be the difference between good and noisy meters.
One  source for supply noise are the switched mode regulators. So looking at there output caps or tests with added caps there might be worth a try.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #167 on: March 25, 2016, 11:41:50 am »
I thought about the foldback amp and I think it is impossible to fold down to 1 with only one Opamp non-inverting without saturating anything if the amplifier should reach 10V at the output.
 

Online Kleinstein

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #168 on: March 25, 2016, 12:38:37 pm »
The amplifier does not need to reach full range at output. The comparator don't need the full amplitude at all, but they also don't care much about 0.7 V more.  All you need is the range between the two comparators. So this is currently 0 V and something like 90 mV,  but I don't see a big problem making this range a little smaller.

So one could use just an non inverting amplifier and parallel the feedback resistor with 2 (or maybe 4) diodes. So close the zero amplification could be something like 10 and at larger voltages it's close to 1. R222 can than be adjusted (e.g. 22 K)  to have the second comparator at maybe -0.4 V instead of the old -90 mV.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #169 on: March 26, 2016, 12:24:19 am »
I placed a fold amp (only breadboarded) into the meter, but no change in the noise :(

I also placed a new cap in parallel to the old one in the 5V switching supply for the floating electronics.

It seems that there is relativley high frequency noise (10th of kHz) in the circuit which produce some jitter on the comparators.
 

Online Mickle T.

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #170 on: March 26, 2016, 05:11:11 am »
I think it is possible to cut the +5V raill from the Floating Power Supply and connect it to external linear one.
Or add a capacitors in parallel with C55 or D53.
 

Offline classicTEK

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #171 on: March 26, 2016, 01:02:51 pm »
Hello All,

I have read in this string about the problems of instability of the LSD.  I may have missed it in all of the very detailed things that were tried already, but did this meter act this way BEFORE you removed the jumper @ LK17?  The only reason I ask is that I just picked one up that is in exceptional condition with factory Schlumberger calibration stickers still intact.  She seems very stable so far as a GREAT 7071.  Should I expect similar degradation of the stability of the LSD if I too remove the jumper?

Cheers,

David
 

Online Kleinstein

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #172 on: March 26, 2016, 02:05:47 pm »
AFAIK the jumper only changes the number of digits displayed, so a kind of choice in software version. It can always be undone.
With less digits displayed, one would not easily see the higher than normal noise, even it is there. So the changed jumper only makes it (more) visible, but it should not influence the noise itself.


For the switched mode supply one should also look at ripple on the input side. A high ESR for C51 could send ripple all the way back through the transformer. An extra cap (e.g. a few 100 nF) at the AC side might be an idea. The same is possible with the earth connected supply.

One could also check the switching waveform (e.g. emitter of TR52) if this looks good. Just to see if it about improving filtering of the normal ripple or maybe an not well working switching transistor (e.g ringing there - so a snubber or changed transistor  would be more appropriate).
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #173 on: March 29, 2016, 02:01:16 pm »
I can't say if it changed with removing the jumper the first time, I don't think so. I read stories about one of the DIP switches for the GPIB adress where used to switch between 7071 and 7081 during the design of the meter. Everything inside my 7071 is labeled with 7081 (including the stickers on the EPROMS) so I think there is really no difference excpect the frontpanel and the selection of the reference diode.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #174 on: March 30, 2016, 06:30:45 pm »
Hello All,

I have read in this string about the problems of instability of the LSD.  I may have missed it in all of the very detailed things that were tried already, but did this meter act this way BEFORE you removed the jumper @ LK17?  The only reason I ask is that I just picked one up that is in exceptional condition with factory Schlumberger calibration stickers still intact.  She seems very stable so far as a GREAT 7071.  Should I expect similar degradation of the stability of the LSD if I too remove the jumper?

Cheers,

David

Hi David,

is it possible to do some measurements in TEST0 mode or in 10V mode with a short at the input? It would be very interesting how much noise a good 7071 will produce.

Thx Philipp
 


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