Solartron 7061 noise

[Kleinstein]

More decoupling capacitors could in theory cause RF resonances between capacitors if the conditions are bad. In this case a few ohms in series with the supply would add damping and essentially avoid resonances. In this case the zener diodes should have enough resistance.
With some chips the lack of local decoupling can slow down switching. Sometimes this helps, but this is an exception.
Adding decoupling is at least one of the few mods to try.

[grizewald]

I will certainly contemplate putting a cap between the two rails right on top of the chopper amp, but for the moment, I'm looking at the PLL and oscillator board.

I had performed the very simple check from the manual (measure voltage at test point, if it's 3V +/- 0.2V you're good) but hadn't looked at the inputs to the PLL chip to see if they were actually locked. So last night I got the Rigol on the test points and discovered this:



(ch1 is MAINS_IN, ch2 is COMP_IN and ch3 is the clock control)

It's not exactly what you'd call locked and regardless of the control signal trying to pull the COMP_IN signal back into phase with the mains, the phase difference was fairly constant, which is more than I can say for the frequency with never settled on anything.

So I twiddled the adjustment on the clock board until the PLL managed to lock the two signals, but the result was less than great. It would correct the phase, but the response was horribly slow. If the COMP_IN signal drifted before the MAINS_IN signal, it would produce a negative signal on the clock control to push it back. If it drifted after, it would produce a positive signal to pull it back again. If the phases were locked, the clock control would remain at the mid point. Sounds good, and that's what it's supposed to do, except I'd expect it to take much less than several seconds to re-lock the phases and I'd also expect the lock to stay locked after maybe a minute at most, but no. It would drift forward and backwards all the time and the clock frequency would never stabilise.

So I've now taken the clock board out of the meter and have it running on the bench with the clock control input fixed at 3V. After a few minutes to warm up and stabilise, it's holding stable. Except the frequency is 52.346 MHz instead of 49.152 MHz. So I'll adjust it to 49.152 MHz, put it back in the meter and see if the PLL can now manage to phase lock the clock to the mains frequency and keep it there. If not, I guess it's time to start tracking down an MV2115 varactor diode.

I looked at the circuit diagram of the 49.152 MHz crystal clock option and was very surprised to find no input for the PLL control signal. A crystal is going to give a more stable frequency, but surely it still needs to be phase locked to the mains for proper AC interference rejection?

[Kleinstein]

The mains locking with the PLL is a 2 sided thing. It can improve the mains hum suppression, but it can also add noise, as the mains frequency is not perfectly stable over a short time. The response of the PLL has to be slow - this is intentional, as the frequency during one conversion should not change much. Some deviation in the phase is the smaller problem. The slow response is not easy to get with analog means.

Most modern meters tend to use no mains PLL, but just a suitable crystal clock and than for some modes do a synchronization of the start of the conversion.

[grizewald]

Well, I managed to get the frequency of the oscillator board as close to 49.152 MHz as I could.



Adjusting the frequency really accurately is next to impossible as you just need to breathe on the adjustment slug to make the frequency change by 50 KHz. After re-installing the board in the meter, I'm happy to see that the PLL now locks with a maximum deviation either side of 300ns, which is well within the +/- 3% specification.

The only negative is that it made absolutely no difference to the amount of noise!

I'm starting to think I might be chasing an impossible goal here. I saw that e61_phil's massive 7081 noise thread ran out into the sand with no resolution. Maybe my expectations are unrealistic, but it seems to me that having a 7.5 digit meter where the last 1.5 digits are effectively a random number is kind of pointless. Maybe that's why Solartron added the program feature, so that one can run a series of readings into the stats program and get it to output the mean instead of even looking at the filtered 7.5 digit reading.

[Kleinstein]

The solartron meters have there limitations. They seem to be nice with quite good linearity, but are also slow and limited resolution when doing fast (e.g. 5 sps) conversions.

For the clock, there is the option to use a fixed crystal clock and accept that the integration time is not exactly a multiple of mains periods.
The PLL part is not easy, though there is no absolute need to have an exact in phase lock. Depending on how the PLL is made there is even an advantage if there is some shift, as this avoids some coupling.

[grizewald]

Surprisingly, I can actually buy a VCXO with an output frequency of 49.152 MHz from stock at mouser. That might be a nice upgrade, delivering both the accuracy of a crystal oscillator and the ability to phase lock it with the mains frequency.

It would need a 3.3V power rail and some level translation, but that's not difficult. I might add one to my next order as they're only €20.

[Kleinstein]

The pull range of an VCXO may not be sufficient for the variations of the mains frequency.  It may work much of the time, but could get unlooked at some times.

[serg-el]

Pay attention to this topic.

https://radiokot.ru/forum/viewtopic.php?f=10&t=129145&hilit=solartron
http://www.ko4bb.com/getsimple/index.php?id=download&file=06_Misc_Test_Equipment/Solartron/Solartron_7081/Solartron_7081_Documentation_pack_-_Mickle.T.zip

https://www.eevblog.com/forum/repair/repair-of-solartron-7081-sn718/

[grizewald]

The pull range of an VCXO may not be sufficient for the variations of the mains frequency.  It may work much of the time, but could get unlooked at some times.

The pull range of the VCXO I found is +/- 100ppm or +/- 4915 KHz. What with all the divisions down from 49.152 MHz to get the 50Hz that the PLL compares to the mains frequency, I'm having a little trouble working out if that's enough! I suspect it isn't.

[Kleinstein]

The mains frequency is under favorable condition much of the time within some +-0.01 Hz (+-200 ppm), but not all the time. Worst case can be up to some 0.1 - 0.15 Hz.
The 100 ppm adjustment range for the VCXO would directly translate to +-50 ppm for mains.
I would consider some 500 ppm pull range as a minimum to get a reasonable good mains lock at least most of the time.

[grizewald]

500ppm sounds right and I guess that's why they went for the MC1648 VCO and why the crystal oscillator option isn't a VCXO; having had a good look around, a VCXO with 500ppm pull range doesn't exist!

[branadic]

I'm quite curious if you grizwald still have the LTC1052 in the meter without any issues. Have  you changed the zener diodes? I saw the



the other day and for some reason after replacing the ICL7650 by an LTC1052 his meter released the magic smoke. Seems like some problem of the rails at or of the LTC1052.
I wonder, is it related to his specific board revision 706105O5Y of the analog board only that this happened or is this a general problem replacing the old chopper amp?

-branadic-

[Kleinstein]

May my guess would be something like an latchup event at the LTC1052 - though I don't know why it should happen.  In the video one of the 15 V supplies was a bit high. With only 7.5 V Zeners the OP would get a nominal 15 V, but with only 2 mA supply current the 7.5 V zeners may have less than 7.5 V drop and thus maybe a supply at the upper limit (18 V limit). The LTC1052 has the specs for +-5 V supply and in the circuit the input and ouput signal is all close to zero. So a +-5 V supply (e.g. 10 V zeners) would be the safer way to run the OP.  The LTC1052 may also be not too happy to work without local decoupling, so it may oscillate and draw a much higher supply current that could increase the heat to the diodes, which may respond with to much supply to the OP while very hot.

A point to check may be the regulator at the supply - maybe oscillation or a poor solderjoint to give spikes with more than +-15 V.

The LTC1052 and ICL7650 are very similar - the main difference is in the noise specs. Even the supply current is very similar.
The LTC1052 is a bit slower, but this should not be a big deal: In the circuit the speed is anyway set by the local FB cap. So the OPs speed would not matter much.

[MiDi]

We discussed that already in the ltz1000 thread, where Dr. Frank and branadics version had same issues.
Only the current limiting of the lt1763 saved it from self-destruction after latch-up.
On power rails with higher current limit it is likely the LTC1052 gets damaged.

[branadic]

Played a bit with Solatron7061 this weekend and monitored some voltages. I spotted large hum in the readings, so I looked into the oscillator of the PLL and bingo, it was way off. So I brought it back into the range where the PLL was able to lock and readings are much better now. Figure 1 and 2 (100 samples) were taken with filter turned off. Figure 3 shows the effect with filter turned on, reducing the repitition rate in 7 digit mode to about 7.4 s.

-branadic-

[grizewald]

I'm quite curious if you grizwald still have the LTC1052 in the meter without any issues. Have  you changed the zener diodes? I saw the

Sorry for the late reply, I haven't been on the forum for ages! (Pressure of work, life etc.)

I had the LTC1052 in the meter for about a month and although it did result in about 30% less noise, it changed the meter's calibration.
Seeing as I'd actually paid to have the meter calibrated, sacrificing my calibration for 30% less noise seemed like a poor deal, so I put the original ICL7650 back in.

Defpom's problem looks like an unlucky infant failure to me, maybe precipitated by the slightly high 15V rails as Kleinstein suggests.

[grizewald]

Played a bit with Solatron7061 this weekend and monitored some voltages. I spotted large hum in the readings, so I looked into the oscillator of the PLL and bingo, it was way off. So I brought it back into the range where the PLL was able to lock and readings are much better now. Figure 1 and 2 (100 samples) were taken with filter turned off. Figure 3 shows the effect with filter turned on, reducing the repitition rate in 7 digit mode to about 7.4 s.

-branadic-

It would be very interesting to see what you get if you repeat the burst mode capture that both Dek and I tried in this thread: https://www.eevblog.com/forum/repair/solartron7061/msg3047176/#msg3047176

[branadic]

Why should I use the burst mode instead of taking continuous readings?

-branad-

[grizewald]

To see if you get the same 50Hz noise as we did when the chopper is running at 1.5KHz.  As you can see in the linked thread, the discussion kind of ran out into the sand, but even so, another data point would be interesting.

[TheDefpom]

I had a poke  around my 7061 (original version with PCB3 and 5) yesterday and scoped the oscillator and PLL etc, basically the same results everyone else is seeing, a PLL that never really seems to lock properly, just constantly adjusting its control voltage and the frequency jumping around by huge amounts.

As I have played with PLL's and VCO's in the past in my designs for CB radio conversion boards, I played around with it a bit and even tried tweaking the tantalum capacitor values to see if I could improve the phase detectors control voltage filter characteristics, but with no real luck, I could influence it and maybe improve it very slightly, but nothing fixed the response and control accuracy.

One thing I did do was replace the Philips 47uF 10V electrolytic capacitor at C422, this capacitor was completely dead (I have found that the same type of Philips capacitors in the Datron multimeters are also always dead, so I always have them as a suspect), anyway, replacing that capacitor appears to have improved the noise on the display slightly, it doesn't appear to be  jumping around quite as much as it was, so that might be something worth checking.

I was hoping to dig right into this PLL/VCO system and try to find a resolution to its instability, I will try again in the near future as I am sure it should be possible to get it locked properly, I have some other ideas for things I could try but after spending a few hours on it yesterday I have put the meter back together and moved onto my other projects.

[Kleinstein]

With increasing numbers of SMPS and PV systems in the grid the mains frequency may show more jitter / variations than in the old times when the supply came mainly from large classical generators. So the PLL may have more difficult conditions than at design time.

To get reduche the effect of wavefrom distorions / higher frequency noise on the main waveform, one could consider a little filtering for the mains signal pic off.

For the PLL it may be worth looking at the VCO control voltage an look if there are some dominant frequencies. Ideally this would not be the case, but a poorly tuned loop filter can show some amplifications / tendency for ringing.

The alternative to a PLL is to have a stable crstal clock and accept a lettle less mains hum suppresion.

[branadic]

When I adjusted the VCO on my unit I used a Keysight 53230A in trend chart mode, this way I was able to observe the PLL trying to lock by "sweeping" the frequency. So the frequency I adjusted the VCO to is not neccessarily 49.152MHz, but close enough to get the PLL to lock.

-branadic-