Electronics > Repair
Troubleshooting intermittent fault in an SRS SR530 reference oscillator PLL
timeandfrequency:
Hello DaJMasta,
You seem to be very close to find the issue.
--- Quote from: DaJMasta on December 18, 2024, 07:13:32 am ---[...] the problem must lie with the comparison stage at the end, so that there's something wrong with the data being driven into pin 3 of U306. At least, that's my understanding.
For the analog switches involved in this loop, there is no toggling when this behavior is occurring. I've also looked at the "phase voltage" control line, and while it's not exactly the 5.12V or -5.12V they seem to indicate, changing through the phase controls on the front panel does make it jump about 5V and neither position seems to make it more or less stable. [...]
--- End quote ---
I could not find in your posts any information that you changed U313, which sometimes is also 'in the loop'.
As you mentionned ealier, C310 (22 pF) presence is not easy to explain. Why put an capacitor between the inputs of a comparator ? HF noise reduction ?
Try to replace it or even try another value (lower or higher value ?)
Did you check the '10.24V REF' signal ? or better the 1.42 VDC reference signal (U314 pin 2) ?
You say "For the analog switches involved in this loop, there is no toggling when this behavior is occurring". I would still give a deeper look in that. IMO, there's something 'external' that is jolting the loop. But for the moment you cannot see it.
As you record very long sequences of a 100 Hz signal, your sampling rate (12.5 MSa/s) avoids catching plainly anything shorter than 160 ns (Nyquist). Even if the 'DG's are rather slow (400 ns), comparators and vanilla logic gates or flip-flops usualy switch within in 10 ns.
I would suggest to improve your trigger condition to be spot on the moment the triangle goes bersek :
- trigger if blue trace (pin 3 of U306) ='1' for less than 1 ms
or
- trigger if pink trace (pin 1 of U306) ='1' for less than 200 us
Then crank up the sampling rate at the maximum value your scope allows and carfully probe again each logic control signal and analog value :
e.g.
pins 8 & 9 of U312,
signals /HI /LO,
/f , /2f,
1.42 VDC ref voltage,
U311 pin 16,
U311 pin 8,
U311 pin 1,
Vvco,
U310 pins 9 & 16 (the others are /HI and /LO),
U306 pin 14
Good luck !
DaJMasta:
The ESR for C329 seems reasonable to me - it's the one in that group that isn't a tantalum, and I looked up comparably sized and rated electrolytics and around 5 ohms seems normal. U305 is currently not socketed, but its function is the signal lost timeout (helps reset the PLL when disconnected and tells another board) and that signal seems to be operating properly.
I think C310 is probably for high frequency noise immunity - it's on a few comparators in similar configurations, and at least the way I think about it, a low value capacitor will be an open until higher frequencies where it will slowly act more like a short, so it balances the two inputs more for higher frequency edges, making it less likely to trigger on that noise. I can't say my explanation is 100%, but SRS has some serious analog design folks designing these, so I am happy to chalk it up to them knowing better than I some esoteric footnote of the LM311's performance.
The 1.42V reference is correct and the 10.24V ref is correct.
I did some scoping around to try to trigger on the narrow secondary pulse and captured some traces.
This is a check of the comparator U314 to see if it's operating correctly. The yellow and blue traces at the bottom are the inputs to the comparator, the purple is its output, and the green up top is the reference input (pin 14 of U306). Looks normal, I'm not seeing the comparator triggering on noise or artifacts that don't seem to be in there intentionally. I don't know if I entirely understand the purpose of the feedback path or the small signal it puts on pin 2, but I don't think it's malfunctioning.
So if the comparator isn't it, then why is it trying to generate a faster sloped triangle wave? I scoped the PLL chip and its output more particularly:
In this one, the top three signals are the same (green is reference input, purple/red is comparator, blue is triangle wave output), but the yellow trace is right on pin 13 of U306, the control line for the programmable current source. Again, this bit of the circuit is a little beyond my understanding, but I think I should be checking values of C336, R316, R317, and C306. Don't have the time to immediately, but maybe tonight.
DaJMasta:
Checked those four passives and the 3 1k resistors nearby. Everything measured good, but the ESR of C336 bounced around a bit at low frequency and then seemed fairly low in comparison to other ceramics I had on hand, so I tried replacing it with an X7R modern ceramic 470pF cap... no real difference. Maybe slightly more stable, maybe that's just my own noticing biases, but the behavior of the triangle wave is unaltered and there are still intermittent errors.
DaJMasta:
Probed all the digital control of the analog switches today and the only narrow pulses I saw on any of them were the ones generated by the PLL controller's !Q output, not even notable noise otherwise. Pulled a bunch more passives and confirmed them, a few will measure odd in circuit but then be totally fine out of circuit.
So I took a more blanket approach and cleaned the bottom of the board with IPA a few times - after the first couple I actually saw less locking behavior, but it was back to normal fully dry and a bit more clean and at the moment it's acting the same again. Pushed in every IC to the sockets, visually inspected and retouched a few joints on the underside of the board.
I tried removing resistor R329 and replacing it with various values, in case somehow the hysteresis for that main comparator had gone out of whack, but while it seemed slightly better with a slightly lower value (39k was better I think but not 27k), it wasn't fixed with any higher or lower value I tried.
Also tried replacing U306's current CD4046BE with the removed CD4046AF, but to no difference.
I'm definitely grasping at straws now.
timeandfrequency:
--- Quote from: DaJMasta on December 20, 2024, 06:55:29 am ---I'm definitely grasping at straws now.
--- End quote ---
It seems still that you're very close to find the issue.
Indeed, the latest screenshots show very interesting things.
Just quicky,
About screenshot #2
SR = 2 GSa/s
TB = 1 ms/div
The yellow trace shows a significant HF noise oscillation (about 300 µs wide). This needs more investigation.
The green trace (at the top), which (at the left side of the screen) was NOT in phase with the triangle wave (blue,) now goes negative synchronously, perhaps with a small glitch (100 µs).
At the exact moment of that green negative pulse, the rate of change of the triangle wave (blue) becomes much steeper. Here somethings's going on, but I don't know yet if it is a cause or a consequence.
We can also see that the pink trace makes this time only a very short low pulse (400 µs), the previous was 2,1 µs wide.
About screenshot #3
SR = 2 GSa/s
TB = 2 ms/div
See also my 'updated' version of this screeshot (attached picture)
Yellow trace : we see the positive and negative pulses, from the phase comparator II.
The two yellow positive pulses clearly produce a significant frequency/period change on the triangle wave (blue).
On the left of the screenshot, the triangle wave has a period of 10.2 ms (between 1 green dot and 2 green dots).
After that, it seems that the frequency of triangle wave becomes quite twice the former value. If we interpolate the triangle wave (dashed red line) we keep the former shape and almost the same period (11.2 ms, between 2 green dots and 3 green dots). This small period difference (+ 1 ms) might show up because the loop has been jolted and cannot immediately return to the old period value.
At this moment, the phase comparator (yellow pulses) tries to compensate with two positive pulses, due to the immense phase difference that occurs when the frequency is doubled.
So the question is now : why does the triangle wave significantly change it's rate of change, and why does the whole loop truly double it's frequency at this moment ?
Has it something to do with the logic signals /f, /2f and 1/2 U313 (the left one) ?
As I'm not fluent with Norton OPAMPs, it's not easy to guess which capacitor is charged and dischard by a constant current in order to generate the triangle wave. Can you see the triangle wave on C306, C307 or C308 ?
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