EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: bostonman on August 20, 2024, 11:45:28 pm
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I have a Stanford Research DS345 and was running some calibration stuff and got stuck on basically part 1 on page 6-6 (https://www.thinksrs.com/downloads/PDFs/Manuals/DS345m.pdf (https://www.thinksrs.com/downloads/PDFs/Manuals/DS345m.pdf)).
Clock calibration (the next part) I was able to adjust (although it didn't exactly state how to 'store' the cal number), but I'm confused about adjusting the 5V reference.
Currently it's 5.00V, however, if I adjust the calibration number, the voltage doesn't change.
My understanding is that changing the cal number will adjust the 5V reference, but I'm not seeing a change.
Does anyone understand what that paragraph is trying to explain?
Other than tweaking the 10MHz (I'm referencing my counter to an atomic clock), I don't see a reason to play with the other stuff. I'm not using my equipment for high performance measurements and think I'll only open a can of worms if I tinker with other settings.
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I interpret it as saying that the 5V reference is fixed and you're compensating for any offset/error at U103 pin 1.
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That's basically how I interpreted it, but the 5V doesn't change as I vary the DAC value.
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I've chased the +5V_REF and it originates from an AD586 5V precision reference.
It can be measured on either pin 1 of U103 as the manual states, or J103 which is the identical point that's fed from the AD586.
My guess is that maybe I needed to disconnect the 10MHz input from my reference because it's preventing the DAC from adjusting the output 5V REF voltage, but the calibration section doesn't make sense.
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No, that's not it. The 5V will never change. It can't because the AD586 is a fixed voltage reference.
SRS used the "J" version of the AD586 which has an output voltage of 4V980 - 5V020. The system assumes that it really is 5V000. Calbyte 1 gives the system the necessary correction factor between the actual output voltage of the AD586 and 5V000.
What's confusing you is that your AD586 actually is 5V00 so the correction factor for you is the default value of 32768. In my DS345, the measured voltage was 4V9986 so I changed the Calbyte to 32759. I don't know if this corrects the bits that the ADC outputs or if it's fixed in software. I don't know how you would actually detect that the system had registered the change (i.e. what would change on the output).
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so I changed the Calbyte to 32759
Per the formula in the manual, that number makes sense.
If I understand what you're explaining, you still measured 4V9986 on J103 even after changing the Calbyte number?
It's a bit confusing because one would assume this voltage should be measured after entering a Calbyte.
Out of curiosity, did you perform any other calibration steps? Seems like one can go down a rabbit hole trying to perform all of them.
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I agree with Ed, the 5V reference will not change, even with changing the Calbyte. The software itself will compensate for any small changes of the 5V reference. It determines this difference by you measuring J103 and plugging whatever voltage you read into the formula. The result is your calbyte which the software will use to compensate for any deviations from nominal.
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I agree that the 5V is fixed based off the AD586.
Maybe I'm misunderstanding, what if I accidentally enter the wrong Calbyte number? I'll never know the 5V reference is off.
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Maybe I'm misunderstanding, what if I accidentally enter the wrong Calbyte number? I'll never know the 5V reference is off.
You will never know unless you can figure out some way to see the change in the output. It could be any of frequency, amplitude, offset, distortion, duty cycle, or something more subtle. It could affect all outputs or only the AWG output.
To answer your other question, I changed Calbyte 0 (Clock) because I was troubleshooting a problem with the internal oscillator circuitry. I didn't change anything else.
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This is a big help, thanks for all the feedback.
I'll double check the Calbyte for the reference voltage. The frequency adjust (Calbyte 0) was about the only thing that made sense.
Since I have it open and referenced to a clock, does an adjustment exist that would bring the Calbyte closer to 0? Currently it's at whichever number which seems close to the end of range, it would make sense to try bringing it closer to 0 so I have plenty of Calbyte room left for future adjustments.
My guess is this isn't easy and involves tweaking things that will throw off other settings.
I plan to perform the Calbyte 0 and reference voltage, and I'm not messing with anything else.
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Since I have it open and referenced to a clock, does an adjustment exist that would bring the Calbyte closer to 0? Currently it's at whichever number which seems close to the end of range, it would make sense to try bringing it closer to 0 so I have plenty of Calbyte room left for future adjustments.
I can't help you with that. My DS345 has the internal OCXO option but once I got it working properly (the frequency was drifting due to a broken resistor), I didn't pay much attention to it because I always use an external reference to sync everything to a house standard (HP 5065A Rubidium).
I remember being disappointed with the performance of the DS345. I was hoping to use it in my Time-Nut investigations but I found out that the nice pristine digitally synthesized frequencies are inaccessible. If you want a square wave, they generate a digitally synthesized sine wave and then use a comparator to convert it to a square wave which adds jitter. :palm: I was eventually able to access the digital signal via the sync output, but only through the AWG function. What a pain!
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Thankfully I haven't needed any of my equipment for such tests, so jitter and stuff isn't much of an issue.
I have another DS345 that unfortunately displays an error and is useless as a frequency generator (may make a good book end). In another thread that I may have started or began contributing to some time ago, someone else and I began exchanging our circuit measurements (our units had identical error messages and voltage measurements), however, he stopped posting. One of his last postings stated he replaced several bad components without fixing the issue; and was dealing with health stuff (why I assume he stopped replying).
Having learned that we have the same error and several components were bad, I somewhat lost interest trying to repair mine. Also, I appreciate companies that supply schematics and part numbers with their units, so complaining about these schematics isn't fair, but SRS did a (in my opinion) horrible job with reference designators.
I spend more time trying to locate components than measuring for anomalies because the numbers are all over the place. One minute they are in the 100s, next minute 700s.
Ideally I'd like to repair the other unit, but, if several components are bad (according to the other poster) and still isn't repaired, I think this issue is beyond the time to repair it.
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Am I missing something in the manual or does it not state how to save the Calbyte numbers?
I assume pressing the "STO" button stores it, but, at one point I pressed it, repowered the unit, and it showed the previous Calbyte number.
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My notes don't mention anything odd. The procedure is shown on page 6-1. Did you flip the switch to enable calibration?
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Yes, the switch is flipped.
I can change the Calbyte number to adjust the 10MHz on Calbyte number 0, so I'm doing it correctly. At one point, either trying to save this number or tinkering with the 5V REF (Calbyte number 1) number, it didn't seem to store.
Maybe I hit the button for factory recall and didn't realize it, but seems an "enter" button should need to be pressed.