EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: nwind on April 15, 2021, 06:22:12 pm
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After burning a few VCSEL lasers recently on one of my test boards I found the suspect.
These VCSEL should be driven with short pulses/bursts with a low (0.1% - 2%) duty cycle depends on the peak current.
Unfortunately to generate bursts control signal I was using 33512A AWG and later I found out that every time when you change the number of of the cycles with the click wheel AWG introduce pause (wrong) and even worse the "pause amplitude" is about half of the pulse amplitude. Since pause is about 3.6ms lasers were driven too hard and inevitably died.
Interesting enough that much older 33250A does the same thing (pause), but at least keeps the output level at 0V which is safer.
Even more interesting that a least expensive 33220A doesn't exhibit this behavior and just seamlessly updates number of the cycles.
Looks to me like 33512A and 33250A FW bug and rather dangerous one.
BTW I've verified this behavior with two different 33512A units with two different FW versions, so it looks like universal bug, not just one unit.
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Interesting... I'm wondering though: did you specify the signal with a peak-to-peak setting + offset or a low / high level? I agree changing the number of cycles should be seemless but if the generator can't for some reason it makes sense to go back to the idle level. In case of a Vpp+offset this should be the offset level, in case of low/high the low level is a logical choice. The latter is pure speculation on my part though.
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Interesting... I'm wondering though: did you specify the signal with a peak-to-peak setting + offset or a low / high level? I agree changing the number of cycles should be seemless but if the generator can't for some reason it makes sense to go back to the idle level. In case of a Vpp+offset this should be the offset level, in case of low/high the low level is a logical choice. The latter is pure speculation on my part though.
For all tested AWGs it was set as low/high level, so testing conditions were consistent with quite a variation from device to device.
TEK AFG31252 has no such issues as well.
This break should not happen at all like with the 33220A little brother, and definitely shouldn't go to 1/2 max amplitude like in 33512A.
BTW I've verified this behavior with two different 33512A units with two different FW versions, so it looks like universal bug, not just one unit.
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Then it is definitely a bug. What does Keysight say about it?
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Then it is definitely a bug. What does Keysight say about it?
Didn't talk to them about it (at least yet), but I would be careful or completely avoid using 33500A for crytical tests. In my case cost of damaged boards was bigger than cost of 33512A. Didn't expect it from Keysight and surprised that nobody catch it before.
Burst mode is a quite common type of the signal.
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Innocent, but dead by now VCSEL
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In my case cost of damaged boards was bigger than cost of 33512A.
Ouch! :'(
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I can confirm that this is indeed a problem on the 33522A generator. It stays at half-level for about 3.5 msec when changing many of the waveform parameters. It has recent firmware version 5.02-1.19-2.00-58-00. Perhaps someone can check 33522B generator, although I am not sure if there is any difference between them. The problem is present when using both square wave and pulse waveform. It reminds me of a similar transient issue that appeared at first on Rigol DG800 generator and also blew up someone's circuit, but Rigol quickly fixed it in the firmware.
Keysight 33522 does not specify a separate "idle value", so the voltage between bursts is not so well defined, but defaults to 1st point of the waveform. When using sine waveform one can set any idle voltage by adjusting the starting phase and when using square waveform one can switch between high and low with starting phase. However, when reprogramming the waveform it goes to 50% point, not to the 1st point of the waveform, I think its just a bug.
In contrast, Rigol DG800 has a separate specification for idle value that can be set as top, bottom or any point of the waveform. And when changing parameters it stays at that idle value.
I would encourage you to contact Keysight directly and demand an explanation. It's surprising that this bug had not been previously reported, but looking back I think it might have caused some unexplained damages in our lab as well.
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Definitely report it to Keysight. I know it's a big company and the website isn't super helpful but they did take the time to respond to a report I made about the 1000-X, so I expect they will take your problem seriously.
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I did it here:
https://www.keysight.com/us/en/contact/support/site-feedback.html (https://www.keysight.com/us/en/contact/support/site-feedback.html)
Strange that I didn't get acknowledgment automated email back.
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That's for issues with the website, not a product.
I think this is the right place: https://www.keysight.com/us/en/contact/support/support-request.html (https://www.keysight.com/us/en/contact/support/support-request.html)
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That's for issues with the website, not a product.
I think this is the right place: https://www.keysight.com/us/en/contact/support/support-request.html (https://www.keysight.com/us/en/contact/support/support-request.html)
You are correct.
Interesting. All pre filled info in Japanese )
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I found out that every time when you change the number of of the cycles with the click wheel AWG introduce pause (wrong) and even worse the "pause amplitude" is about half of the pulse amplitude.
Can be worse on some of their other generators, the ones built into scopes seem to use arbitrary mode for a lot of their functions and have pauses like that at any value within the range (I blew up some power circuits in a similar way to what you did, expecting a clean transition when changing duty cycle).
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I did report it and they contacted me by phone (was a weird conversation) and by email:
"R&D advises you try using the pulse function instead of the square wave function to see if performance improves. Let us know what you find. Some scope screen shotes will help as well if you can provide those."
I really hope they will fix the FW because it affect so many users and it's a bad bug for "professional" equipment.
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I tested both pulse and square and it was the same. Its easy to make screen shots, it's not an occasional problem. They should be able to easily reproduce it themselves.
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I tested both pulse and square and it was the same. Its easy to make screen shots, it's not an occasional problem. They should be able to easily reproduce it themselves.
Agreed. I sent them scope shots anyway
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Respond after bunch of emails I've got from Keysight: :phew:
We were able to duplicate your issue on some instruments, but not all. R&D and the product engineer are looking into it to see if it might be a firmware issue or something else. For the meantime, the work around is to turn the output off before changing anything in the burst menu.
Regards,
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Thanks for update. It's interesting that the issue is not present in all instruments, so maybe its a subtle bug. They should be able to fix it with a firmware update.
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Thanks for update. It's interesting that the issue is not present in all instruments, so maybe its a subtle bug. They should be able to fix it with a firmware update.
I've confirmed it on two 33522A. Don't know what they mean.
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Any update on this issue from Keysight?
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Total silence since April 28th