Products > Test Equipment
Looking for the serive manual of an HP 70951 optical spectrum analyzer
<< < (4/8) > >>
_Wim_:

--- Quote from: zrq on June 24, 2023, 01:02:16 pm ---I'd be a bit surprised if it was an overload that caused the problem. The photodiode will likely die before the electronics.

--- End quote ---

One of the read relays selects between the internal photodiode (including initial amplification) and the external photodiode input. I suspect some previous owner overloaded the external photo diode input (max 10V and max -10mA).The internal photodiode would not be damaged is this case, as the reed relay provides sufficient isolation. This would also explain why the very sensitive amplification was not damaged (external signal is only injected after this part) but some parts of the following sections are damaged. Off course, this is just a theory, and a schematic of this amplifier section would be very handy!

Worst case I could brute force and replace all active components in the transimpedance section, but for now I still want to find out what was exactly damaged.
_Wim_:

--- Quote from: nctnico on June 24, 2023, 01:18:16 pm ---Can you burn out an optical attenuator? I suppose there are some mechanically operated optical attenuators somewhere in the signal path. And with the device working for low level signals, it could be these are switched in / out of the path. I'd check whether these attenuators still work.

--- End quote ---

As far as I can see (and also browsing through the manual) I do not think this OSA has an optical attenuator fitted.
nctnico:
I just checked the specs and the maximum input level is +20dBm which is far more than I expected. So yes, the problem could be in transimpedance section. I'd try to retrace the schematic and make more measurements before going for a brute force replacement of random parts. IF you can't find the service manual. Isn't there are similar machine from HP from the same era for which the service manual exists? It is likely the electronics are similar enough to get an idea of how ranges are controlled and how the amplifier works.
zrq:
I have never seen an OSA with optical attenuator.
Is it possible to manually inject a current and see how the TIA respond in different ranges? (maybe you have already tried this)
_Wim_:

--- Quote from: zrq on June 24, 2023, 01:37:20 pm ---I have never seen an OSA with optical attenuator.

--- End quote ---

They do exist, my other OSA (Anritsu MS9710C) has one built in, it allows to increase the range from +10dBm to +23dBm.


--- Quote from: zrq on June 24, 2023, 01:37:20 pm ---Is it possible to manually inject a current and see how the TIA respond in different ranges? (maybe you have already tried this)

--- End quote ---

That is indeed my next plan (currently not in the lab). I plan to use the keithley 220 in program mode continuously stepping between 2 currents. This way is should be easier to trace where the actual signal is. Ideally however I would have a board extender, as currently probing is not easily possible (apart from the IC towards the top, it means pulling the board out, making the connections with micro grabbers, reinstalling the board (and hoping the grabbers to not disconnect), measuring, repeat all over again...
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod