Electronics > Repair
INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
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Fraser:
Looking at the pictures of the MMIC gain block stage, there is evidence of rework in the MMIC supply rail and associated bias components (R67,R69 & R71) The MMIC may not be running in optimum conditions and suffering as a result.

As a side note.... I do not know whether the rework was carried out by a service centre, but if so, I would send the tech for retraining ! A pretty poor soldering job on the reworked components. The MMIC installation is very untidy and still has flux residue present. Not good tradecraft at microwave frequencies. Worth removing that flux with IPA even if the stage is working. I would also tidy up the soldering but that may not really be necessary if the stage is operating correctly.

Additional comment: I raise the possibility that R67, 69 & 71 are selected on test in order to 'calibrate' the gain stage and this may be why the bias resistors have been reworked.

Useful app note here:

http://www.avagotech.com/docs/5967-5924E

http://www.hittite.com/content/documents/mmic_amplifier_biasing_procedure.pdf

http://www.ece.ucsb.edu/~long/ece145a/Lab2_145A/BiasingMMICAmps.pdf

Aurora
videobruce:

--- Quote from: grumpydoc on March 15, 2015, 12:48:56 pm ---
--- Quote ---Isn't -3db half the level
--- End quote ---
-3dB is half the power, but if you reduce voltage by a factor of two then power is quartered which is a 6dB drop

--- End quote ---
Not to get too far OT or hijack the thread, but since you brought it up;
Most SA's that I have seen seem to have a default max 50vdc input level, But the RF levels are different between models with the same max 50vdc. Why?
Fraser:
An RF spectrum analyser is an ac device and uses capacitor coupling at the input.

To answer your question .... DC blocking capacitor voltage rating versus the sustained ac signal overload capability of the input protection.

Aurora
G4JMO:
Thanks Aurora for your input. Yes, the MMIC re-work does look untidy work for an official re-work doesn't it!..and the biasing does look like select on test from the resistor chain appearance.
So, I spent the morning checking out the gain from the input through to the output of the MMIC.
Accounting for external cable losses etc, the test setup had 0dBm at 2GHz injected into the input N connector. I then removed the interconnecting coupling caps to measure the signal down the chain.
With C101, 103 removed and coax tapped onto the output of the Pi attenuator, I measured -19dBm. Looking at the Pi components, this seems high..and coincidentally 6dB more than I calculate it should be. But I'll be upsetting the match somewhat with my soldered coax tap and the removal of the coupling caps. I tried to keep the coax tap as short and tidy as possible.
I put the caps back in and then took out C102,104 and tapped onto the output of the switch, U26. I measured -22dBm. This suggests a loss of about 3dB through the six switches. At 0.5dBm per switch, this seems about what I'd expect.
I put the caps back in and then removed the two caps from the output of the MMIC and tapped onto this output. I measured -10dBm, suggesting the MMIC is providing 12dB gain. This is about 3dB lower than the data sheet suggests, but again, I'll be affecting the matching with my coax tap and removal of the coupling caps.

So here's the big surprise. I refitted all the components, and decided to re-check the error on both the 2GHz signal and the 100MHz internal cal signal....
Weirdly, the 2GHz signal was now showing only about 0.9dBm error (reading about -1.9dBm) taking into account the external cable loss of about 1dB.
I switched on the internal 100MHz calibration signal and measured 1dB error.
With all the screening covers screwed back on, the unit is still showing only about 0.9dB error, which is within the tolerances of my test kit and the unit.
Very odd... but I seem to have a correctly working unit after all of that! At least I learnt something about my analyzer :)  ...but not why it had a 6dB error before stripping down!   :-//

Fraser:
Interesting news.

A possible interstage coupling capacitor dry joint ?  The 6dB error in the pad is interesting. In such a situation I would have lowered the test frequency to one less affected by cable mismatch etc. 2GHz is one heck of a tricky frequency to carry out a path loss test. I would have plumped for 100MHz or 50 MHz myself as you are looking for an error that is evident at the calibration output running at 100MHz  ;) Far less demanding of your coaxial cables and test equipment  ;D

I hope she keeps working Ok for you

Best Wishes

Fraser
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