| Electronics > Repair |
| INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value |
| << < (5/13) > >> |
| videobruce:
--- Quote ---We could trace and find the proper distributor to assist you soon. --- End quote --- Since when distributors repair equipment?? :-- |
| videobruce:
--- Quote ----6dBm means exactly half the signal amplitude --- End quote --- Isn't -3db half the level?? |
| G4JMO:
Here's the RF PCA I've not had time to do a thorough circuit analysis yet but here's a brief description (Refer to GSP-827 Front end photo... RF input through capacitor into resistor pad R91,92, R93 (75R, 120R, 93R respectively). Then there's a cap to ground and what appears to be 2 x SOT23 back to back diodes marked 'EOF'...probably limiters. The signal then feeds via C103,101 into U22 marked S169. This is a Skyworks Phemt GaAs SPDT DC-2.5GHz switch. Signal continues into a second S169 (U23), a third S169 (U24) and then into U21 marked S131 418. I think this is another Skywork's switch. Then onto another S169 (U25) into U26, another S131 418. The signal then gets +16dB gain from U18, an NBB400 MMIC. Looking at this components solder seems to suggest it has been the subject of some attention before!. I plan to isolate it, power it and feed a signal into it and check how much gain it is giving. I'd be surprised if it was this component, being so far down the chain. I guess the first things to check are the protection diodes. Regards Andrew |
| Fraser:
Do not be fooled by the position of the MMIC. I have seen many such amplifiers destroyed by excessive signal level. I have also seen partial failure as some are multi stage inside the package and suffer failure of one stage. The fact that this has received attention in the past increases my suspicions surrounding this component. It is worth checking how much current it is drawing and comparing the treading with the manufacturers recommendation. This can help identify failure and (when repaired) check that the MMIC is correctly biased in the design. I managed to cook an MMIC in a test piece by passing broad band noise through it. It is not just the peak RF signal level that can be harmful, it is also the combined energy of a multitude of signals in a broadband signal source that causes high dissipation in the amplifier. If this does turn out to be the failure point, I recommend some further testing after repair to see whether some modifications are in order to protect the MMIC. Some interesting reading from NASA: https://electrondevices.grc.nasa.gov/files/T-06-Dec-1996-JPL-96-25.pdf NBB400 datasheet is here: http://www.rfmd.com/store/downloads/dl/file/id/29224/nbb_400_data_sheet.pdf http://www.farnell.com/datasheets/1499972.pdf Aurora |
| grumpydoc:
--- 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 |
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