I received an Agilent N9340B 3Ghz Handheld Spectrum Analyzer with a note that it passes all self-tests but does not respond to input signals. I took the gamble that it’s the RF input connector got disconnected somehow.
I opened up the case and noticed that the 40Mhz cable was unplugged, so I was half-correct. I connected it and got a signal at the precise frequency, but the amplitude doesn’t look quite right. It’s around -20dB off. When I scanned it across the full 3GHz band, I noticed the amplitude roll-off when I scan below 800Mhz, and I got very little signal left when I get to somewhere near 10Mhz.
I tried running a user calibration with a 50Mhz CW source but it failed amplitude calibration. Apparently the unit is not fully working. No self-test errors though.
So I opened up the unit and the RF section. The front side of the board doesn’t have any visible signs or unusual smells, so I suspected the improper gains is caused by the input attenuator HMC307:
I was about to order the chip, but because of the lead time, I decided to just take a picture of everything and analyze it off-line:
After removing the screws holding the N-type terminal so I can get to the back side of the board for taking pictures, I noticed the RF out connector just fell off the board with the pad:
That means the RF out is not touching the board. I never would have suspected that it’s the problem, since this unit does not have the tracking generator option enabled and hence the RF out port is pointless. But for the sake of completeness, I resoldered the connection after I put the board and the connectors back to the RF module slab. It’s ready to be stowed away for another day when the attenuator chip arrive.
Guess what? Once I put unit back together, I turned it on again and everything works perfectly! The power level is flat and within 1dB of what my 8648C pumps out. I did the user amplitude calibration again, it passed, and everything was spot on!
My suspicion is that the path gap created a capacitor between the board and the type-N terminal, which messes up the termination and created reflections. The bottom section of this picture is where RF out meets RF in:
I didn’t study RF as my EE speciality, so if you are a RF design expert, please let me know what you think the reason might be in the comments section.
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