Hi all,
I got hold of a heavily optioned but also very broken Agilent spectrum analyser for a decent price. First thing I noticed was that someone has had a go at fixing it as well; I can hardly blame them
. Anyway, to cut a long story short, the FFC between the main board and the keyboard had some shorts across it, and this blew out the main CPU, video also seems to be dead now. CPU unfortunately is an obsolete part which is also a 324 LFBGA, but I found some and replaced it. I only discovered the keypad problem after I killed the new CPU by plugging the keypad in, so I ended up having to solder two separate chips; that was a stressful day! Also burned myself finding out which chip was shorted (200C apparently)
So right now, the spectrum analyser works over USB and Ethernet; which I am pretty happy with; I don't need it to be portable. However, it has just become clear to me that there is a problem with the RF frontend; the input signal disappears once I go above ~2.1GHz. At 2.2GHz it is completely gone. The noise floor is the same across the whole span however.
I can't find a service manual for this (even called the keysight service center, they said that it doesn't exist), nor even a block diagram of what mixer configuration or IFs it uses. I wouldn't say that this sort of RF is my specialty, so I'm putting it to everyone; do you have any hypotheses of what could cause this behaviour?
My first thought is that it is doing the downconversion through multiple paths, and one of them is dead. But given that this is a compact modern analyser, 3GHz BW seems like it could be done in a single path and I'm sure they would try to do so.
I'm very busy with other stuff at the moment, but I'm going to open it up again and take some pictures in the future. First though, I need to get hold of another spectrum analyser to fix this one...
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