| Electronics > Repair |
| E4433B RF sig gen repair |
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| microbug:
Good news: I had another U16 (MSA-0686) on an old PCB from an HP 8922M GSM test set. Bad news: I swapped it in and the symptoms are exactly the same. The input and output of U16 look identical to before. The bias tee DC voltage is around 3.5V at U16, which looks about right (datasheet). The 9.1V rail measures 8.6V. I checked each resistor and both capacitors in the bias tee, and they all look ok. Inductor is a short so I am assuming it's fine. Edit: I checked U15's output and it looks a lot like U16, i.e., seems like it might be broken. I will now have a look at U12, which is MSA-0386 that drives both U15 and U16. |
| microbug:
I have found that by pressing the synth board down into its RF gasket (as it would be when assembled fully), the PLL unlock error goes away AND the U12 output switches between a single, stable peak at the correct frequency, and a mess of peaks all over the place. Sadly, pressing the synth board down doesn't change the output of the whole instrument, which remains a fixed 2.72GHz at -3dBm. So it seems I've just solved a problem I created myself -- and a problem I identified earlier (the PLL losing lock) |O I put the synth board back in its shielding block and tightened the screws. Now I get no frac-N unlocked error and the coherent carrier output looks perfect -- one peak at the desired frequency, and some harmonics visible (which should be filtered out later on the output board). Main peak between 1-3dBm. The problem must have started when I opened the synth board RF shields. So I'm back to suspecting the I/Q modulator chip. I'll have to poke around the output board again, and see if I can find the source of the 2.72GHz signal -- it's not coming from the synth board. Perhaps the modulator chip is self-oscillating. Edit: screenshot of the coherent carrier output attached. |
| vaualbus:
It is still strange to me that the only fault could be the I/Q modulator.... I guess looking, at the schematics, you could bypass the IQ modulator chip? The output is AC coupled to next amplifier I am expect the chip to have some loss so if you have a 3db attenuator maybe you could insert that there to avoid demage the next stage chip; Also I see that the IQ modulator output is differential and is converted back to a single ended signa by two transformer (T50, T51) maybe one is open? I still do not get why the output is that magic 2.72Ghz signal at all. |
| R-1125F:
You didn't mention what options you have. Is it possible that the digital generator boards are somehow interferring with the IQ modulator? |
| vaualbus:
Doesn't really make sense thought anyway? The IQ is at IF frequency, how they ended up at a so high frequency? If the IQ modulator is a multiplier of some sort, it would produce some spurious armonics at multiple of the carrier frequency, so why you are stuck at 2.7Ghz? So, let's said it failed how the hell the output is stuck at 2.7Ghz, if the input signal is at 4Ghz as it was shown in the first pictures of this thread? It is still puzzling me, either the IQ is oscillating, and we see the result of it or who knows. What I guess is possible to do is to bypass it temporary with a coax cable, the output is DC coupled into the next stage MIMC amplifier so I guess it should "work"? Or maybe better if you have another sig generator you can try feeding a signal after the IQ modulator and see if you get any output at all! |
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