Electronics > Repair
E4433B RF sig gen repair
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microbug:
I must apologise to R-1125F. On checking again, the coherent carrier output is looking pretty suspicious.

After boot, the sig gen is set to 4GHz. Notice the peak at 2.72GHz, which is the same as what I'm seeing at the main RF output.


Sweeping the output through the 250kHz-4GHz range, it got worse. The 2.72GHz peak is always present, and is particularly bad around f_out = 2.434GHz.



The correct carrier does always seem to be present, and is always the loudest signal (amplitude of 1-3dBm).

I have forgotten to bring an N-type to SMA adapter home, so can't check the actual RF output today. I'll have a quick look at the synth board...

If anyone knows what the coherent carrier output should look like (in terms of images present and carrier amplitude), do let me know. I will have a dig through the service manual if not.
vaualbus:
I think you can look at the coherent output in one of te TSP repair video. it had a failed generator with ALC error and it foe sure measure the coherent output.
The video should be around 4 years old(never mind it's already 9years, wow time is passing so fast!)  let me post here the link:
https://youtu.be/RSCGAY5z_44?si=z0MPqTE5SF31ma4G
microbug:
Found the coherent output discussion, at 18:30 in the video. Looks like my levels are ok (his were -6 to -3dBm, mine are 1-3dBm).

There are spectral images in the video, but otherwise his spectrum looks fairly clean. My spectrum is generally bad with the 2.7GHz signal, and at 2.434GHz is particularly bad, with multiple peaks that shouldn't be there. There's nothing like that in the video. That does lead me to suspect the synthesiser board.

The fact that this 2.7GHz signal is always present narrows the search down to areas of the synth board that are common to all frequencies. There are some switched bandpass filters on the synth board, in tight bands between 250MHz and 1GHz. They are quite aggressive (11th order). I would expect these to cut the 2.7GHz signal out. Either they aren't working, or the signal is appearing after the filters. I need to sweep the frequency slowly through the full range and monitor the 2.7GHz signal level. This should reveal whether different filters are switching in and out.

Another clue: the PLL out-of-lock LED (DS1) is on in some frequency bands. There are at least four different bands where the PLL is out of lock. I will look closely at U201 (UPB584G -- NEC 2.5GHz div2 prescaler, marked Agilent 1GC1-4209), because its datasheet warns of unwanted self-oscillation if the input is too small.

I also have been trying to install Windows XP to get the Agilent Falcon service software working (using this post). Have had some difficulties with GPIB-USB-HS drivers and the Agilent software crashing.
microbug:
Here are some pictures of the synth board. The right side is the phase locked loop and splitter. The middle generates FM baseband to mix with the VCO tuning voltage (I think). The left includes a /2 divider and two doublers, and a lot of switched filters. The gold part near the connector is the output buffer.

Edit: the label that says 'microstrip to output section' should be 'microstrip to filters/dividers/doublers'. The output section comes after these parts.
vaualbus:
Well more interestingly is that, if the PLL get unlocked, why the instrument never report such error?
Maybe check the reference 10Mhz signal and the different supplies on the board.
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