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8753C/87054A 6 GHz troubleshooting

Question asked by n8ur on Apr 28, 2013
Latest reply on Apr 30, 2013 by n8ur
Just acquired an 87054A test set to use with my 8753C.  Everything is just fine in 3 GHz range, but when I go to 6 GHz, I get a "CAUTION: POSSIBLE FALSE LOCK" message.

I have done the following troubleshooting:

1.  When setting a CW frequency of say 2 GHz in the 3 GHz range, I get a nice single response on a specrum analyzer connected to Port 1 or the R output (through a power divider).  However, going to 6 GHz range and setting any CW frequency (e.g., either 2 or 4 GHz) I see the signal dancing all over the place.  It's definitely not locked and seems to be sweeping from DC-ish through 3 GHz.  I don't see any sign of output above 3 GHz.

2.  RF output level from the 8753C seems to be reasonable; using a power meter inserted via power divider on the RF OUT to RF IN path I'm seeing signals that are within a dB or so of where they should be, and I can generate more than +20 dBm output.

3.  I've tried applying 2 GHz at +10 dBm (as much as I can crank) from a separate signal generator to the RF IN port of the test set and then looking at the R output (through a power divider) in both 3 GHz and 6 GHz ranges.  I think this may be the giveaway: in both ranges, the R port has a 2 GHz output at about -12 dBm but the 4 GHz output is lower than -60 dBm.  So there's no difference in the output signal when switching frequency ranges, as if the doubler isn't being engaged.

I've done some other tests as well but those three seem to indicate that the switch around the doubler probably isn't working and is never being switched in line.  The receiver is expecting a signal at 2X the frequency it actually sees, and therefore is unhappy and doesn't lock.

My biggest concern right now is trying to determine whether the problem is in the test set, since the dealer gave me a warranty and I may be able to make this *his* problem.  But I don't have other analyzers or test sets to swap around, and therefore I want to do as much as I can to avoid sending back something that isn't broken.  (By the way, I've swapped RF and control interconnect cables and that didn't make any difference.)

Does my diagnostic make sense?  Are there any other tests I should perform to definitively rule in or out one of the boxes?