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Network Anaysis - a possible fault uncovered

I'm not sure if this faut I describe below accounts for the curious problems I've been experiencing, but it might be relevant, possibly... When I'm displaying (in rectangular mode as opposed to polar) a sweep from 4Mhz to 1.3Ghz I get a pretty flat horizontal line right across the screen as perhaps you might expect (with no load attached to the test bridge and the port left open. Now if I stick my finger inside this port and probe around; I get very little response from the trace. Likewise if I now short the port, there's very little variation in the trace. I'd have thought making major changes to the electrical characteristics at the test port would give rise to some pretty extreme waveform changes; but this isn't the case. It seems like it's missing a large amount of senstitivity and I can only think maybe the y-amplifier might have gone low-gain. Does that make any kind of sense in terms of the problem I'm describing? Can anyone suggest a test to confirm or eliminate this possibility? Thanks, Paul

Reply to
Paul Burridge
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Hose the test signal output directly into the, um, I think it's A and B ports on that instrument, one at a time, and see if you get responses when they are selected for display. You can just put a filter in-line between the generator and the receiver channel you're testing and see at least roughly the response you're getting. Check the receiver gain settings and display scaling to make sure they are working properly, assuming you get something reasonable displayed. I'm assuming you can display A and B independently and not just as a ratio to the R(eference) input, but I don't recall the details of that instrument off the top of my head. Clearly it wouldn't be the amplifier that drives the display y-axis, as you're getting deflection in both directions with the polar display.

Divide and conquer: is the analyzer OK? If it is, then look into the T/R test set. If it isn't, look into the analyzer...

Reply to
Tom Bruhns

Ahem Paul, usually there is a "power" bottom somewhere, usually under a software menu, where the output can be switched off. It not neccessarily defaults to "on". This feature lets you keep the analyzer in an operating state while it allows you to tweak the circuit without being radiated.

I'd grab a spectrum analyzer and check whether the output port has a sensible signal. The signal level is usually adjutable in a menu.

Rene

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Reply to
Rene Tschaggelar

Thanks, Tom. I'm working through the test procedures in the (rather

*large*) manual and am noting down the tests the machine doesn't pass. I just assume the y-magnifier circuits are seperate (at *some* point) between the polar and rectanguar trace display circuitry and that might help to pin the problem area down somewhat. I'm very good at troubleshooting (given a decent schematic) but have no experience with this particular instrument.
Reply to
Paul Burridge

Thanks, Rene and nice to hear you're still posting here, despite all the political BS that's sadly still thrown around by some of our regulars. I'm afraid this unit doesn't have any software as such; it's circa 1980 vintage (which is why it appealed to me as it's truly servicable with no custom chips etc.). As for the spectrum analyser, I sold my last one about 4 years ago and regretted it within 2 weeks. I've never replaced it. :-( However, I still have some pretty mean scopes, so all is not quite lost! What do you suspect the problem is: signal level or spectral purity? Paul.

Reply to
paul

Paul, just have a look at the waveforms with the scope. Does the amplitude make sense, does the sweep make sense ?

Rene

Reply to
Rene Tschaggelar

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