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Old 28th Nov 2017, 4:03 pm   #9
cmjones01
Nonode
 
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
Default Re: Directional coupler - terminated and unterminated

Then I tried looking at the output from the 1st LO using my sampling scope. I couldn't get a connector on to J35 because there's only room for a right-angled SMA connector and I don't have one in my armoury of cables. I couldn't get an elbow to fit. I could, however, get the connector on to J38, the output to the first mixer, with the attached result (after an initial schoolboy error: sampling scope doesn't work usefully if the oscillator frequency is sweeping...). I'm not sure if it's legible in the photo but the scope says the signal is 297mV RMS, which I make to be 1.8mW at the scope input, minus 3dB for the splitter to the scope's trigger input. So a 1st mixer drive level of about 5 to 6 dBm, I reckon.

This is without a terminator on J35. Putting a terminator on J35 made no discernible difference to the signal on the scope.

Having spent ages wrestling with SMA connectors on hardline coax in confined spaces to put it all back together, I now find that the analyser has lost 10dB in sensitivity and there's loads of breakthrough from the 50MHz calibration source, even with nothing connected. Aargh.

Chris
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