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Old 28th Nov 2017, 2:57 am   #5
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: Directional coupler - terminated and unterminated

I think the coupler/sampler section is there for locking the first LO. I think the 2nd LO stuff will be off to the right of what we can see in the small snippet of the overall circuit.

These analyser plug ins are old school and (relatively) low cost and I'd expect the 2nd LO to be a free running oscillator with a narrow sweep range and it presumably locks the first LO with the sampler on narrow spans and then sweeps the second LO.

I'm going to guess the first mixer runs with about a 10mW drive level so the LO level at the aux coupler will be 10dBm plus 3dB for the T attenuator plus maybe 1.5-2dB for the loss in the two couplers and the cables/connections. So it could be up at about +15dBm at the input to the aux coupler? With a 10dB coupler here it would manage +5dBm at connector J35. But that is all just a guess.

If the coupling factor for all the couplers is as low as 3dB then I'd expect the input VSWR of the aux coupler at the P1 mainline input to be up near 3:1 if J35 was left unterminated. That would be a bit poor and may cause lots of mismatch ripple across the LO1 range.

Also the input to the aux coupler would be +10 +3 + 3 +3 + maybe another 1.5dB = +20.5dBm. The output at the aux connector would then be +17.5dBm when an external connection was made here. That seems quite a high level.
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Regards, Jeremy G0HZU
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