[hpsdr] Using Mercury to monitor final amplifier transmit signal

[Tom Cathey]

Jean-Claude,

What Joe said.

I think you need to find your own sampling level “sweet spot.”   Dynamic range requirements are  dependent on how low into the noise you need to see the crud up the band.  If the signal is too strong, you may generate spurious.  If it is too weak, you will not see the stuff that is way down near the noise floor.  Experiment with pickup levels and see what works best for your particular set up,


I found that just the SO-239 jack “inner stub” mounted on the sampler chassis is all that was needed to sample enough signal for the Mercury DUP function.  I tried a ferrite xfmr and that was way too much signal. In addition, I added a 10 turn 2K pot for a variable atten at the change-over relay to give a wide range of signal level coverage – when using from 10w to 1500 w of power output.


Also be careful with stray pickup in the Merc changeover relay box.  I needed to use beads and .001 bypass caps for the DC relay leads. They were bringing in an extra 20db of stray pickup when unfiltered..  You should be able to disconnect the sampler coax and see the transmit signal drop way down, like at least -30 dB or more. Otherwise there is stray pickup around the intended sampler box. Draw the plan out and you will see what I mean.

All in all,  this new “DUP” IMD monitoring system has become my main station monitor – in addition to a standard o’scope.  Nothing like watching a spectrum analysis of our overall FINAL transmitted signal on the air. But it must be a signal that is sampled without stray pickup or it will be invalid.   

When testing, you will find there are amplifier drive, loading, biasing  and reduced power output “sweet spots” that give better IMD than others. 

Tom, K1JJ







From: Jean-Claude JCA. Abauzit 
Sent: Sunday, February 10, 2013 10:51 AM
To: hpsdr at lists.openhpsdr.org 
Subject: [hpsdr] Using Mercury to monitor final amplifier transmit signal

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What is the safe level in dBm to use when monitoring the transmit signal? Here I have Hermes and I suppose the max level on RX is the same as for Mercury.

 

Thanks,

Jean-Claude PJ2BVU

 
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