I used the spectrum analyzer to look of noise in the converter (auto-oscillation or something… ?) but could not find anything. The unit seemed to work fine on the VNA but not in the receiver. I made every measurement carefully, I had no explanation for this. Very interesting, enabling 10dB attenuator only had a 2dB impact on sensitivity, and the 20dB attenuator about 13dB. I reinstalled the module with a SMA connector instead of the female N and went to check sensitivity with various combinations of the attenuators. But how could this be ? Strange attenuator behaviour Sorry, I did not capture the plots on the VNA. Surprise Surprise! The attenuators worked JUST FINE and there was little more than 1dB loss in that unit when they were bypassed. I took it out (not easy) and connected it to the VNA to confirm (that is what I thought) that even with both attenuators bypassed, the signal was being attenuated a lot more than reasonable. By connecting the RF generator directly after the CONV-B UNIT, sensitivity went up to -123dBm so the problem was in that module. After the second relay, there is a 20dB attenuator and then a 10dB one. Signal from the antenna is sent to the down-converter for frequencies above 1GHz, or bypassed by the two relays otherwise. The converter unit (CONV-B UNIT) in the VNA This was not right, I had to find why that was. The specifications say better than -113dB which is not very far, but the much simpler/cheaper receiver was almost 10dB better at -118dB. A trip to the lab and I found the sensitivity SINAD was only -110dBm. Moderate signals were readable on both but I could hear clear voices on the other than I could not even notice on the 8500. I was happy with the IC-R8500 until one day I used a splitter to connect another receiver along this one to the same antenna at the same time, and I found it was performing a worse than the other.
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