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I am not an RF engineer but play one at work. I am working in uhf SatCom. In the lab, In order to test two radios communicating over a satellite, I built a small RF circuit out of cots components. I basically put a mixer between the two radios and inject the delta of the uplink and downlink frequency ( one radio also has discrete inputs and outputs so I use a circulator to split that out). Finally there are some attenuators so I'd do not blow anything up.

Here is the strange thing. I can use this circuit for most of my testing when I operate in SBPSK 2400bps. Also in CPM 2400 - 56kbps. BUT, if I operate in FSK 16k, the receiver will not get the transmission. No RX shows up.

Then I noticed that if I unscrew the N connector JUST enough ( most likely when the center connector is no longer engaged) but the shell is still on, reception all of the sudden works perfectly fine. I am currently TX at 24dBm and I have 33dB of attenuators. The receiver sensitivity is -10dBm to -80dBm. I "think" I tried it earlier with a different setup with 50dB attenuation with the same results.

My theory is that it is now tiny antennas vs. a solid connection. What is causing this? And is there another RF component that I should put in to properly replicate this condition? If so what is it called?

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  • \$\begingroup\$ Is this a combination of LO phasenoise degrading the FSK, along with overloading the frontend? \$\endgroup\$ – analogsystemsrf Aug 14 '17 at 5:10
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    \$\begingroup\$ You need to somehow "see" the baseband signal on both ends, either with oscilloscopes or spectrum analyzers. Maybe you are overdriving the receiver. \$\endgroup\$ – filo Aug 14 '17 at 5:17
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    \$\begingroup\$ I have a spectrum analyser. On TX I can see my receive power (~-30dBm) but I would not know how to look for something like phase noise. I guess I should see what the signal looks like twith the connector connected and disconnected. \$\endgroup\$ – kuhnto Aug 15 '17 at 5:21
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I ended up increasing the attenuation by adding additional attenuators. Once I was at -80dB I would receive just fine.

Sounds like I was overdriving the receiver.

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