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Output low-pass filters measurement with NanoVNA #qdx
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Andrey (KK7EEV)
Hi!
I am trying to understand loss of power on output on all waves: 80m drops from ~5w at T1 output to ~3.6w at antenna output. I know the loss itself is not a big deal, but I am curious to understand the reason :) I have QDX v3 I decided to measure low-pass filter performance using NanoVNA. I'm not too experienced at this, so it's possible that I did something wrong. I calibrated OSLIT. Hooked up S11 to C9 at point of connection with L5. Hooked up S21 to antenna output. Powered up QDX and selected 80m band. Graphs I'm getting for impedance and attenuation not disastrous - they actually very similar in shape to Hans' simulation in manual, but devil is in details. where it supposed to be flat and at 0db with impedance ~50ohm (3mhz-4mhz range) I am getting it jumping between -3db and -8db, with impedance jumping between 25 and 50. Higher in freq Hans' simulation shows much stronger drop in dB than I'm observing too. do I do something wrong? did anyone try measuring actual performance of filters? my next step would be to de-solder all filter components and measure them individually and try to reproduce filter on breadboard. thanks! Andrey |
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Syd
Out of a NanoVNA Test board kit I connected a small adapter PCB that I can plug in LPFs and BPFs and test with a NanoVNA. I also jury rigged a couple of connectors to 2 test cables that I plugged into the NanoVNA and the LPF, BPFs and they also tested OK with the NanoVNA. So, if things are not looking good measuring the LPF in circuit, I would disconnect the coupling cap to the filter and tack solder in an input from the NanoVNA to the LPF eliminating impedance mismatches and test again knowing that only the LPF is what you are testing.
73 wt1v |
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Andrey, the location you are injecting the signal into the filter is the middle of the filter and the filter is not even in the circuit unless the rig is on and 80 meters is selected, and if not transmitting, the other filters probably aren't turned "off" completely either. All this leads to bad VNA readings. If you want to test this 80 meter filter itself you will probably need to lift the cathode ends of D1 and D4 and connect the VNA leads to the two holes where these diode leads are removed from, and ground as well obviously. Good luck .. 73 ... Ron Out of a NanoVNA Test board kit I connected a small adapter PCB that I can plug in LPFs and BPFs and test with a NanoVNA. I also jury rigged a couple of connectors to 2 test cables that I plugged into the NanoVNA and the LPF, BPFs and they also tested OK with the NanoVNA. So, if things are not looking good measuring the LPF in circuit, I would disconnect the coupling cap to the filter and tack solder in an input from the NanoVNA to the LPF eliminating impedance mismatches and test again knowing that only the LPF is what you are testing. |
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