Receive Filters

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G8GKQ
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Receive Filters

Post by G8GKQ » Sat Sep 16, 2017 8:46 pm

I went out /P out Portsdown Hill today, which is a very challenging site RF-wise. I managed to receive some pictures from G4GUO on 146.5 MHz. The receive filter I used has a 3dB bandwidth of about 700 KHz, a 20dB bandwidth of 1.75 MHz and a 40 dB bandwidth of about 4 Mhz.

Despite this, I still had problems from a co-located APRS TX (should be 40 dB down) and some other transmitters. I could have done with a sharper filter! The preamp+filter response plot below is centred on 146.5MHz, 500KHz/div and 10 dB/div.

Interestingly, the leandvb receiver in the Portsdown build (RTL-SDR and Raspberry Pi with touchscreen) performed better than a MiniTiouner because it was able to lock-up faster when the interfering signals went off. A lot of the time, the interferers started transmitting again before MiniTioune had locked.

Dave, G8GKQ
Attachments
G4GUO on leandvb.jpg
G4GUO on leandvb.jpg (75.7 KiB) Viewed 2047 times
146 Filter.jpg
146 Filter.jpg (66.99 KiB) Viewed 2047 times

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