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Medium Wave Filtering with RSPduo

medium wave filtering

Your wideband SDR receiver can be crippled by strong local broadcast signals. Medium wave filtering can provide the solution. Either a bandstop or high pass filter does the trick.

Recently, I have been exploring challenges associated with using wideband loop antennas and wideband SDR receivers. The biggest challenge at my QTH is medium wave overload from local AM broadcast signals. Solutions call for wideband rejection filtering.

By this, I mean some sort of wideband device that reduces strong medium wave signals while passing weaker shortwave signals without reducing HF sensitivity. With the RSPduo, your first line of defense is what SDRplay calls a “medium wave notch”. According to the specifications, the MW Notch will reduce AM broadcast signals by at least 40 dB across 540 to 1490 kHz.

And guess what? It really does!

Check the top of the picture above in green, labelled Notch. My eight strongest local broadcasters, which normally swamp me with -20 to 0 dBm signals, are reduced by 40-50 dB. I don’t know the details, but SDRplay has designed and implemented bandstop medium wave filtering that is really effective. With the MW Notch engaged, you can enjoy full spurious free dynamic range and low noise figure across all of shortwave. No more spurs.

I made these diagrams using the RSP Spectrum Analyzer with the RSPduo. You will find this free companion software provides a very useful tool for radio experimentation. You can use it with any SDRplay radio spectrum processor.

Medium Wave Filtering Outboard Solution

Last week, I tested the performance of my outboard DIY 1.7 MHz high pass filter with NanoVNA. My test showed medium wave filtering of more than 30 dB at the low end of broadcast band, rising to around 20 dB at 1400 kHz.

So I switched off the MW Notch and inserted my outboard filter ahead of the RSPduo. You can see the HPF filter performance at the lower end of the above picture in blue, labelled Filter. Medium wave signals are reduced by 35-40 dB at the lower end of the band, and by 25-30 dB further up.

Unfortunately, both the MW Notch and outboard HPF filters are so effective that they prevent medium wave DXing with either turned on. That will require a different approach to be described next.

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