A five dollar module from eBay lets you experiment with front end attenuation to avoid SDR overload from local broadcasters and enjoy medium wave DX.
You may find this counter-intuitive, but sometimes you want a receiver that is less sensitive. That’s why most better ham and SWL receivers have an attenuator switch, providing 10-20 dB attenuation before any active devices. Front end attenuation is particularly important for your SDR to avoid overloading the analog-to-digital converter.
If your SDR doesn’t have front end attenuation, you can easily add this feature with some fixed attenuator boards available on eBay. For example, the Fixed RF Attenuator, shown above, features SMA connectors, 10/20/30 dB attenuation and can handle signals of +23 dBm (200 milliwatts or 9 volts p-p). It’s response is flat from DC to Daylight and it only costs US$5.
Now, some people don’t like inserting front end attenuation because they add noise and increase the Noise Figure of your receiver. That’s true, but only on frequencies where internal noise matters, such as above 20 MHz. Down on medium wave, your noise limitation is external – atmospheric and man made noise. So, if you add a 20 dB attenuator ahead of a receiver with a 10 dB Noise Figure, you will have changed your Noise Figure to 30 dB.
However, since your atmospheric and man-made noise is probably about 30 to 50 dB higher than this, you won’t even notice the noise you have added to your receiver.
On the other hand, adding 20 dB front end attenuation will increase your spurious free dynamic range (LNA and ADC) by 20 dB. That’s usually enough to prevent Medium Wave overload in your SDR and let you use a wideband active loop for AM BCB DX without problems from strong nearby stations. Of course, you will need to remove the pad when you move back up to hear weaker shortwave signals.
Front End Attenuation – RSPduo Performance
Without front end attenuation, medium wave DX with my RSPduo is quite difficult. You can get rid of overload by reducing LNA gain by -42 dB, but this raises the noise floor up to around -80 dBm. Adding 10 or 20 dB attenuation seems to work best for getting a good SNR on medium wave DX stations. You will also need to do -12 to -18 dB LNA gain reduction, though.
All of this depends on your local noise situation, the quality of your antenna and of course the strength of your local AM stations that cause overload. Running the RSPduo as a 14 bit ADC gives you more headroom to play with as well.