Some front end filtering helps you use wideband loops with wideband SDR receivers on HF while holding overload from nearby broadcasters at bay.
Well, here is my last article on the challenges of using a wideband loop with a wideband SDR receiver. Wideband active loops, like my AAA-1C one meter model, put out very strong signals from local medium wave broadcasters. If you aren’t careful, these strong AM band signals will drive your analog to digital converter (ADC) into non-linear overload.
Simply put, even if you are listening at 10 or 15 MHz, you must make sure medium wave signals do no exceed your ADC dynamic range. Some receivers use diodes to limit signals entering the ADC. But non-linear diode clipping often causes as many problems as it solves. Plus, if your receiver is not direct sampling, mixers used for frequency shifting can overload as well. Keep in mind the bit depth of your ADC. Your 16 bit ADC will have 24 dB more dynamic range than a 12 bit device, and that can make a huge difference.
I can listen all across HF on my Flex 6300-loop combination without any problems even though there is no bandpass filtering (see green above) in the signal chain, thanks to 16 bits. Everything works fine. Similarly, switchable half-octave bandpass filters in the 14 bit Perseus (see orange) knock down medium wave signals by more than 15 dB. If that’s not enough, the step attenuator steps in.
Perhaps the best solution is provided by the RSPduo with its 40 dB medium wave band reject filter. You still get some artifacts from the mixer, but ADC overload is completely gone.
All of these built in solutions make the RSPduo, Perseus and Flex 6300 just fine for HF DX using a wideband loop. And you will find that none of these solutions raise the receiver Noise Figure or reduce shortwave sensitivity, except if you need to add attenuation with the Perseus.
Reduce ADC Overload – The Afedri Challenge
Like the RSPduo, the Afedri AFE822x dual channel receiver is a great piece of kit for spatial filtering and diversity reception. But, other than lack of truly dedicated software, Afedri cannot handle HF listening in the with a wideband loop. As you can see in blue above, your Afedri signal chain is wideband direct sampling with a 12 bit ADC. No bandpass filtering. To avoid MW overload on HF, you need to reduce LNA gain substantially, either manually or with AGC enabled.
What you have left is about 20-30 dB less gain for your shortwave listening after you prevent medium wave overload. I can solve this problem easily by inserting a 1.7 MHz high-pass filter at the antenna input. This allows me to run the Afedri on HF as if the local AM broadcasting stations were not there. As with the other solutions, the outboard filter does not add to Noise Figure or reduce sensitivity, but maintains the radio’s spurious free dynamic range above medium wave.
An alternative solution to a DIY HPF might be the cheap Distill AM Barebones Bandstop Filter for around US$10. You may need some SMA connector adapters, but the 20-30 dB medium wave reduction should be more than enough to keep things sweet.