SDRplay Community Forum

Thanks glovisol, very informative.

I tested the RSPduo in the same conditions and in the same lab position. The metal case of the RSPduo shines here in its quality, the uploaded pic shows it: no FM broadcast feedtru at all.

The filter has now been assembled into the die cast box and I could upload the final measurements. The filter attenuation in the stop-band has increased to between 45 and 50 dB and it is on average 6 db better than the commercial filter. Also, looking at the comparison, white for the designed filter and yellow for the commercial filter, the FM Broadcasts feedthru seems far worse for the commercial filter, but at 90 dB down anything can happen, with FM signals stealing directly into the RSP-1A.

In any case both the commercial and the designed filter are a useful addition to my RSPduo accessories and I must use one or the other when scanning the Airband or the VHF 2 m Ham Band to get rid of overload FM Broadcast noise.

Very nice post.
Another useful trick in low pass filters is to replace part of the capacitors to ground by open quarter wave stubs of coaxial cable.
You cut the cable to notch out a strong local signal .
Normally there is not enough capacity so you add a real one to compensate.
Or you add another open stub on a higher frequency.

Hi F1BJB,

If you look here:

https://www.sdrplay.com/community/viewt ... =12&t=4307

you will find the very first experiment I did a short time ago to test the new Spectrum Analyser software with a band-stop filter quickly made with coaxial stubs. Twentyfive years ago I was designing coaxial filters made with equal element series and shunt sections directly derived from tabulated low pass filter data. In a future project I plan to demonstrate the design technique of such filter types.

Cheers,

glovisol