Thursday, July 05, 2012

The Afedri SDR-net - The Effects Of A Low Pass Filter

Those who have read my impressions of the Afedri SDR-net, will know that the receiver is susceptible to overloading, due to its modest 12-bit ADC and lack of band pass filters. Hence, in many cases one will have to reduce the internal VGA gain, which again reduces useable sensitivity.

While a bank of band pass filters would be the best solution, the second best is making or buying a filter which eliminates the strong signals - if they are away from the spectrum you want to monitor. I decided to test a low pass filter, so I could eliminate SW signals while listening to MW. I ordered a tailor-made filter from Clifton Laboratories, with a cut-off frequency of 2100 kHz.

I put it together today, and you can see the results below. The Afedri had been set up with max VGA gain, but I had reduced the gain by 10 dB in SDR-Radio. Still, I would have a sensitivity of around -104 dBm. The pictures show how the MW spectrum displayed before, and after I engaged the low pass filter. Not only was there a visual difference, the audible difference was huge.

In addition to the low pass filter, I also used a combined high pass filter (cut-off frequency 500 kHz) and 100 kHz notch filter, designed and built by Dallas Lankford. There is only one major LF source though, the 250 kW Loran C station on 100 kHz, 14 km away. I didn't do any before-and-after test of this filter, since removing it will kill MW reception regardless of receiver.
MW spectrum no low pass filter

MW Spectrum with low pass filter
I probably should have reduced the Afedri's VGA gain another 3-5 dB, but I suppose you get the picture, so to speak. A quick check of the 49 meter band showed that there were no signals left, while before the signal levels were very high indeed.

I carried out the test with a 30 meter longwire and a 25 meter long coax feed line.


UT2UU said...

Good job! Thank You.

low vhf tv dx qrp 28 mhz said...

thank you for the tests .. most helpful .. regards david