First off, I needed to check which software was well suited for filter analysis. On this occasion I settled for the ‘Frequency Response Analyzer’. Output 1 of the Red Pitaya was swept over the full frequency range. I fed these signals into the circuit and on the output I measured the result using Input 1 of the Red Pitaya. Next I performed the filter test for the 20 Meter band. Everything was connected as described above.
The result was really not so bad, although the undulating undershoots disturbed me. These came about because the filter was not terminated with the necessary 50 ohms.
A better result was achieved after I terminated the filter with a grounded 50 ohm resistor.
Here you can see the filter curve of the 30 meter low-pass filter with the 50 ohm resistor.
This looks so much better and I’m very pleased with the result. At the first harmonic around 20 MHz the filter displays an attenuation of around 30 dB.