Measuring the Frequency Response of a Bandpass Filter with the RTL-SDR

Lacking both an expensive signal generator and spectrum analyzer, Hans used his RTL-SDR together with a simple diode based wideband noise generator to measure the frequency response of a 137 MHz bandpass filter.

To do this he used the RTL-SDR Scanner software which allows you to create a composite spectrum over a frequency range wider than the maximum 3.2 MHz of bandwidth the RTL-SDR provides. The wideband noise generator was cleverly constructed out of a diode operating in it’s reverse breakdown mode.

Apart from the obvious excessive spurs, this method worked quite well and the shape of the filter is clearly visible.

Bandpass Filter Response
Bandpass Filter Response
Wideband diode noise source
Wideband diode noise source
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I think BeardieBob is right. YOUR drawing is missing a 10K-100K ‘R2’ between the Base and -.
His LATTER TWO comments are also correct.


I really liked this item. I cannot afford a spectrum analyser and I only need simple measures. However, the original site of Hans at now has a correction to the noise generator schematic. You may like to update your page here. Do keep on going. Your whole website is superb.