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.
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