EMRFD Message Archive 13798

Message Date From Subject
13798 2017-04-13 13:59:38 k1rf_digital_stev... More on LC bandpass filter measurements
I just uploaded the file "Bandpass Filter Measurement anomalies" in the K1RF folder.  I had summarized this problem in the recent "bandpass filter testing" thread but I thought I would write up something in more detail.  This problem was bugging me for quite some time and I finally figured it out.  Basically, you must have a VERY clean sinewave signal when testing relatively wide LC bandpass filters if you are trying to measure bandpass filter response at frequencies less than half of the bandpass filter's center frequency. Even relatively "small" harmonic content of the signal generator's fundamental frequency will cause "jaggedies" in the frequency plots but only on the lower edge of the frequency response.  The file explains in detail what is going on. So a square wave type signal generator must be fed to a lowpass filter with steep attenuation such that little harmonic content remains at the lowest frequency of interest used for testing the bandpass filter. The problem is not evident with lowpass filter testing. The problem is not evedent when testing return loss of  the bandpass filter.  The problem is not evident when testing narrowband crystal filters. It shows up in spades when testing wideband LC bandpass filters.

Steve K1RF