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Measuring filter shape factor
On Wed, 11 Jan 2006 21:23:09 GMT, "Dale Parfitt"
wrote: "Michael Thorpe" wrote in message news:43c4e407.4277000@news-server... Amongst all these political propaganda posts, I wonder if there is a little room for a technical question related to radio please? If so, here goes: How does one measure the IF filter shape factor in a shortwave radio? I am technically minded and do have a signal generator, etc., but I am not sure of the exact procedure. Do I get the audio level at the peak frequency, then detune both sides off the peak for -3dB of the demodulated audio, note down the width between these points, then detune to obtain for -50dB of the demodulated audio, get the width at these points, and then the ratio of the two widths is the filter shape factor? I would appreciate a link to the exact procedure if there is one (don't seem to be able to find this in Google) or an expert advice please. I would also appreciate advice as to what actual values of filter shape are considered poor, good, excellent, etc... Thanks in advance, Michael Hi Michael, Although I have never given it much thought, I don't believe one can fully characterize a filter while it is installed in the receiver- certainly, parameters such as insertion loss would not be easily measurable. Perhaps programs like Spectran may give some rough insight. My method is classic- the filter is removed, properly terminated and connected as the DUT on a scalar network analyzer. If you have a vector network analyzer available you could also observe group delay- an important factor in narrow filters. See Paul Kiciak's (N2PK) site for his clever VNA- ideally suited for filters. Dale W4OP Look at the interstation atmospheric noise with a spectrum analyzer? |
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