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Measuring filter shape factor
Michael Thorpe wrote: 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 You need to generate a swept frequency response of the IF filter Take a look at the following article on filter selectivity: http://www.radio-electronics.com/inf...electivity.php The shape factor is traditionally defined as BW(-60dB) SF =--------------- BW(-6dB) (see the third figure in the article illustrating a typical filter response and the meaning of the two bandwidths) For an ideal filter (see second figure), this bandwidth ratio would be equal to ONE (1.0). The closer the shape factor is to 1.0, the sharper the selectivity. For example, if you look at the specs for the Eton E1 on the following link (Monitoring Times review) http://www.monitoringtimes.com/html/...k-etone1xm.pdf the shape factor for the 7 KHz filter would be: SF = 12 KHz/7 KHz = 1.71 (not a very sharp filter, but not bad for a radio of this type) The other two filters have even worse shape factors. With DSP receivers, these filters can be made extremely tight and shape factors can be very close to 1.0. Hope this is helpful. Perhaps Pete can chime in with a detailed measurement procedure. RK |
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