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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 Thanks, Dale, for your input. I would have no problems measuring filters when taken out of a receiver. But this is not always possible, for example with modern DSP software-defined receivers. There you have no choice, the filter is no longer just a component you can desolder, so you do need to measure the entire receiver. Parameters such as insertion loss is immaterial in such cases - all that matters is the filter shape. Michael |
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Measuring filter shape factor
In article 43c63d58.6926500@news-server,
Michael Thorpe wrote: 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... 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. Thanks, Dale, for your input. I would have no problems measuring filters when taken out of a receiver. But this is not always possible, for example with modern DSP software-defined receivers. There you have no choice, the filter is no longer just a component you can desolder, so you do need to measure the entire receiver. Parameters such as insertion loss is immaterial in such cases - all that matters is the filter shape. But when the whole system is put together, it's no longer "filter shape factor", it's some other parameter, "adjacent signal rejection" or something like that. Mark Zenier Googleproofaddress(account:mzenier provider:eskimo domain:com) |
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