CW Filter Shape Factors

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  • Updated 6 years ago
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Has anyone with a signal generator measured the CW filter shape factors by measuring the -6/-60 dB points. It may be a little tough to do with the 10 hz frequency step and the S-meter not being calibrated in decibel level, but has anyone attempted this. I'm only asking because, after years and years of using 1.4:1 shape factor crystal filters, I'm not hearing what I expect to hear from "brick wall" digital filters. By using the term "brick wall" filters, I am assuming that this definition implies a much better approximation to the 1:1 shape factor of a "perfect" filter than is possible with analog filters. This is why I would like to know if anyone has measured this using the 6500 narrow CW filters.
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Charles - K5UA

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  • unsure but hopeful.

Posted 6 years ago

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Steve - N5AC, VP Engineering / CTO

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Official Response
Off the top of my head I can't tell you if the shape factor is governed strictly by the number of taps in an FIR filter or if the chosen bandwidth plus the number of taps will set this.  What I can tell you is that the number of taps in the filter goes down by a factor of two at each of these crossings at the present time and I know that will affect the shape factor.  So there are two possibilities:

1) shape factor is independent of filter bandwidth chosen (I know how this sounds, but I believe it is a possibility since all that we are analyzing is one side of a skirt) in which case you will also see a 1.2 for the 100Hz filter.  If this is the case than all filters in each range will have the same shape factor (0-400, 401-1000, 1001-1500, etc).

2) The other possibility is that the shape factor is bandwidth dependent in which case there will be a progression from 0-400 and then a jump to 401, a progression from 401-1000 and a jump at 1000, etc.

Let me know which it is because I'm curious too!