[Elecraft] 250 Hz and 400 Hz Filter Measurements

[Tom W8JI]

> These measurements are QUITE valid, and represent the response of the 
> radio
> from antenna input to audio output. The wide and narrow measurements on 
> the
> same page are the same data, plotted to different scale -- the narrow 
> plots
> to show filter bandwidth, the wide plots to show behavior well down the
> slope of the filter curve. The dynamic range of the measurement is at 
> least
> 75dB.

I'm still mulling this over but........the curves of the "filter response" 
are very similar to the output distribution I've seen when I was using a 
multiple carriers to measure IMD in CATV amplifiers. The problem is that was 
over 30 years ago, so my recall might be flawed.

I'm having a little trouble understanding how the equivalent of an infinite 
number of carriers in a passband can be used to measure filter shape when 
the distortion in every stage after the filter can cause mixing and produce 
intermodulation products outside the filter passband.

If I wanted to know the deeper attenuation response of a filter or system 
with a filter, I would use an input signal that would not fill the system 
after the filter with multiple frequencies (that are unattenuated since they 
are inside filter passband) that could mix and display intermodulation 
products. I think there is significant risk using broadband noise through 
the filter might display intermodulation distribution more than filter skirt 
response.

My inclination would be to turn off the AGC and sweep with a single tone. 
I'm not so sure a measurement like this isn't just a measurement of 
odd-order IMD in stages following the filter.

73 Tom