[Elecraft] 250 Hz and 400 Hz Filter Measurements

[Tom W8JI]

> Yes, think about this using common sense.  Assuming typical band noise 
> might
> typically be -120 to -130 dBm, and the internal noise floor of the radio 
> is
> -137 dBm, you cannot expect to measure 60-70 dB down from such a low level
> noise source without running into internal noise floor problems.

I'm not sure how much this problem manifests itself in the K3 noise 
measurement system, but you guys might not be seeing noise floor in the 
receiver at all.

One way to measure intermodulation is to use a broadband noise source and 
notch the signal from one area prior to injecting the signal into amplifier 
stages. The noise, from intermodulation products of the noise mixing with 
noise, fills the "hole" notched on the broadband noise. I recall the 
passband displayed for the filter bandwidth noise test when the notch is 
turned onto a passband of noise as being about like the display of typical 
IMD.

I think that measurement is also measuring the total IMD of every stage (at 
least in part) from the roofing filter to the soundcard, not just bandwidth 
or background noise.

I just can't imagine measuring a filter followed by multiple stages with 
limited IM performance, especially including a DSP system and audio 
amplifier, that way in an effort to determine bandwidth.

Is anyone else here familiar with notched noise IMD measurements?? I only 
used them a few times in the 80's, so my recall is limited.

73 Tom