Posted by
Kok Chen on
Jul 18, 2010; 9:10am
URL: http://elecraft.85.s1.nabble.com/250-Hz-and-400-Hz-Filter-Measurements-tp5300671p5308187.html
I just did a quick experiment.
First, I lied to my K3 that my 400 Hz filter is really a 2.8 kHz filter. I then turned AGC off and set the K3 WIDTH setting to 2.4 kHz.
This way, I will be able to see the response of both the 2.4 kHz DSP filter and the 400 Hz roofing filter, as Dr. Grebenkemper KI6WX had described.
Sure enough, with band noise as input to the K3, I see a similar spectrum as Jim showed here
http://audiosystemsgroup.com/K3FilterStudy-250HzRoof.pdfIn my case, it is a 400 Hz wide hump sitting on a wider 2.4 kHz noise pedestal.
I switched to antennas of various gains and directions, and as expected, the peak of the crystal filter hump rises and falls, while the wider DSP filtered noise floor remains constant (lots of spectral averaging of my FFT output :-). Remember that I have turned AGC off.
I then changed to using the Elecraft N-gen as the noise source. The noise became stronger now than using band noise -- the peak of the 400 Hz hump is now about 30 dB over the DSP noise pedestal. But the wideband noise pedestal (presumably the internal K3 noise, filtered by the DSP filter) remained at about the same magnitude.
When I changed the antenna input a dummy load, the 400 Hz hump disappears into the wider noise pedestal. But again, the 2.4 kHz noise pedestal did not change width nor amplitude. In my case, the DSP pedestal is a little over 20 dB higher than the noise floor outside of the DSP pedestal.
Not finding a louder noise source, I resorted to the CW signal from the Elecraft XG2, set to 50 µV output. As I tune across the carrier I can see it rise up to 70 dB above the DSP noise pedestal and then falling back to the DSP noise pedestal. The shape looked very reasonable for a crystal filter.
So I can definitely see at least 70 dB worth of decent 400 Hz filtering coming from the roofing filter.
The lower amplitude noise humps (from 0 to 30 dB over the DSP noise pedestal, depending upon the strength of the noise source), like the plots by Jim, are in my case, the result of the band noise (and N-gen noise) not being strong enough for me to see the full dynamic range of the roofing filter. The XG2 showed that the filter floor of the roofing filter is at least over 70 dB below the peak 50 µV signal.
So, I would like to suggest that Jim try using a stronger noise source than band noise. Even an N-gen (with K3 preamp on) was only giving me 30 dB over the DSP noise floor. Perhaps a receiving preamp would be useful (no, I don't have one to try, otherwise I would :-).
I think Jim will see what John was talking about, i.e., the plot with 25 dB to 30 dB hump over a wider pedestal is simply an artifact that the noise source to ping the roofing filter is only 25 dB to 30 dB louder than the internal noise of the K3 between the crystal filter and of the DSP filter. Perhaps, the variation of the hump over the DSP pedestal as you vary the input noise might be persuading enough without resorting to using extremely strong noise sources.
73
Chen, W7AY
P.S., now I need to go undo my filter settings, or I might wake up tomorrow thinking that my K3 had been bricked to only seeing a 400 Hz passband! :-)
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