[Elecraft] 250 Hz and 400 Hz Filter Measurements

[Joe Subich, W4TV]

 > Sure enough, with band noise as input to the K3, I see a similar
 > spectrum as Jim showed here
 >
 > http://audiosystemsgroup.com/K3FilterStudy-250HzRoof.pdf

Gentlemen, this is the typical noise response of a distributed
filter system.  The noise peak is front end noise through the
roofing filter and the broader "pedestal" is the noise generated
within the receiver following the roofing filter.

This greater the gain in the 2nd IF (or 3rd IF for upconversion
receivers like the FT-1000MP) the more pronounced the pedestal
and the less the "hump" produced by the roofing (or 2nd IF)
filter.  Jim/Chen, you can see this very effect in your Yaesu
rigs by forcing it to use the widest available 2nd IF filter
with a narrow 3rd IF filter (easy to do on the MP) and using
band noise to excite the radio.

This "hump" effect is one of the reasons for such approaches
as minimum gain IF stages following the last IF filter, post
IF filtering (e.g., a filter just before the product detector)
and active audio filtering following the product detector.
In all cases, these design approaches were intended to remove
the broadband noise generated in the last IF stage.

When operated as designed with the DSP providing the last
frequency selectivity, the K3 actually provides matched
filtering as the last stage in the IF and the first stage
of AF and shows no "pedestal."  The FT-1000 family, on the
other hand, still shows a noise pedestal with its cascaded
filters due to the excess gain in the 3rd IF unless one
follows W8JI's recommendations and reduces the IF gain or
enables DSP demodulation/audio DSP.

73,

    ... Joe, W4TV

On 7/18/2010 5:10 AM, Kok Chen wrote:
> 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.pdf
>
> In 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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