[Elecraft] 250 Hz and 400 Hz Filter Measurements

[Guy Olinger K2AV]

That measured 464 vs 333 is very close to what I have measured, and
why I set these two filters as 450 and 350 in the K3 utility setup.
In contesting lingo former is regular run width, the latter is tight
run width.   It is important to keep that guy up 350 Hz OUT of the IF
in a tight situation.  Opening up the pre-digital gain some to hear
someone at the noise level needs that down 70 skirt positioned
correctly to deal with the 70 dB wanted/unwanted differential and
keeping the unwanted from adding to IN-band crud before DSP can deal
with it.

Calling the 333 filter "250" is left over from Inrad supplying it for
a dual filter setup in the FT1000MP where the 250 was the COMBINED
result of TWO filters in successive IF's.

With the 50 Hz incremental DSP width, identifying a filter by a
combined bandpass number no longer makes sense.  If one sets the DSP
to 350 or 300 using the 333 filter, you will get a very decent 250-ish
overall passband.

With the flexibility of the K3, it is left to the operator to figure
out the result when combining 333 and 350.

I would like to have an 8 pole Inrad roofer that is designed
specifically for the optimum RTTY bandwidth presuming that the DSP was
set just a little wider for uncrowded and pulled in one step for
crowded.  What number do you call that?  I don't know.  I'd put it in
the narrow slot in the KRX3.

73, Guy.

On Sun, Jul 18, 2010 at 2:33 PM, Jim Brown <jim at audiosystemsgroup.com> wrote:
> On Sun, 18 Jul 2010 02:10:38 -0700, Kok Chen wrote:
>
>>So, I would like to suggest that Jim try using a stronger noise
> source than
>>band noise.
>
> You're exactly right, Kok. I wasn't pushing the K3 front end hard
> enough. I found a much stronger noise source (a nasty switching
> power supply that runs some low voltage lighting), fed it through
> a DXE preamp and into the K3. The K3 was set for max RF gain, but
> all the user gains were adjusted to minimize any obvious
> overloading of the signal chain. I could, for example, hit the
> audio chain harder and see significant harmonics and IM.
>
> The new data is at the same link as the old data
>
> http://audiosystemsgroup.com/K3FilterStudy-250HzRoof.pdf
>
> Executive Summary
>
> The plateau effect WAS an artifact of my not driving the K3 hard
> enough. My higher level noise source was enough to move that
> plateau down to about -60dB (referenced to the peak of filter
> response). Another point relative to the dynamic range of this
> measurement -- this is a somewhat impulsive noise signal -
> individual, un-averaged, measurements show peaks 6-10dB greater
> than the averaged data, so the K3 is, indeed, being rather
> robustly excited.
>
> IM would show up mostly as LF noise. The wide plots of response
> with 250Hz DSP IF show LF noise to be more than 66dB down. The
> small broad peaks at about 1.4kHz and 2.65kHz are also probably
> IM, but they're at least 78dB down.
>
> As to the use of broadband noise as a source -- the real world of
> contesting and DX chasing does not consist of a few big sine
> waves, rather, there are often several signals, plus noise, within
> a few kHz of bandwidth, and for many hams, that noise can often be
> nearly as strong as a strong signal. If you can figure out how to
> use it as a measurement tool, noise is a FAR better representation
> of the real world than even the world's best sine wave generators!
>
> The new data DOES show tha the Inrad filters, as integrated into
> the K3, are well behaved at their skirts.
>
> The curves showing the roofing filter response with a 1kHz wide IF
> clearly show that the 250 Hz filter is about 22% narrower than the
> 400 Hz filter in the range where my data can be trusted (above
> about -48dB).  That's 333 Hz vs. 464 Hz at -6dB, 501 Hz vs. 645 Hz
> at -30dB, 620 Hz vs 771 Hz at -48dB.  As a roofing filter, it is
> clearly a 22% improvement the 400 Hz filter. That does, however,
> fall far short of the 38% improvement suggested by the ratio of
> the nominal bandwidth of these filters, 400 Hz and 250 Hz. I think
> many of us still want a real 250 Hz filter!
>
> The curves showing the cascaded response of the two filters with
> the 250 Hz DSP IF shows very little narrowing of the response by
> the narrower filter. To see significant benefit from cascading,
> one would need to set the switching point of these two filters to
> wider bandwidths   perhaps 500 Hz and 350 Hz.  K2AV noted that
> many users have chosen this path, and it does make sense. If you
> have both filters, it might also make sense to set the 400 Hz
> filter to 400 Hz, allowing you to hear a bit more bandwidth when
> you're running, and set the 330 Hz filter to 350 Hz so that you
> can quickly narrow it down when the going gets rougher.
>
> Thanks to all those who have commented on my previous measurements
> and shown me the error of my ways. Two things I learned long ago:
>
> 1) You learn a lot when you stick your neck out and say what you
> think you know. When you're wrong, or when there are things you
> haven't learned yet, someone will tell you. If you don't have an
> ego problem, that's a good thing.
>
> 2) He who does nothing does nothing wrong.
>
> 73, Jim Brown K9YC
>
>
>
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