http://elecraft.85.s1.nabble.com/250-Hz-and-400-Hz-Filter-Measurements-tp5300671p5310353.html
I chose the 250 and 500 filters for my narrow ones. I set the 250(370) at
400 and the 500(565) at 600. Works great for me. Just played in the RTTY
individual mark and spaces traces in the clear form each other?. I would
think it might be possible. Anybody done this?
>
> Jim,
>
> > 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.
>
> This is because your choice of a 250 Hz DSP bandwidth makes the DSP
> the dominant filter. The DSP filters have a skirt selectivity (slope)
> of about .6dB/Hz which can be seen in the upper skirt of the 400 Hz
> filter (nearly the entire 60dB range is within the -6dB passband of
> the 450 Hz wide filter).
>
> > The new data DOES show tha the Inrad filters, as integrated into the
> > K3, are well behaved at their skirts.
>
> No, your data - particularly the first two graphs show only the skirt
> selectivity of the DSP filtering. The third graph begins to show the
> skirt selectivity of the "250 Hz" (300+ Hz) crystal filter but the
> DSP bandwidth would need to be considerably wider - its "corners" must
> be wider than the expected -80 dB points of the filter being measured
> to avoid coloring the results.
>
> > 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.
>
> I don't know that "benefit" is necessarily the correct word to use
> ton describe cascading. The better term would be "effect." The K3
> still exhibits benefits from the distributed filter design - the
> roofing filter protects the 2nd IF and DSP from overload, AGC
> pumping and IMD from strong signals outside the passband of the
> DSP filter.
>
> However, the roofing filter is not intended to contribute significant
> selectivity within the DSP passbad. That's not the function of a
> roofing filter - just like the VHF filter in an upconversion based
> receiver is not intended to provide significant baseband selectivity.
> In an upconversion receiver the baseband (ultimate) selectivity is
> provided by the 2nd/3rd IF filters (FT-1000MP/MKV, etc) or DSP (IC-
> 756Pro/ProII/ProIII, FT-2000/5000/9000). In either case the sole
> design function of the "roofing" filter is to protect the IF chain
> from strong out of band signals.
>
> 73,
>
> ... Joe, W4TV
>
>
> On 7/18/2010 2:33 PM, Jim Brown 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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