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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