Joe,
I believe that has been taken care of for you in the K3 - but the low
limit could still be restricted in the firmware. With the DSP IF at 15
kHz, the answer to your question will depend on how the demodulation is
done in the DSP - but remember that DSP can produce very steep (and
deep) filter skirts, so the problem may not exist in a particular DSP
implementation. With DSP, there is no fixed 'signal path' like there is
in an analog receiver even though the processing may be represented that
way on a block diagram for simplicity of explanation..
The mathematical functions can all be merged if it is more convenient to
do it that way, and there is no real need to process one function first
and then do the next - that is the choice of the DSP programmer, and is
why one DSP receiver can sound different from another - it all depends
on the skills and creativity of the DSP programmer, and his decisions
about which techniques to use. All my statements here are, of course,
theoretical and not a reflection of that actually used in the K3 I know
Lyle and Wayne have done a good job, because I hear good things from the
Field Testers, and it is unimportant to me exactly how they have chosen
to implement a particular function.
As a parallel example, consider the process for calculating power when
one knows the resistance and the peak to peak RF voltage. It is quite
possible to work at the solution one step at a time - divide the Peak to
Peak voltage by 2, then multiply it by the square root of 2 to obtain
the RMS value, then square the result and divide by the resistance.
Each step takes time, but if one reduces the formulas necessary to do
the same computation, and plug the values into the resulting formula,
the peak to peak voltage can be directly squared and divided by 4 times
the resistance to obtain the same result in much less time than the
stepwise approach. The same principles work in any mathematical
computation, including those used for DSP - no magic, it is all in the math.
The problem of crossover to the other sideband is a situation that does
exist with component oriented filters, and needs an awareness of it when
aligning the IF filters in a K2 for a low pitch (which was the main
focus of my statements) - watch the response on the low frequency end of
the passband to avoid it.
73,
Don W3FPR
Joe Subich, W4TV wrote:
How does (if it does) this change when the second filter is a DSP
filter/demodulator working at IF instead of at audio? For example,
in the K3 with the 500 Hz (565 Hz SF=3.1) roofing filter set for
a 250 Hz pitch the IF will have significant bandwidth "below zero
beat." If the DSP is set for 200 Hz bandwidth, will it include
response at the "audio image" or does the K3 DSP "filter before
demodulate?"
73,
... Joe, W4TV
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