W3WPR:
> While we are on the subject, I might say something about very low
> pitches (since it has been mentioned) - there is a limit to how
> low you can go and still retain the single signal reception because
> a significant amount of the filter passband crosses over from zero
> Hz pitch to the other sideband. In the extreme, you end up with the
> response like a Direct Conversion receiver where the same pitch can
> be heard (at equal amplitudes) on both sides of zero. One must
> consider the IF filter skirt slope when deciding how low the filter
> can be set while still retaining single signal reception.
W4TV:
>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?"
It the K3 works like Orion (as I expect), the bandwidth
"below zero beat" is truncated by the DSP, and there is no response
on the other side (unlike in analog rigs). This is practically why
a wide filter such as a 1.8 kHz actually has lower bandwidth. For a
500 Hz pitch, the actual bandwidth becomes 1400 Hz...900 Hz above the
filter center (at 500) and 500 Hz below it. The other 400 Hz below
zero beat is truncated by the DSP. This is one of the advantages
of doing things in the DSP...you can do tricks that are impossible
in an analog rig.
I've been using my TS-930S while waiting for my K3 and
had completely forgotten about the below zero beat response
until I started using it again. Practically this is not
really a problem in analog rigs if you use a typical 500 Hz
bandwidth, even with very low pitch (e.g. Pitch or Fc= 250 Hz,
upper cutoff = 250+250 = 500 Hz and lower cutoff = 250-250 =
0 Hz). The below zero beat response can be quite noticeable
if you use an SSB BW filter since its wider passband extends
the response below zero beat.
73, Bill W4ZV
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