Bill W5WVO wrote:
In WSJT's FSK441 screen, there is a useful little window that plots
* the amplitude of the noise floor and the decoded signal burst, if any,
* against audio frequency. Since FSK441 uses four tones ranging in
* frequency from 882 Hz to 2205 Hz, the net passband (after all stages)
* should ideally be flat from about 850 to 2250 Hz, given good signal
It needs to be substantially flat across each sub-carrier, but could
slope across the band. That means that a sharp cutoff at the edge may
be worse than having significant passband ripple.
The reason for having multiple sub-carriers is to better tolerate
frequency selective fading, and group delay variations, so the
modulation method is already designed for total channels which do not
have a flat frequency response.
Incidentally, if the tones are spaced by 441Hz and there is no wasted
spectrum, the total passband should be more like 661.5 to 2424.5.
* frequency accuracy. However, the WSJT FSK441 software is designed to
* find and decode signals as much as +/- 400 Hz from the nominal receive
* frequency, which extends the requisite passband from around 450 to
2650 Hz.
The sub-carrier frequencies contain an arbitrary offset that has been
chosen to put the group in the middle of the passband of typical SSB
filters. That's the only reason that the particular frequencies are
chosen. You could equally consider them to be --661.5, -220.5, 220.5,
and 661.5 Hz.
The reason that the decoder can cope with up to 400 Hz off frequency is
probably because there are receiver filters around that will allow that,
but I don't see that that is a reason for always making the receive
filter that wide. The system has been clearly designed to work with
filters that are well behaved over a much narrower frequency range.
I suspect the 400Hz slop is based on a 2.7 kHz filter, allowing some
slop for carrier offset errors and on the basis that the upper and lower
channels will be degraded by the filter roll off.
Is the K3's passband flat over this range of audio frequencies using
* the 2.7/2.8 kHz filter, or if not, can it be made effectively flat by
* the use of custom equalization settings? I should think the answer would
* be yes, but I'd like to hear from somebody who has perhaps done it and
* seen the resultant flat noise profile in the FSK441 display.
You also need constant group delay across each sub-carrier, and unless
the decoder de-skews the individual sub-carrier streams, across the
whole group. If the equaliser users infinite impulse response filters,
it may actually exacerbate group delay problems (although it might also
cancel some of the group delay variation from the IF filters).
If the decoder is good, it will have an adaptive filter that will
automatically equalise the signal for good digital decoding.
[ Single line paragraphs have been arbitrarily wrapped.]
--
David Woolley
"The Elecraft list is a forum for the discussion of topics related to
Elecraft products and more general topics related ham radio"
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