[Elecraft] [K3] Passband "flatness" for FSK441

[David Woolley (E.L)]

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
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