[RTTY] K3

[J. Edward (Ed) Muns]

> Like Joe, I feel sure that the K3 generates its "direct" FSK entirely in
> DSP. However, we don't have any details about the implementation of
> frequency slewing, or about the levels of keying sidebands compared with
> the AFSK alternative.

Apparently you missed my post yesterday ...

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> I suspect Lyle is generating both PSK31 and FSK in the DSP and not 
> creating external audio.

Yes.  In the 15kHz IF.  One frequency that is shifted for MARK and SPACE.

Ed - P49X (W0YK)

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There is one frequency that is generated in the 15 kHz IF and MARK/SPACE are
created with "phase-continuous frequency shifting".



> Jumping back to the discussion about narrow roofing filters for RTTY,
> W4TV pointed out that Elecraft don't presently offer a filter with a
> true bandwidth of about 250Hz. The so-called "250Hz" 8-pole filter has a
> -6dB bandwidth of almost 350Hz, which is almost the same as the 400Hz
> filter, while the 200Hz 5-pole filter is too narrow for practical use.
> 
> Joe mentioned a simple modification to increase the bandwidth of the
> 200Hz filter to about 260Hz. Here is a summary.
> 
> The schematic of the 5-pole K3 filters is on page 9 of:
> http://www.elecraft.com/manual/K3_Schematics_Oct2008.pdf
> 
> The modification was suggested by Wayne, N6KR, and involves changing
> both C4 and C5 from 1000pF to about 800pF. The modification involves
> removing and replacing two 0604 SMD chips.
> 
> Lower capacitance will give a wider bandwidth. I used 830pF (680 +
> 150pF, 5% NP0) which gave a bandwidth of about 260Hz. The center
> frequency remains almost exactly the same. The measured results are
> shown at:
> http://www.ifwtech.co.uk/g3sek/misc/kfl3-200_mod260.gif
> 
> The increase in bandwidth to 260Hz makes the filter much more practical
> for RTTY under heavy QRM conditions, without losing its effectiveness as
> a roofing filter for CW.

Unless you tell the K3 that this 260 Hz crystal filter is, say, 500 Hz so
that it switches in with the DSP bandwidth much wider, the cascade effect of
the crystal filter and a narrow DSP bandwidth (250 or 300 Hz) will be
narrower than 200 Hz.  Furthermore, there is no need for a crystal filter
that narrow.  The DSP can select 200, 250 or 300 Hz bandwidths and the 370
Hz bandwidth of the so-called 250 Hz bandwidth 8-pole filter is plenty
narrow enough to protect the DSP at these narrow DSP bandwidths.  In fact,
there will be no operational difference by using any of the 250, 400 or 500
crystal filters for RTTY.  A 260 Hz crystal filter doesn't make any sense
for RTTY.

Ed - P49X (W0YK)