[Elecraft] K3: KRX3 question

[Ed Muns]

> If you worked RTTY you would find out that the "250Hz" filter 
> is ideal for recovering weak ones and for 20M during RTTY 
> contests.  The dual 
> passband filter fits very nicely within it.   I don't care what 
> bandwidth you call it.  Don't eliminate it as a choice.
> 
> I was astounded the difference it made during yesterdays DL 
> RTTY contest.  Fine tuning + the selectivitiy made the 20 KHz 
> RTTY band seem like 100KHz wide while doing search and 
> pounce.  A/B comparisons between it and the 400Hz filter did 
> indeed show significant benefit.  
> Spectrogram spectra, with a band loaded with signals, 
> strengthen this A/B comparions conclusion.
> Some offending signals simply fell off the edge of the 250 Hz 
> filter slope.  Part of this benefit is undoubtedly due to the 
> capture effect of FM like modes.  Strongest wins.

What DSP bandwidths were you using and where did you have your crystal
filters engaging?  There is only a 65 Hz bandwidth difference between the
"400" and "250" crystal filters and both are wider than the what is needed
for 170 Hz shift RTTY.  The crystal filter function is to protect the DSP
from strong nearby signals and a 65 Hz delta is irrelevant at 400 Hz.  So
there will NOT be any discernible benefit on RTTY or any other mode if you
do the A/B comparison with the same DSP bandwidth (with or without the DTF).
For example, if your 400 Hz filter engages at DSP bandwidth of 400 Hz and
your 250 Hz filter engages at 250 Hz, then you are really A/B comparing DSP
bandwidths of 400 and 250 Hz, not the crystal filter differences.

What can make a difference in large RTTY pileups is running the DSP down as
low as 200 Hz which rolls off the outer edges of the tones, but eliminates
enough of the pileup to sometimes be a net advantage.  In general, running
the DSP at 300 Hz with the DTF engaged is ideal for 170 Hz shift RTTY
because there is little roll off of the desired passband.  Of course, the
250 Hz DSP bandwidth is a step between these two settings and also a viable
alternative in some situations.  The "250 Hz" crystal filter at an actual -6
dB bandwidth of 370 Hz is a good roofer for these three DSP bandwidths, but
the "400 Hz" crystal filter at an actual BW of 435 Hz is essentially the
same as far as protecting the DSP from nearby strong signals.

73,
Ed - W0YK