[Elecraft] RTTY Filter and low end of BFO range

[W3FPR - Don Wilhelm]

Leigh,

Most if not all K2s will have a problem tuning for a 2200 Hz tone on both
sidebands, but 1000 Hz should be possible with some work on the BFO.

What you are asking to do is to have a BFO range of more than (IF center
frequency +/- 1000 Hz + filter width) - the center frequency is the real
problem - it will depend on which matched crystal set you have installed in
your K2.

You can certainly try Bill's capacitor addition, but be aware that the BFO
could become unstable if the crystals are pulled too far - if you end up
with 'muddy sounding' signals, it is not working for you.

You could also change or pad C173 and C174 (observing the relative size
ratio) to shift your entire BFO range since you have excess range on one
side and accomplish the same thing - note that your BFO high and low
frequencies could be shifted a bit from those indicated in the manual if
your IF center frequency is off to one side of the average.

73,
Don W3FPR


> -----Original Message-----
>
> I have enough BFO range as described in the manual and am able to reach
> 4912.97 on the low end.  Unfortunately, while that is low enough for me
> to center the RTTY filters on 1000Hz for one of the sidebands, it is not
> for the other.  On the "bum" sideband, the narrowest RTTY filters are
> centered around 7700-750Hz.  Both CW and CWR filters at 600Hz are ok.
> ...
>
> On the other sideband, I have no trouble, and so I am fine on the lower
> bands, as I can use R, but on the higher bands, I have to switch
> sidebands for digital modes.  That's not a a problem for some modes such
> as PSK but it is a problem for MFSK and its variants.
>
> Are other K2s able to center RTTY filters on 1000Hz on both sidebands?
> Would this small cap Bill suggests help mine without affecting
> stability?  It looks like I need a 250Hz-300Hz move down on the low
> end.  I don't know if that is reasonable and an effect of the wider
> KSB2, or if I have some other, undiagnosed problem.
>
> 73 es TNX,
> Leigh / WA5ZNU
>
> On Sat, 7 May 2005 7:25 pm, Bill Coleman wrote:
> > I recently had to completely reconfigure my filters, as I added the
> > RTTY mode. In order to accommodate my chosen center frequency of 1500
> > Hz for AFSK RTTY, I had to modify the BFO to get sufficient range. I
> > had already added 47 and 100 pf caps across C174 and C173, respectively
> > when I made the A->B mods and the BFO stability mod. I changed these to
> > 56 and 120 pf, but this did not move the BFO frequencies much at all.
> > Then I added a 3 pf cap from X3/X4 junction to ground. This moved the
> > lower frequency about 800 Hz without affecting the upper frequency.
>
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