[Elecraft] Elecraft K2/100 in RTTY and PSK31 Modes

[Julian, G4ILO]

Vic K2VCO write:

> I have been able to completely eliminate this shift.  What I do is first 
> do CAL FIL for the appropriate bandwidths, for example 1.6, 0.8, 0.4, 
> and 0.2 KHz, using Spectrogram.  Then I tune in a steady signal (the 7 
> MHz birdie) and go through the four selectivity positions, noting the 
> change (if any) in the location of the pip on the Spectrogram display. 
> Suppose I find that the frequency in the 0.8 and 0.4 positions is 10 Hz 
> higher than the others.  In that case, I change the bandwidth slightly 
> (let's say to 0.78 and 0.42), and move the BFO frequency one 'click' to 
> compensate.  When the new filter parameters are saved, more often than 
> not the VCO offset is calculated so that the pip comes out on target. 
> You may have to try several combinations of bandwidth and BFO position, 
> but it has always been possible for me to have it come out right.

This is interesting. A while ago someone else claimed to have completely 
eliminated the shift, and I recall that it drew one sceptical response, 
with which I was inclined to agree.

My experience is that while it might be possible to eliminate the shift 
when lining up on one frequency, such as the birdie, when you tune to an 
actual signal, the problem is back again. The reason, as I tried to figure 
it out, is that in order to use a narrow filter at a particular center 
frequency, the BFO has to be shifted a certain amount, and then the VFO 
shifted the opposite way by the same amount to compensate and keep the 
Tx/Rx frequency correct. Because of the limited BFO and VFO resolution, the 
BFO and VFO will never both move by exactly the same amount every time, 
depending on what frequency the VFO is tuned to. I'm guessing that the VFO 
is the actual problem here, since the BFO frequency is the same in all 
cases and should therefore always move by the amount originally set up.

I'm not sure if my explanation is correct, but the effect is quite 
noticeable in actual use, where on one station in the 14.070-072 region, 
switching in the narrow filter may produce hardly any shift at all, whereas 
on another station on a slightly different frequency, the shift may 
approach 20Hz. Consequently I've given up trying to eliminate the shift, 
and learned to live with it.

73,
--
Julian, G4ILO. (RSGB, ARRL, G-QRP)
G4ILO's Shack: http://www.qsl.net/g4ilo





  ==== Original message ====

> Julian, G4ILO wrote:
> 
> > The main issue is the limited VFO and BFO frequency resolution, which 
> leads 
> > to a slight audio shift when you change selectivity. 
> 
> I have been able to completely eliminate this shift.  What I do is first 
> do CAL FIL for the appropriate bandwidths, for example 1.6, 0.8, 0.4, 
> and 0.2 KHz, using Spectrogram.  Then I tune in a steady signal (the 7 
> MHz birdie) and go through the four selectivity positions, noting the 
> change (if any) in the location of the pip on the Spectrogram display. 
> Suppose I find that the frequency in the 0.8 and 0.4 positions is 10 Hz 
> higher than the others.  In that case, I change the bandwidth slightly 
> (let's say to 0.78 and 0.42), and move the BFO frequency one 'click' to 
> compensate.  When the new filter parameters are saved, more often than 
> not the VCO offset is calculated so that the pip comes out on target. 
> You may have to try several combinations of bandwidth and BFO position, 
> but it has always been possible for me to have it come out right.
> 
> Before bothering to do this, however, it is imperative to do the BFO 
> drift mod (included in K2's from serial no. 3000).  Otherwise, the 
> alignment will drift.
> 
> > The other apparent issue with the K2/100 is drift, due to the higher 
> > temperatures if the PA is mounted internally. There's a mod to 
> compensate 
> > for this, but I don't know if it stops it sufficiently to be 
> satisfactory 
> > for data use. 
> 
> I don't either, being a CW operator, but I will say that it provides a 
> BIG reduction in the amount of drift.  This mod is applicable to K2's 
> earlier than #3446.  I recommend this mod for all K2/100's, regardless 
> of your favorite mode.  I've also added an auxilliary fan -- a 3" (about 
> 7.6 cm) square 12v DC fan which I stuck on the right side of the K2/100 
>   heat sink with double-sided tape just for the heck of it.
> 
> Vic K2VCO