[Elecraft] BFO setting: bandpass versus frequency shift

[Don Wilhelm]

Steve,

You are seeing the correct image with Spectrogram.  When you change the 
BFO, the filter passband shifts within the audio spectrum.  The filter 
passband is not changed - and you will find that fact important when 
setting the BFO frequencies.  If you have a 700 Hz passband that you 
center at a pitch of 600 Hz, you will receive signals 350 Hz below and 
350 Hz above - resulting in a passband in the audio range from 250 Hz to 
1050 Hz.

You will also find that with filter widths wider than 2 times the 
sidetone pitch, you cannot literally center the passband without losing 
the single signal reception quality.  For wider CW filters, set the 
lower slope of the passband at about 200 Hz.  That is similar to setting 
the SSB filters, where the lower frequency corner of the passband should 
be set near 300 Hz.

The K2 takes care of preserving the pitch as you change from one filter 
to another, although a shift of about 20 Hz may be heard when changing 
filters due to the limits of the DAC resolution.

If you are using digital modes, I recommend that you set the RTTY filter 
set on (in the secondary menu) - that gives you 4 independent filters 
for data modes and allows you independent control of the compression 
level between SSB use and RTTY use.  My recommendation for the RTTY 
filter setting is to set FL1 the same as the SSB FL1, and use filter 
widths of 1.00, 0.70 and  0.40 for FL2, FL3 and FL4.  The narrow filters 
can be set at a pitch of 1000 Hz if your BFO range is adequate, but you 
can use 800 Hz just as well if the BFO range is lacking.  In operation, 
you would tune your desired signal with the VFO until it appears near 
1000 Hz, and if there are strong signals other than the one you wish to 
copy in the passband, you can narrow the filter to keep those srong 
adjacent signals from taking over the AGC in the K2.  Try it, you may 
like it.

When positioning the filter passbands, it is easiest if you do not have 
any single frequency signals being received.  A wideband noise generator 
like the N-Gen is ideal, but lacking that you will likely find enough 
noise energy on a dead spot in some band to give a nice display of the 
passband - turning the preamp on helps in obtaining a good passband display.

73,
Don W3FPR

Steve Kallal wrote:
> I am starting to notice that the adjusting the BFO for a given bandpass
> response also shifts the frequency. Of course frequency shift is what a BFO
> setting is all about. I guess the K2 doesn't have an IF shift like most
> rigs. If I set the BFO for each filter width, to give the same frequency,
> what will happen to the bandpass response.
>
> I have these questions because of my new KSB2 option. But I also noticed the
> problem with the CW filters in the CW mode. I can live with a little bit of
> shift on CW and SSB. But I can't really live with it on the digital modes. I
> understand from reading the KSB2 docs, that tx is always from the OP1
> filter, including the RTTY mode. So it is essential that I get BFO on the
> other filters to match OP1 as close as possible. That probably means the
> crystal filter characteristics will determine the center frequency of the
> bandpass. Is this correct?
>
>