BFO setting: bandpass versus frequency shift

[Steve Kallal]

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?

The real question I have is how to adjust the frequency shift accurately. I
have a idea to float. I have the XG1 signal source for 7040 kHz at S9. I can
use MixW to measure the beat audio frequency down to 0.1 Hz. I assume I can
set the BFO for each filter to the same audio frequency as OP1. I understand
I may not be able to get right on the same frequency due to the resolution
of DACs. I can correct the shift between tx and rx with MixW. But the
frequency shift might change from filter to filter.

Right now I've set the filter widths in RTTY mode for 2500, 1000, 500, & 250
Hz for starters. I set SSB for OP1, OP1, 2200, & 1800 Hz.

Do I have a handle on the issue, or am I missing something?

73,

Steve N6VL
K2 #2289


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[Don Wilhelm-4]

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?
>
>  

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