K2 filter center frequency shift with band?

[Chris Kantarjiev K6DBG]

I'm playing in the RTTY Roundup this weekend, using a simple AFSK setup
with my K2. For some reason, when operating on 15m I need to shift
the Mark frequency to get the signals to fall inside the filter.

This doesn't match up with how I understand the K2's filters to work;
I expect the center frequency of the filter to be the same regardless
of the band.

What am I misunderstanding?

73 de chris K6DBG


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[Julian, G4ILO]

Chris Kantarjiev K6DBG wrote

I'm playing in the RTTY Roundup this weekend, using a simple AFSK setup
with my K2. For some reason, when operating on 15m I need to shift
the Mark frequency to get the signals to fall inside the filter.

This doesn't match up with how I understand the K2's filters to work;
I expect the center frequency of the filter to be the same regardless
of the band.

What am I misunderstanding?

Nobody has answered this, so I'll try, though most of what I knew about the K2 has been overwritten by K3 stuff. The K2 effectively swaps sidebands above 17m so USB on 21MHz and above uses the filter settings for LSB on 17m and below. I think your USB and LSB filters don't have symmetrical BFO offsets. Rather than try to explain, I suggest you look at the relevant text in the manual which is on page 95 in my copy: "Setting up Filters for SSB/RTTY."

[Don Wilhelm-4]

Chris,

I see that Julian has given you some information already - the sideband
reversal thing is correct..
I must ask just how much you need to move the mark frequency.  If it is
only a few tens of Hz, then it is simply a matter of adjusting the BFO
for LSB.  If it is in the range of kHz, then it is due to operating with
the wrong sideband.  You must adjust the mark frequency in the
application to match the filter center frequency in the K2 to use narrow
filters.

I highly recommend that you activate the RTTY filter set because it
allows the compression to be set independently of the setting used for
SSB.  I usually set the RTTY FL1 filter the same as that used for SSB
FL1, but also set FL2 thru FL4 to 1.0, 0.7, and 0.4 kHz with the filter
center at 1000 Hz.  Greater audio frequencies may exceed the K2 BFO
range capability, and 1000 Hz is a compromise that is easy to remember.  
Tune the desired signal to 1000 Hz (with the VFO) on the waterfall
display and you are ready to switch in a narrow filter when that
rock-crusher comes on close to your desired signal.  The same applies to
all other digital modes, not just to RTTY.

73,
Don W3FPR

Chris Kantarjiev wrote:

> I'm playing in the RTTY Roundup this weekend, using a simple AFSK setup
> with my K2. For some reason, when operating on 15m I need to shift
> the Mark frequency to get the signals to fall inside the filter.
>
> This doesn't match up with how I understand the K2's filters to work;
> I expect the center frequency of the filter to be the same regardless
> of the band.
>
> What am I misunderstanding?
>
> 73 de chris K6DBG
>  
>

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[Chris Kantarjiev K6DBG]

Julian, Don:

You hit it right on the head - I'd forgotten about the sideband
reversal issue. Hitting CW RV puts the audio signal right where
I expect it when using 15m.

I was shifting the MARK frequency by about 900 Hz (using the
values built into MMTTY, from 1175 to 2000) which was a little
bit too far.

If I do a lot of RTTY (unlikely), I may revisit how the filters
are set up and try to get them symmetric; I don't recall being
very careful in the setup with I first tried to get RTTY going.

Thanks!

73 de chris K6DBG
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