[ICOM] CW Filters on RTTY

[Vladimir V. Sidorov]

Gents,
I've been using an IC-756 with 2 x 250 Hz filters for a few years and it was
really nice. Virtually every signal was copyable, except the 200 Hz shift
signals. For cases like this I had to turn one of the filters off, then it
was no problem to copy.

Indeed, it does not work just like this, when you simply install two
filters. A precise adjustment is certainly needed.
What I did myself.
First, I tuned to a load station with a wide filter for a clear reception.
Then, staying on tuned on the same station the first (9 MHz) filter was
switched on. Normally the station simply disappears, and I had to adjust the
respective PBT to regain the reception. When done, the 2nd filter goes on.
Same procedure.
I hate "clock arrow" games, but OK. The typical PBT adjustments were approx.
9 o'clock, 1st filter, and 1 o'clock, the 2nd one.

The result was wonderful. I might slightly move from a load station in a
contest and easily copy a weak DX.

Well, right now my life is not so easy anymore... The 756 has gone, am
running the 775, and it's not that easy anymore to fool it around and call
the CW filter SSB one...

So, for me it worked well. Your mileage may vary.

73,
Vladimir EU1SA


---



> I just recently modified my IC-765 receivers to allow use
> of the 250Hz CW filters in the RTTY position (switchable.)
> Before the mod, I could only use the SSB filters, or the
> 500Hz CW filters in RTTY mode.  I could choose just the
> 9MHz 500Hz filter, or both the 9MHz and 455kHz 500Hz filters
> with the "IF SHIFT" switch.  Now I have choices of 500 or 250
> Hz bandwidths, with single or cascaded filtering for each.
> These are stock Icom filters.
>
> I have found that when using cascaded 250 Hz filters, the
> passband is not really wide enough to easily tune in a RTTY
> signal.  The audio spectrum scope confirms this.  While it is
> possible to RX RTTY with cascaded 250's, it requires a very careful
> adjustment of the PBT to center both RTTY tones in the passband.
> Sometimes I can't even tune in a signal, regardless of the PBT
> position.  I would not recommend cascaded 250's as a default
> RTTY bandwidth, unless you have control over the skirts.
>
> Using a single 250Hz filter seems to widen the skirts enough
> to allow fairly easy tuning with sufficient rejection of
> close-by signals.  The only problem is there is more blow-by
> than using the cascaded 250's together.  My preference after
> a few weeks of experimenting is to use cascaded 500 Hz filters
> with the PBT tweaked around the RTTY tones of 2125 and 2295 Hz.
> For tough QRM, the single 250 Hz filter usually does the job.
> If QRM is really severe, I will try the cascaded 250's, and
> carefully tweak the PBT to center the tones.
>
> 73
>
> -Kirk  K4RO
>
>
> On Mon, Oct 11, 2004 at 07:37:05AM -0700, John Geiger wrote:
> > Good morning to list members (or good whatever,
> > depending on what time you receive this message).  I
> > have a questions about narrow filters and AFSK.  I do
> > RTTY using MMTTY in the LSB position, which generates
> > AFSK instead of true FSK.  One nice thing about Icom
> > radios is that you can use the CW filters on SSB once
> > you figure out how to confuse the software.
> >
> > Now here is my question, will a 250hz CW filter work
> > on AFSK, or is that too narrow.  In the 706 manual it
> > says that the 250hz CW filter can be placed in line
> > for RTTY narrow, but in the Icom 740 manual it says
> > the 270hz CW filer is too narrow to use on RTTY.
> >
> > So, has anyone used the 250hz CW filter on RTTY yet?
> >
> > 73s JOhn NE0P
> ----
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