[ICOM] CW Filters on RTTY

[K4RO Kirk Pickering]

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