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Topband: IF Filters for CW

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Subject: Topband: IF Filters for CW
From: (Ford Peterson)
Date: Wed, 5 Dec 2001 02:03:28 -0600
Once again I look to the Topband reflector for technical advice...Hello to

I am going to work the ARRL 160 contest this weekend (CW) and want to beef
up my IC-746.

The IC-746 comes from the factory with SSB filters installed for a so-called
"wide" setting.  These filters are installed at both 9 MHz and 455 kHz IFs.
Without optional filters installed, the "narrow" setting is identical to the
"wide" setting on CW.  This is a quad conversion receiver.

Several optional IF filters are available for the IC-746.  You can install 2
more filters for the 9 MHz IF and 1 more filter for the 455 kHz IF.  Here
are the available options:

9 MHz optional filters:
FL-101 cw narrow filter            250 Hz/-6dB
FL-232 RTTY/CW narrow filter 350 Hz/-6dB
FL-100 cw narrow filter            500 Hz/-6dB    (Installed)
FL-223 SSB narrow filter        1.9 kHz/-6dB
FL-103 SSB wide filter            2.8 kHz/-6dB

455 kHz optional filters:
FL-53A cw narrow filter            250 Hz/-6dB
FL-52A CW/RTTY narrow filter 500 Hz/-6dB
FL-222 SSB narrow filter            1.8 kHz/-6dB
FL-96 SSB wide filter                    2.8 kHz/-6dB
FL-257 SSB wide filter                3.3 kHz/-6dB

The menu selector on the IC-746 will only allow a "wide" and "narrow"
setting for each mode (RTTY, CW, SSB, etc).  The narrow setting can be menu
selected to include one or both of the "optional" filters installed when the
"narrow" button is pressed--one extra filter at each IF.

I have a 500 Hz filter at the 9 MHz position already intalled and use it all
the time on RTTY/CW.

Long ago I read some article by Urlich Rhode, and I am left with the
impression that 2 - 500 Hz filters in the IF chain leaves a filter somewhat
less than 500 Hz.  I can't find the reference, but my recallection is about
350 Hz to 400 Hz.  I operate all modes (CW, SSB, RTTY, PSK, etc) and want to
beef up the IFs for this weekend but don't want a filter that is going to
make RTTY difficult.

I am seriously considering the FL-53A 250 Hz filter at 455 kHz.  When in
"narrow" mode on CW, the 9 MHz will have a 500 Hz and the 455 kHz will have
the 250 Hz.  Is this going to be too narrow (ringing, missing non-zero beat
QSOs, etc.)?  What is the effective IF bandwidth when you have a 500 Hz and
a 250 Hz filter in the IF chain (or 500 Hz and 500 Hz for that matter)?  Any
IC-746 owners out there with two "narrow" filters installed--what's your

Respond direct and I will summarize on a future post.


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