[ICOM] CW Filter "Blowby" revisted

[Philip Atchley]

Hi,

I've briefly mentioned the "CW Filter Blowby" my IC-751 exhibits.  I
directed the question to Inrad as they are third party sellers of
filters and are pretty knowledgeable on the subject.

Here is the question I asked them and their reply.

73 de Phil,  KO6BB

****************************************
Phil,

One of the major reasons for cascading filters is to eliminate blowby.
For
example:  In the 1000MP the isolation provided by the 8 MHz IF is only
65 dB
by itself.  The 455 kHz IF has 95 dB, so when they are cascaded no
blowby
can be heard.

The best thing to do is to add a 455 kHz filter.

George, W2VJN

> Hi,
>
> I have a recently acquired Icom IC-751 (not A) transceiver which has
> the
> optional FL-63 9MHz 250 Hz CW filter installed, and I'm considering
> upgrading and/or adding a 455KC Narrow CW filter.
>
> But first a couple questions.
>
> When I select the Narrow CW filter there is an excessive amount of
> filter
> "blowby".  In the CW mode, as I tune away from a "strong" carrier I
> can hear
> that carrier all the way out to nearly 5KC away. In a "waterfall"
> display
> on the computer it's plainly visible. I've checked the filter
> installation
> and it appears to be good (and the jumper is in the correct spot).  If
> I
> switch in the "wide" SSB (cascaded 9MC and 455KC FL-44) filters and
> use
> the PBT to narrow the bandwidth I DON'T get the blowby.
>
> Questions.
> 1. Is this a common problem with the Icom IC-751/FL-63 filter combo.
> 2. Do I have a bad filter?
> 3. I'm pretty sure that installing a Narrow CW filter in the 455KC
> slot
> would eliminate this, but would just replacing the FL-63 with an Inrad
> equivalent eliminate it also?
>
> 73 de Phil,  KO6BB

73 de Phil,  KO6BB
991 Different NDB's heard to date.

http://www.geocities.com/ko6bb/
Merced, Central California, 37.3N  120.48W  CM97sh