[ICOM] CW Filter "Blowby" revisted

[Philip Atchley]

Hi Don,

Yes, but I think that they were just using that as an example as they 
knew off the top of their head the actual specs of the 8MC IF in the 
FT-1000.  I would expect that the 9MC IF used in Icom Radios would be 
similar, or perhaps have even more "blowby" due to the higher frequency 
involved.  I've read elsewhere that blowby around filters is largely due 
to the residual capacitance of the filter switching diodes when biased 
off (filter not selected).

Perhaps in "normal" use this may not be noticed by 'most' operators, 
However, when chasing LF beacons I typically preface the radio with a 
30-40dB tunable preamp.  It's not at all unusual for me to be trying to 
hear a beacon near the noise floor that is anywhere from 100 Hz to 5KC 
from a S9+50dB carrier! This places very tough demands on a receiver and 
some fare better than others.  The R-75 WITH cascaded filters is a 
favorite of LF beaconeers.

Example:  If the first IF filter, let's say 250Hz wide, has only 60dB of 
isolation, and a carrier 2KC or so away is S9+50dB, that means that 
AFTER the filter that carrier is STILL in the area of S7-S8!

Anyway, it appears that as soon as I'm able I'll be getting a 455KC 
narrow CW filter 8^)

As an aside, my homebrewed receiver with it's 11Hz bandwidth (3dB point) 
had EXCELLENT nearby rejection.  But it was a "one trick pony" suitable 
ONLY for CW.

73 de Phil,  KO6BB


> Phil,
> Am I reading that wrong, or did you ask about an Icom 751 and got an 
> answer about an FT-1000MP???????
> 73,
> Don, WB5HAK