[ICOM] 756PROIII_ROOFING_FILTER

[Igor Sokolov]

Hi George,
With great respect to Adam's opinion I have different experience
here.
I have operated Orion and 7800 in some major contests and IMHO use of
narrow roofing filters is pretty well justified. You get much less
IMD with narrow roofing filter in Orion and therefore can pull out
weak signals which you do not hear on the radio with wide roofing
filter. The reason why 500 and 250 Hz roofing filters in Orion give
worse figures then the 1 Khz filter is explained by the fact that
some extra gain is introduced to compensate for losses in  the narrow
filters (500&250) which in turn spoils superior IMD performance that
Orion sports with 1 kHz filter.

73, Igor



> Hi George,
>
> This article should address your question.
>
> http://www.qsl.net/ab4oj/icom/ic756pro_notes.html#roofing
>
> The 15 kHz bandwidth was chosen to pass all emission types in use,
including
> FM (occupied bandwidth 16 kHz at -26 dBc). A narrower roofing
filter will
> reduce the statistical likelihood that a strong signal outside the
passband
> of the DSP IF filters will pass down the analogue IF chain and
overload the
> analogue/digital converter (ADC) ahead of the DSP.
>
> The IC-7800 also has selectable 15 and 6 kHz roofing filters. Icom
probably
> made a cost-based decision in the case of the IC-756Pro3.
>
> I have operated an IC-7800 on 40m, 17m and 20m SSB, and did not
notice much
> difference between the 15 and 6 kHz filters in terms of ability to
copy weak
> SSB signals in the presence of strong signals.
>
> Cheers for now, 73,
> Adam VA7OJ/AB4OJ
>
>
> -----Original Message-----
> From: icom-bounces at mailman.qth.net
[mailto:icom-bounces at mailman.qth.net] On
> Behalf Of George
> Sent: 06 November 2004 05:27
> To: Icom Mailman
> Subject: [ICOM] 756PROIII_ROOFING_FILTER
>
> Can anyone tell me of the advantage of a narrow roofing filter in a
> receiver?
> I note that the ProIII is still sticking with a 15kc roofing filter
and the
> Ten-Tec Orion has filters that are narrower and selectable at that.
> I read the ARRL product review on the Orion and it did note that
there was
> an advantage to narrow roofing filters to a degree. Their tests
indicated
> that too narrow filters such as, 500 cycles isn't necessarily
better.
> Since Icom is sticking with a fixed 15kc roofing filter is it their
opinion
> that a narrower filter isn't needed for adjacent strong signal
operation.?
> Also the very top end Yeasu 9000 also has selectable narrow roofing
filters.
> Is there any merit to the narrow roofing filter question?
> If it were not needed why does the Orion offer selectable filters?
> Thanks.
> George
>
>
>
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