[ICOM] 756PROIII_ROOFING_FILTER

[Adam Farson]

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