[ICOM] 756PROIII_ROOFING_FILTER

[Jan C. Robbins]

I quite agree, Igor.  Your observations are also entirely consistent 
with independent testing done by a number of specialists, including Rob 
Sherwood (the best in the business; see his extensive--and 
convincing--data at www.sherweng.com).  Tks es 73!  Jan N0JR

Igor Sokolov wrote:

> 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
>>
>>
>>
>>----
>>Your Moderator: Dick Flanagan K7VC, icom-owner at mailman.qth.net
>>Icom Users Net: Sundays, 1700Z, 14.316 MHz
>>Icom FAQ: http://www.qsl.net/icom/
> 
> 
> ----
> Your Moderator: Dick Flanagan K7VC, icom-owner at mailman.qth.net
> Icom Users Net: Sundays, 1700Z, 14.316 MHz
> Icom FAQ: http://www.qsl.net/icom/
> 
> 

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