On Thu, 2007-03-01 at 14:19 +0100, Rick, NJ0IP / DJ0IP wrote:
> Yesterday afternoon the OMNI VII arrived.
>
<SNIP>
>
> There has been a lot of speculation on the reflector as to whether the term
> "Distributed Roofing Filters" is real value-add or just marketing hype.
> Coupled with careful management of amplification (signal strength) from the
> antenna input through to the second mixer, it becomes very effective, but
> one needs to understand this. It's a slightly different approach. It
> clearly gets the job done, but I think Ten-Tec will have some missionary
> work ahead of them.
>
> Their main problem is, they did too good of a job in selling the industry on
> the concept of "roofing filter" when they brought out the ORION.
>
>
The name "roofing filter" is new. The concept of getting selectivity
close as possible to the antenna was put forth in a QST article in the
1960s. However multiple conversion receivers have been the norms for
most western Pacific designs (and Collins designs) where the selectivity
is isolated from the antenna by several noisy IMD limited mixers. Almost
universally, receiver makers have neglected to make the second mixer
(behind the relatively broad first IF) stand up to strong signals in
that IF pass band which they could have by using a higher level (costs
money) mixer to allow for the gain between the mixers. Worse, some have
neglected to properly terminate the mixer ports for best conversion
gain, NF, and strong signal handling even when prior products showed
they knew the need.
Much receiver performance has also been compromised by oscillators with
excess phase noise to the point that in some Yaesu (FT-736) fixing the
mixer terminations has no effect on the weak or strong signal
performance of the radio. E.g. see the 2006 Proceedings of the Central
States VHF Conference.
Most TenTec do put the selectivity right after the properly terminated
first mixer (save for the Paragon and the general coverage portion of
the Orions) which has a great deal with their great receivers. And while
the band select crystals of the Omni VI and earlier radios don't have
the to the Hertz precision of the synthesizers, they also don't have the
broad and strong phase noise of the typical very frequency agile
synthesizers.
"Distributed roofing filters" is probably hype, but "distributed
selectivity" is a useful concept putting the receiver bandwidth
determining filter right after the first mixer with similar selectivity
right before the product detector helps increase the audio S/N for all
signal strengths. There is a German 2m transceiver that does that,
called the Hohentweil. The predetection filter eliminates the opposite
sideband IF stage noise from all those gain (and mixer) stages after the
roofing filter.
>
> Next update will be after testing on "real antennas" at DL1A.
>
>
>
> 73
>
> Rick
>
> DJ0IP (NJ0IP)
>
>
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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