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Re: [TenTec] OMNI D

To: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: Re: [TenTec] OMNI D
From: Ken Brown <ken.d.brown@hawaiiantel.net>
Reply-to: ken.d.brown@hawaiiantel.net, Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Tue, 12 May 2009 09:28:03 -1000
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
Hi Cal,

     There are two common way of generating a SSB signal. There is the 
phasing method, and the more common filter method. In the filter method 
a double sideband signal is generated by a balanced modulator, and the 
unwanted sideband is rejected by a filter, leaving only one sideband. 
Almost every SSB transceiver uses the filter method, and shares the 
sideband selecting filter used in transmitting with the receive function 
in the transceiver. In the Ten-Tec Omni VI radios this filter is 2.4 kHz 
bandwidth 8 pole crystal filter with a center frequency of 9.0015 kHz. 
The transmit signal for SSB and CW modes goes through this filter, and 
in the case of SSB this filter's characteristic determines the opposite 
sideband rejection, and along with the audio circuitry and your 
microphone, it determines the bandwidth of your transmitted SSB signal. 
This filter also determines the maximum bandwidth of your receiver, 
because it is always in the receive signal path, regardless of other 
optional filters in either the 9 MHz IF or the 6.3 MHz IF. (Please note 
that in FM mode this filter in neither in the transmit or receive path) 
Normally this "default" filter is never changed, and will always 
determine your SSB transmit bandwidth. Optional filters in either the 9 
MHz IF or the 6.3 MHz IF only affect receive function, as they are not 
in the transmit signal path.
     It is possible, but generally not advisable, to change the default 
filter. There are two reasons I have heard of for doing this. One is to 
produce "High Fidelity" SSB. There are a number of good reasons not to 
do this. The most obvious is that it consumes more spectrum in our 
crowded bands. Another reason not to use a wider filter in the transmit 
signal path is that it can degrade the carrier rejection. The balanced 
modulator is not perfect, so some carrier is generated and then 
attenuated by the opposite sideband rejecting filter.
     A second reason for changing the default filter would almost only 
be considered by people, such as myself, who don't operate SSB, or have 
another radio to use for SSB. By putting a narrower filter in the 
"default" filter position it is possible to improve the receiver 
selectivity. Selection of the filter has to be done with care, so that 
the bandpass coincides with the bandpass of any other optional filters 
in the 9 MHz IF. The SSB transmit bandwidth would become narrower than 
normal, making the SSB signal probably not pleasant to listen to. It 
could possibly make for a good pile-up busting SSB signal, if the 
filter, BFO frequency and operators voice were all matched just right.
     All of the above applies to the Omni VI radios. There could be some 
differences in the Omni D. I am not familiar with that radio. I am 
certain however that as long as you do not change the stock "default" 
transmit and receive 9 MHz IF filter, your transmitted signal will not 
be changed.

DE N6KB
> I have been the happy user of a TEN TEC OMNI D for a little over 2 years.
> Gets great signal reports on every band I work from 80-10 meters.  Have a 
> D-104(trans amp) mike, MFJ Versa tuner, feeding ladder line to a double zep 
> 60' up between some very tall trees. It has all 3 filters. They are great in 
> filtering down the qrm.
>
> Someone told me recently that changing the filters also changes the 
> transmitted audio as they are switched down to filter the received audio.  
> I've read and re-read the manual and don't find anything that says the 
> filters make any changes to the transmitted audio?
>
> Comments anyone?
>
> Cal Zethmayr
> W4GMH@COX.NET
>
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>   

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