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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