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turn off the transceiver before starting or
shutting off the dc source equipment. These
recharging devices often generate large
voltage spikes that can damage the
transceiver.
A WORD ABOUT GROUNDING
A good ground system is essential for
optimum operation of any HF transmitter.
The best solution is to connect all the station
equipment chassis together using a heavy
gauge of flat ground braid. Use a short
length of braid to connect to a ground rod. If
you are not using a linear amplifier, a less
ideal ground may suffice. A ground
connection to a copper cold water pipe was
often suitable, but that is now a violation of
the National Electrical Code. Be aware that
many modern water connections use plastic
pipe, and are not suitable ground
connections.
Antenna type and its proximity to the station
are also factors in choosing ground
methods. With good resonant antennas
located away from the station, the AC
ground in your house wiring might be
adequate.
PHILOSOPHY OF DESIGN
Ten-Tec's goal with the OMNI-VII
transceiver was to provide an HF transceiver
that meets several disparate criteria. 1) Be
easy for the average amateur to operate.
2) Have a feature set that experienced
amateurs expect a top of the line piece of
equipment to include. 3) Offer Amateur
Radio's first completely Ethernet controllable
HF rig, with an eye to remote operation
becoming more and more popular as time
passes. 4) Include general coverage HF
receive capability plus 50 MHz.
The filtering system in the OMNI-VII uses
what we refer to as "distributed" roofing
filters. The definition of roofing filters varies;
in Amateur Radio terms the term has come
to mean a crystal or mechanical filter that
allows for the preservation of available
receiver performance for realistic on-the-air
situations, as opposed to lab conditions
when only two signals at a given spacing are
present for test purposes.
588 manual
Part #74409
Printed in USA
General coverage HF transceivers today are
of an upconverting, VHF level first I-F,
followed by two or three more conversion
stages. This VHF I-F first stage is
somewhat problematic due to physics to
create an adequate mode-appropriate
roofing filter. At the same time, some
amateurs are reluctant to use a ham bands-
only HF transceiver that may have a low
frequency first I-F and be generally more
suited to high performance applications than
a general coverage HF rig.
The question was how to combine a general
coverage receiver with a mode-appropriate
system of filtering appropriate for high-end
Amateur Radio receiver performance.
We refer to the roofing filters in the OMNI-VII
as "distributed" because filters are spread
across both the first and second I-F stages.
The conversion stages in the OMNI-VII are
70 MHz first I-F, 455 kHz second I-F, 14 kHz
(DSP) third I-F. A monolithic filter at 20 kHz
bandwidth is present at the first I-F stage
between the first and second mixers.
Selectable second I-F filters at bandwidths
of 20, 6, 2.5 kHz, 500 Hz (optional) and 300
Hz (optional) can be cascaded with the first
I-F monolithic filter at 20 kHz bandwidth.
Bandwidth filtering is done in DSP at the
third I-F and is controlled by the BW encoder
on the transceiver front panel.
The net effect of using 455 kHz second I-F
filters is to increase blocking dynamic range
over what the receiver would be capable of
without the second I-F filters installed. Third
order intercept point essentially remains
constant. The front-end AGC in the
transceiver is after the 455 kHz I-F filters;
having them installed prevents the radio
from attempting AGC action on a signal that
is outside the bandwidth of the 455 kHz I-F
filter.
SSB operators will not require additional
filters; additional filters can be installed
optionally by CW or digital mode operators.
50 MHz transceive operation at 100 watts
output power is included in the OMNI-VII.
Additional receive capability to 48 MHz has
been provided for monitoring of VHF
European TV coverage and other signals of
Second release – August 2007
5
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