User's Manual

www.axcera.com • 724-873 -1500
UHF Solid State Transmitter / Translator
UHF Solid State Transmitter / Translator
Exciter
The exciter, designed and
built by Axcera, contains the
circuitry to convert the input
video and audio signals to a
combined, modulated RF
signal which drives the
power amplifiers. In the
translator version, a receiver
replaces the modulator
circuits to accept an off air
RF input and convert it to IF.
An optional Frequency
Correcting Receiver (FCR) is
available, which uses a
phase locked loop to correct
frequency errors from the
incoming signal. This option
is especially useful in
multiple hop translator
systems.
Video/Visual Modulation
The video signal is processed
in several ways prior to
modulation. Sync tip
clamping is provided to
restore proper DC level.
Sync and white clipping are
also included to limit video
transient faults. Back porch
clamping is also available for
some scrambling systems.
The video signal is then
applied to a double balanced
diode modulator, providing
modulation capability to 1%
at standard intermediate
frequency (IF). A SAW filter
is employed for precise
sideband filtering with
minimal group delay error.
Audio/Aural Modulation
The audio signal is applied to
a very wideband, linear FM
modulator which operates at
intercarrier frequency (4.5
MHz for system M). The
high performance modulator
readily accepts the full range
of multichannel sound
signals. Standard aural IF is
achieved by heterodyne
conversion of the modulated
intercarrier signal with the
visual IF.
IF Processing
The visual and aural
modulated IF signals are
combined and applied to IF
processing stages. These
stages provide outstanding
signal precorrection to yield a
very linear transmitter
output. Amplitude linearity,
incidental carrier phase
modulation (ICPM), and
frequency response
correction are all adjustable.
Upconversion
The IF signal is upconverted
to final channel frequency
through heterodyning with a
very stable local oscillator.
The oscillator is crystal
controlled, and embedded in
two ovens for tight stability.
The exciter is controlled with
an Automatic Level Control
(ALC) loop which ensures
stable signal levels. After
upconversion the signal is
amplified to provide the
exciter output.
Power Amplifiers
The 600 watt power
amplifiers are high gain units
producing 45 to 50dB of
gain. Operating in parallel in
the 1 and 2kW versions,
these trays provide
redundant paths from the
exciter output to the
bandpass filter. Redundancy
is enhanced with
independent power supplies
and cooling for each
amplifier assembly.
Furthermore, the output
stage of each amplifier tray
employs eight transistors in
parallel for added
redundancy.
A high degree of protection
is provided with each
amplifier. Individual
circulators, overdrive
protection, VSWR cutback,
and overtemperature
protection are all included.
AGC around each amplifier
ensures that the transmitter
output remains stable.
An integrated output
bandpass and trap filter is
included to provide superior
out of band rejection. This
network also adds lightning
protection through the
quarter wave stub of the trap
filter (DC and lightning short)
and the DC short circuit of
the band pass filter.