Harris Platinum Series, Technical Manual

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are recombined using four two-way combiners, before passing
to the four 15 kW PA cabinets (see 15 kW system, above).
The visual cabinet outputs are recombined in pairs, using two 3
dB quadrature hybrids. Finally, the pairs are recombined with a
final 3 dB hybrid.
1.2.9.5 Parallel Visual Drive Paths
Parallel visual drive paths are available as an option on many
models. This option eliminates the possibility of a single-point
visual drive failure.
In some lower power transmitters, the option simply consists of
using a two-way divider and a two-way combiner, plus two
driver modules, to replace the single driver module found in
standard systems. In the event of the failure of one driver, the
AGC module increases the system gain to allow full-power
transmission to continue.
In transmitters 15 kW and higher, a switchable 6 dB pad is inserted
before the two-way splitter, and this pad is controlled by the slave
controller in the cabinet containing the visual driv chain. If one
driver fails, the slave controller senses the failure and disables the
pad to compensate for the decreased system gain.
In some higher power systems, where the drive chain consists
of a cascaded driver and PA module, both the driver and PA are
duplicated in each path.
1.2.10
Aural Signal Path
Again, the basic topologies of the aural paths in the various
models are similar. As in the visual path, the exciter aural output
passes to an AGC module which monitors a sample of the aural
system output. The AGC module output then drives the aural RF
chain. (Refer to the transmitter block diagram in the drawing
package for this discussion.)
1.2.10.1
Exciter
Either monaural audio and SCA, or externally generated com-
posite stereo, is fed to the exciter and modulated onto an IF
frequency, which is lower than that of the visual IF by an amount
equal to the difference between the desired aural and visual
carriers. The modulator is a voltage-controlled oscillator whose
center frequency is held constant by a phase-locked loop (PLL).
IF group delay correction (optional) can be used at this point to
improve stereo separation in systems where notch diplexers are
used. The IF signal is converted up to channel using a mixer and
the same LO as in the visual chain. The resulting signal is
bandpass filtered and amplified, becoming the exciter’s aural
output. As in the visual path, if dual exciters (optional) are used,
each exciter’s aural output is routed to the exciter switcher.
1.2.10.2
Dual-Carrier Systems
In systems where dual aural carriers are generated, the exciter
path takes a different form. The two signals are modulated onto
two different IF carriers, and the modulated carriers are added
together. Linearity pre-correction is added to prevent intermodu-
lation of the two carriers. The resulting signal is mixed up to
channel with the same LO as used in the visual chain, and is
bandpass filtered and amplified, becoming the exciter output.
1.2.10.3
AGC Module
As in the visual path, the exciter switcher output or single exciter
feeds an AGC module, which holds the aural transmitter gain
constant by controlling aural RF drive based on samples of
exciter drive and transmitter aural output.
1.2.10.4 Phase and Gain Module
As in the visual signal path, in higher-powered systems, it may
become necessary to feed parallel signal paths through one or more
phase and gain modules. These modules allow the gain and phase
of each path to be trimmed, so that the proper phase and amplitude
relationships are obtained at the final combiner input.
1.2.10.5 Aural RF Amplifier Chains
The aural RF amplifier chains vary in complexity depending on
visual peak power output, 10% or 20% aural power, and single
or parallel paths. The following configurations are used in the
various transmitter models:
•
1 kW Systems, 10% or 20% Aural
In these systems, the aural AGC output drives a driver module,
which alone produces enough output power to serve directly as
the transmitter aural output.
•
2 kW Systems, 10% or 20% Aural
The AGC module output passes through a preamp, to a driver
module. The driver produces enough output power to serve as
the transmitter aural output.
•
5 kW Systems, 10% or 20% Aural
In a 5 kW system, the aural AGC output is fed to a driver module,
whose output drives a PA module. The PA module output
becomes the transmitter’s aural output.
•
10 kW and 15 kW Systems, 10% Aural (standard)
In these systems, the aural AGC output feeds a driver module, whose
output is split in a two-way splitter and sent to two PA modules. The
PA outputs are recombined in a two-way combiner, whose output
passes through a harmonic filter before reaching the optional diplexer.
•
15 kW Systems, 20% Aural (optional)
In 15 kW systems with the 20% aural option, the aural AGC
output is split in a two-way divider, and the two signals are input
to two driver modules, whose outputs are recombined in a
two-way combiner.
The drive signal is then split four ways in a four-way splitter,
whose outputs drive four parallel PA modules. The outputs are
recombined in a four-way combiner and passed through a har-
monic filter before being sent to the optional diplexer.
•
20 kW Systems, 10% Aural (standard)
The aural AGC module output passes through a phase and gain
module. Each of the two outputs passes to an aural chain in one
of the PA cabinets.
Once inside the PA cabinets, the two signals are sent to driver
modules. Their outputs are split using two-way dividers, and the
resulting outputs feed an array of four PA modules. The two PAs
in each cabinet feed two-way combiners, and the combiner
outputs feed a 3 dB hybrid used as a final two-way combiner.
The resulting signal passes through a harmonic filter before
being sent to the optional diplexer.
Section I - Introduction
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WARNING: Disconnect primary power prior to servicing.