Harris Platinum Series, Technical Manual

Page: 53 / 158

SECTION IV
THEORY OF OPERATION
4.1
Introduction
This section provides detailed, technical descriptions of the
operation of various circuits in the transmitter. For a more general
description of transmitter operation, refer to Section I.
4.2
RF Circuits
4.2.1
Exciter
For detailed descriptions of the circuits contained in the HX-1V
exciter, please refer to the exciter manual.
4.2.2
Exciter Switcher (optional)
The exciter switcher consists of two sections: the logic is
mounted in the accessory tray above the exciters, and the relay
panel is mounted on the cabinet side wall. (Refer to the exciter
switcher logic board and relay panel schematics.)
The visual and aural RF outputs of each exciter are detected, and
the detected DC samples are routed to the exciter logic board.
When the switcher is in automatic mode, if one of the output
samples drops below a preset threshold, the logic board signals
the relay panel to switch to the other exciter. However, this
switching action will only occur if the second exciter’s outputs
are both above its preset thresholds as well. These threshold
controls are marked “RF Presence” on the front of the switcher
logic panel.
In manual mode, either exciter may be selected regardless of its
output level.
LEDs indicate automatic mode, exciter A or B active, and faults
(output below threshold) on exciter A or B. Besides switching
the aural and visual RF signal paths, the relay panel also switches
the I/O paths between the main controller and exciters, so that
power adjustment, mute, and VSWR foldback commands from
the control system are automatically routed to the active exciter.
4.2.3
Hybrids
Hybrids are a special RF circuit, whose properties are useful for
combining RF power from two sources, and for attenuating and
phase-shifting RF signals.
The AGC module, Phase and Gain module, hybrid combiners
and dividers, and hybrid (notch) diplexers all incorporate hy-
brids. Therefore, it is important to understand the rules under
which hybrids operate, before trying to analyze the subsystems
in which they are applied.
A 3 dB quadrature hybrid is the most commonly encountered
type. It is typically drawn as shown in the diagram below. The
four ports are (1) input, (2) 0
o
output, (3) -90
o
output, and (4)
reject. This type of hybrid is symmetrical, which means that the
device operates by the same rules for waves incident on either
pair of ports.
Other types of hybrids, with different amplitude and phase
relationships, also exist and are useful for non-symmetrical
combining, subtracting one signal from another, etc.
The rules for the operation of 3 dB quadrature hybrids are listed
below. Rules number 4 and beyond may be derived from the first
three. Other types of hybrids follow similar sets of rules.
SPLITTING
1) A signal incident upon any one of the four symmetrical ports
will split equally between the opposite and cross ports.
2) In a 3 dB hybrid, each output terminated in Z
o
(normally 50
Ohms) will receive an amount of power 3 dB below the power
into the input port. If both outputs are terminated in 50 Ohms,
half of the input power will be passed to each output port. In this
case, the voltage amplitude at each output port will be 0.707
times that at the input port.
3) The signal at the opposite port will be in phase with the input,
while the signal at the cross (diagonal) port will be in quadrature
(phase shifted by -90
o
).
4) Any mismatch at an output port will result in power reflected
back into the hybrid. Signals reflected back into the hybrid by a
mismatch are treated in the same manner as input signals incident
upon that port.
COMBINING
5) Two equal-amplitude signals applied in quadrature to adjacent
ports will sum and reappear at one opposite port.
6) The sum port will be the port directly opposite the input port
with lagging relative phase, and diagonally across from the input
port with leading phase.
7) Any amplitude difference, or any phase difference other than
90
o
between the two input signals, will result in power appearing
at the fourth, or “reject,” port.
HYBRIDS IN ATTENUATORS AND PHASE SHIFTERS
A special case occurs if a signal is input to port 1, and the two
output ports see mismatches whose impedances are identical in
magnitude and phase. By rule 1, the power is split and appears
in quadrature at ports 2 and 3. Since the impedances at ports 2
and 3 are identical, the signals reflected back into ports 2 and 3
have the same amplitude and are still in quadrature. By rule 5,
the reflected signals add together, with the sum appearing at port
888-2365-001 4-1
WARNING: Disconnect primary power prior to servicing.