Harris Platinum Series, Technical Manual

Page: 54 / 158

4, the reject port. This is the basis for attenuator and phase-shifter
operation.
An adjustable attenuator can be made from a 3 dB hybrid and
two 50 ohm potentiometers ganged together. Power is input to
port 1, and ports 2 and 3 are each terminated with one section of
the potentiometer, so that each sees the same impedance. Port 4
is taken as the attenuator output.
If the pots are both set for 50 ohms, ports 2 and 3 see matched
terminations, and the pots absorb all of the power, with no power
being reflected from them back into the hybrid; this gives maxi-
mum attenuation. If the pots are set for zero ohms (a short
circuit), the signals that appear at ports 2 and 3, still in quadrature
and equal in amplitude, are completely reflected back into the
hybrid. Thus all of the input power appears at port 4, and the
attenuation is minimum. Intermediate settings give intermediate
values of attenuation.
In our applications, PIN diodes in parallel with 50 ohm resistors
are used instead of potentiometers. The amount of bias current
through the diodes controls their RF impedance. The impedances
seen at ports 2 and 3 are the parallel combination of the 50 ohm
and PIN diode impedances. When the PIN diodes are biased
“off”, they appear as open circuits, resulting in 50 ohm loads seen
at ports 2 and 3. Thus, all of the RF power is absorbed in the 50
ohm resistors, giving maximum attenuation. If the diodes are
biased “on,” they appear as short circuits, resulting in short
circuits appearing at ports 2 and 3. This causes complete reflec-
tion of the RF power incident on ports 2 and 3, and the RF
recombines at port 4, resulting in minimum attenuation. Varying
the bias current in the PIN diodes varies their impedance and,
hence, the amount of attenuation.
An adjustable phase shifter can be constructed in a similar
manner, by using equal reactive terminations (i.e. ganged vari-
able capacitors) instead of resistive terminations at ports 2 and 3.
The amount of reactance determines the phase difference be-
tween a signal incident on one of the reactive loads, and the
resulting reflection back into the hybrid. Since no power is
dissipated in the reactive loads, the power appears unattenuated
at port 4, but phase-shifted by an amount determined by the
reactance at ports 2 and 3.
4.2.4
AGC Module
(Refer to AGC schematic.)
An RF sample of the PA output is fed to J1, detected by D6,
buffered, and applied to the noninverting (+) input of the com-
parator section of U2.
The exciter output is input to J2 and sampled by directional
coupler DC1. The sample is detected by D5, buffered, and
applied to R51, the exciter reference adjust potentiometer. The
arm of R51 is connected to the inverting (-) input of the compa-
rator.
The comparator’s DC output, pin 7 on U2, represents the differ-
ence between the exciter sample and the transmitter sample. This
difference is integrated by R30 and C16, buffered, and applied
to base of Q1, a Darlington transistor whose emitter drives a PIN
diode attenuator.
R1 through R9 form fixed pi attenuators, used to bring the exciter
drive level to within the desired range. U1, a 3 dB 90
o
hybrid, is
connected to PIN diodes CR1 through CR4 and resistors R11 and
R14 to form a variable attenuator. (Hybrid/PIN diode attenuator
operation is described above.) Q1 controls the amount of forward
bias supplied to the diodes, which controls the amount of attenu-
ation.
Switch S1 disables the AGC by providing a fixed DC voltage to
Q1, from a voltage divider formed by R26 and R27.
Set control R21 adjusts the amount of compression by the AGC
during normal transmitter operation.
4.2.5
Phase and Gain Module
(Refer to Phase and Gain schematic.)
The phase and gain module consists of two subsystems: the RF
path, and the control/logic circuit.
4.2.5.1
Radio Frequency Path
The major elements in the RF portion of the circuit are hybrids
U1 through U5. One is used as a power splitter, two as variable
attenuators (as in the AGC module above), and two as variable
phase shifters.
The signal entering J1 passes to port 1 of U1, and is split equally
between the two main signal paths. This first hybrid split also
establishes a quadrature phase relationship between the two
paths, which will be necessary when the PA cabinet outputs are
recombined in a hybrid combiner.
With both paths properly terminated, little power is misdirected
to R1, the reject load. Both paths behave in an identical manner.
Each path contains an adjustable attenuator and an adjustable
phase shifter.
Hybrids U2 and U4 are configured as variable attenuators. (Hy-
brid/PIN diode variable attenuator operation is discussed in 4.23
above.) Parallel networks consisting of 56 ohm resistors and PIN
diodes provide the variable terminations. In the upper path on the
schematic, pin 1 of hybrid U2 is the attenuator input, and pin 6
is its output. R57 controls the current in Darlington transistor Q1,
whose emitter biases PIN diodes CR1, CR2, CR39, and CR40.
As R57 is adjusted clockwise, the current through Q1 increases,
reducing the impedance of the PIN diodes, which reduces the
amount of attenuation. In the lower circuit path, R58 controls the
attenuation through the attenuator formed by hybrid U4 in an
identical manner.
Hybrids U3 and U5 form the basis for the variable phase shift
networks. (Hybrid phase shifters are discussed in 4.23 above.) In
the upper path on the schematic, U3 pin 1 is the phase shifter RF
input, and pin 6 is the output. Logic signals at points A through
L are used to turn PIN diodes CR3 through CR15 on or off. Each
PIN diode is an RF switch, which either grounds or floats one
end of a capacitor C4 through C19. The remaining end of each
capacitor is connected to the hybrid.
4-2 888-2365-001
WARNING: Disconnect primary power prior to servicing.