4-32 888-2463-001 3/6/08
WARNING: Disconnect primary power prior to servicing.
Diamond Series™
Section 4 Theory of Operation
R50 supplies an adjustable voltage to the PA Module Control Assembly. This voltage is
used to electronically trim the nominal phase shift through each PA Module by means
of the Control Assemblies interface to the Phase & Gain Vector Modulator. R50 voltage
is measured at E1, 0-4.5v will represent 0 - 90 degrees of phase shift. Initial set up is
2.25 volts for 45 degrees of shift.
4.9.3 Pallet Circuit Description
The Harris UHF MRF183-based PA pallet consists of 3 different sections. The
description and operation of these sections will be detailed below.
4.9.3.1 Input Circuit
The input circuit is formed by:
a. Input 3dB hybrid. Its function is to:
1. Split the input signal to drive two separate pairs of push-pull transistors.
2. Isolate the input circuit of a push-pull pair from the other.
3. Improve input return loss. This is a necessity due to the highly reactive low
input impedance’s of these devices. By keeping a symmetry in the layout,
most of the reflected input power will be absorbed by the isolation load,
R1, offering a good match at the input of the pallet.
b. Input Balun transformer. It transforms the unbalanced input (one signal refer-
enced to ground) into a balanced one (two signals 180 degrees from each other).
These two signals will drive the gates of the push-pull pair, Q1/Q3 and Q2/Q4.
c. Input matching circuit. Consisting primarily of stepped microstrip lines and
capacitors, this circuit’s main function is to up-transform the input impedance’s
of the devices. It also contains series R’s to lower the input Q and avoid possible
instabilities.
d. Gate Bias Circuit. A temperature-compensated network biases the amplifier into
class AB, with a quiescent current of 500 to 600 ma per push-pull pair, for a total
of 1 to 1.2 Amp per pallet. At this Idsq, Vgs is approximately 3.96 Volt. The Idq
is adjusted by R5 and R6 and measured at the DC input. Thermistors R9 and R10
provide temperature compensation by lowering the voltage dropped over them as
the temperature increases, this in turn lowers the gate bias voltage.
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