A TTL “LOW” control signal from the Modulation Encoder will
switch PNP transistor Q5 ON and switch NPN transistor Q7 OFF.
A TTL “HIGH” control signal will turn Q5 OFF and Q7 ON.
Figure C-4b. shows the equivalent RF drive circuit when the RF
amplifier is ON. Transistor Q5 is ON, which completes the RF
ground path through the secondary of transformer T1 and pro-
vides RF drive to the gates of parallel MOSFETs Q3/Q10. The
RF drive circuit for Q4/Q11 is the same, except that the RF
ground path is through control transistor Q6.
Figure C-4c. shows the equivalent RF drive circuit when the RF
amplifier is OFF. Transistor Q7 is ON, which clamps the positive
half-cycle of the RF drive from transformer T1 slightly above
ground. This prevents parallel MOSFETs Q3/Q10 from switch-
ing ON. The RF drive circuit for Q4/Q11 is the same, except that
CR8, Q6, and associated components are used.
C.2.5
RF Transformer Primary Current: Amplifier
Off
When an amplifier module is OFF, there is no current flow from
the supply through the combiner transformer primary and the
module does not supply any power to the combiner. Current will
still flow through the combiner secondary, however, unless the
total combiner RF output is zero. This combiner current will
induce RF voltages in the toroidal transformer primary windings
on all inactive modules.
If the combiner transformer primary sees an open circuit, induced
voltages can damage amplifier MOSFETs, and high RF voltages
in the unloaded primary can cause an arc which can crack the
toroid. The “quad” amplifier configuration and reverse diodes in
the MOSFETs provide an RF current path, as described in the
next paragraphs.
This explanation is based on simplified diagram, Figure C-5.
When the polarity of the induced voltage in the combiner trans-
former is as shown in the diagram, Q1/Q12 is OFF and Q2/Q9
is ON. A low-impedance RF current path is available through the
reverse diodes in Q1/Q12, and bypass capacitors C1, C3, C4, and
C2.
When the polarity of the voltage induced in the transformer
winding reverses, Q1/Q12 turn ON and Q2/Q9 will turn OFF.
The current flow will now be through the reverse diodes in
Q2/Q9.
C.2.6
Oscillator Sync Signal
When the VSWR protection circuits turn all RF amplifiers in the
Power Amplifier stage OFF, “ringing currents” will continue to
flow in the output network, and in the RF combiner secondary,
for several cycles. For maximum MOSFET reliability during a
high VSWR, Q1/Q12 and Q2/Q9 in all PA Modules must switch
in phase with output network ringing currents. This is accom-
plished with the Oscillator Sync circuitry, including an output
network current sample and circuits on the Oscillator.
C.3
Circuit Description
Refer to Schematic 839-7855-080 in the Drawing Package.
C.3.1
Supply Voltage
The supply voltage for section A enters the module through
P1-23, 24, 25 and 26; the supply voltage for section B enters the
module through P1-29, 30, 31, and 32. The low side of each RF
quad amplifier returns to ground. The supply voltage is +230
Figure C-4. RF Amplifier Module, Control Section
Operation (817 2099 034)
Section C - RF Amplifier Modules
888-2297-002
C-3
WARNING: Disconnect primary power prior to servicing.
