Specifications

as determined by the function required. The following discussion
applies to all RF Amplifiers, regardless of their location or use.
G.4 Block Diagram Description
The RF amplifier module is a Class D, Quad Bridged configura-
tion.
Refer to Figure G-1 for the following discussion.
G.4.1
RF Flow
G.4.1.1 RF Drive
Two separate but identical RF drive sinewave signals (exactly
equal in Phase and Amplitude), enter the module, and each is
applied to an RF Transformer. Each RF Transformer, in turn,
feeds pairs of MOSFET Switches with signals that are 0 and 180
degreesout ofphase, referenced to thephase of the input RF drive
signal.
G.4.1.2 MOSFET Switches
Refer to Fig. 1, RF Amplifier Block diagram for the following
discussion. The module is divided into two halves, Section A and
Section B. Section A consists of a pair of MOSFET switches
(with each switch consisting of two paralleled MOSFETs) which
are in series from the Supply Voltage to ground. Section B is
identical, but is driven 180 degrees out of phase with Section A.
A Combiner Transformer Primary is connected across the two
pairs, in between the MOSFET switches. This is referred to as a
Quad Bridged Configuration. Each MOSFET Switch is alter-
nately driven into cutoff or saturation depending on the phase of
the signal from the RF Transformer, thereby acting like a switch.
The MOSFETs require a positive voltage on the Gate with
respect to Source to turn ON. Therefore the MOSFETs Turn ON
for one half of the RF input cycle, and turn OFF for the other
half. For example, (See Figure G-2) for the first half cycle of the
RF Drive input, if the upper switch in Section A is ON then the
lower switch is OFF. Section B would be exactly opposite, in that
the lower switch would be ON and the upper switch would be
OFF. For the next half cycle of the RF Drive input, the lower
switch in Section A would be ON and the upper switch would be
OFF. Section B again would be the opposite and the upper switch
would be ON and the lower switch would be OFF. The output
signal across the Combiner Transformer Primary, for either half
cycle is then switched between ground, (about zero Volts) and
the positive supply voltage at an RF rate. Since the amplifier is
a bridged configuration, the effective voltage across the Com-
biner Transformer Primary will be approximately twice the
supply voltage.
G.4.1.3 On/Off Control Switches
The Turn ON/OFF Signal is applied to Control Switches that are
connected to the drive signal on the lower pair of MOSFET
Switches and ground. When the Turn ON/OFF Signal is high
(positive), the module is turned “OFF” and the Control Switches
are closed. Drive to the lower pair of MOSFET Switches is
removed and therefore they remain in an open condition. When
the Turn ON/OFF Signal is low (negative), the module is turned
“ON” and the Control Switches are open. This allows drive on
the lower pair of MOSFET Switches.
G.4.2
Cable Interlock
A single trace on the printed circuit board connects two pins of
the corresponding edge connector together. If the module is
missing or not properly connected to the edge connector, the
Cable Interlock string will be broken and the transmitter cannot
be turned on. Front Panel Module/Cable Interlock LED will be
lit.
G.4.3
Fuse Open Detector
RF Drive and Supply Voltage Fuses are located on the module
for protection of other circuits in the transmitter. Should any one
of the fuses open, a lower than normal voltage is present on the
Cable Interlock line that will activate Fuse Fault detection cir-
cuits.
G.5 Detailed Circuit Description
Refer to the RF Amplifier Schematic Diagram (839-7930-001),
for the following discussion.
Refer to other Simplified Diagrams as requested.
G.5.1
RF Drive
G.5.1.1 Primary
RF drive for section A enters the module at P1-49 and RF drive
for section B enters at P1-53. There is a separate RF fuse (F3 and
F4) for each section, so that if a section should fail, the fuse opens
and the failed section is disconnected from the driving source.
A network in parallel with each RF drive transformer primary
broadbands the input circuit, so that no component changes are
required for operation at any frequency in the AM broadcast
band. For T1, this network consists of all the inductors, capaci-
tors, and resistors drawn to the left of T1. The same is true for
the components drawn to the right of T2.
When the module is turned “OFF”, the voltage at the base of Q15
and Q17 is positive. Q15 is saturated and connects R7 and C5 to
ground, meanwhile Q17 is turned off.
When the module is turned “ON”, the voltage at the base of Q15
and Q17 is negative. Q15 is turned off and opens the ground for
R7 and C5, while Q17 is saturated and grounds the base of Q15.
This action presents the same RF impedance to the driving source
when the module is “OFF” that it has when the module is “ON”.
The same is true of Q16, Q18, R8, and C6 for section B.
G.5.1.2 Secondary
Each RF drive transformer has two pairs of secondary windings,
which provide two pairs of drive signals, 180° out of phase, for
the upper and lower MOSFET pairs in each section. On the
schematic diagram, small circles at one end of each transformer
winding indicate RF phasing.
Tranzorbs (Back-to-back zener diodes) CR1, CR2, CR3, and CR4
protect the MOSFET gates against overvoltages,including possible
G-2 888-2339-002
WARNING: Disconnect primary power prior to servicing.