SECTION V
TRANSMITTER OVERALL
5.1.
Introduction
This section of the maintenance manual
will present the principles of operation for
the individual sections of the GATES Se-
ries™ AM Transmitter. Included will be
information on AC Power Flow, RF Power
Flow, and miscellaneous parts of transmit-
ter not covered in sections A through K.
5.2.
Personnel Protection
Ext e nsive interlocking and safety
switches have been provided on these trans-
mitters because of the low impedance high
current capabilities of these power supplies.
The rear access panel to these transmitters
are provided with three safety switches. The
first safety switch is operated by a small pin
protruding through a hole. As you start to
remove the rear panel, the interlock inter-
rupts the control voltage to the primary
contactors allowing them to de-energize.
Upon further removal of the rear panel, a
HV discharge switch will discharge the en-
ergy storage capacitors through large resis-
tors for current limiting. Upon opening the
rear panel even further, a switch also oper-
ated by the rear panel shorts the power
supply discharging the voltage remaining
on the filter capacitors.
WARNING
THE NORMAL PROCEDURE IN TRANS-
MITTER TURN OFF SHOULD BE FOL-
LOWED IN DE-ENERGIZING THIS TRANS-
MITTER. TURN OFF THE HIGH VOLTAGE
BY DEPRESSING THE OFF BUTTON. IF
YOU MUST ENTER THE TRANSMITTER,
SET THE REMOTE/LOCAL SWITCH ON
THE CONTROLLER BOARD TO LOCAL
AND ALLOW THE POWER SUPPLY TO
DISCHARGE AS INDICATED BY THE
FRONT PANEL METERS. REMOVE THE
REAR PANEL SLOWLY TO ALLOW THE
INITIAL RESISTOR DISCHARGE MECHA-
NISM TO FUNCTION. UPON OPENING
T H E R E A R PA N E L F U RT H E R T HE
POWER SUPPLY WILL BE SHORTED TO
GROUND AND MADE SAFE. A GROUND-
ING STICK IS PROVIDED IN THE TRANS-
MITTER TO ASSURE THAT ALL VOLTAGE
HAS BEEN REMOVED UNDER FAULT
CONDITIONS.
If the above warning is not heeded sub-
stantial damage may be done to circuit card
foil, grounding switches, and the capacitors
themselves. Always allow the voltage to be
discharged prior to opening the rear panel.
If immediate access is an absolute necessity,
it is recommended that the high voltage be
turned off, the rear panel be opened ap-
proximately 2 inches and be allowed to
remain in this position for 2 seconds before
being opened further.
5.3.
Principles Of Operation
5.3.1.
FET’S in the GATES Series™
All of the FET’s used in the GATES Se-
ries™ operate in a switching mode of op-
eration. In a switching mode, the FET’s are
either completely ON or completely OFF.
The type of FET’s used in the GATES Se-
ries™ transmitters are well suited for this
application because they have a very low
and consistent ON resistance, very high
OFF resistance, and very fast switching
times. The benefit of these characteristics is
high efficiency and excellant modulation
linearity.
5.3.2.
PDM Theory In Brief
Pulse Duration Modulation (PDM) is a
high efficiency type of modulator system
wherein the modulator operates in a switch-
ing mode. Since the FET’s have very low
ON resistance, very high OFF resistance,
and very fast switching times, the modula-
tor efficiency is relatively high.
The basic makeup of the modulator sys-
tem used in the GATES Series™ transmit-
ters is with the PA, the PDM filter, and the
PDM Amplifiers in series. The PDM Am-
plifiers purpose is to switch the PDM Filter
inputs to ground at a 60 kHz rate. For the
sake of understanding, a relay could be used
for this purpose if it could switch at a 60
kHz rate.
The PDM Filter, which is between the
PDM Amplifiers and PA, removes the
switching frequency so that the signal pro-
vided to the PA is high level DC and audio.
The percentage of time that the PDM Am-
plifiers are ON varies with changes in pulse
width or duty cycle. This change in duty
cycle is what causes a change in PA voltage,
and is how the PA is modulated. The higher
the PDM duty cycle, the higher the resulting
PA voltage.
The voltage for the PA is the difference
between the high voltage supply (about 260
volts) and the output of the PDM Filter. For
a normal full power, unmodulated condition
the PDM Filter Outputs will be 100 to 105
volts less than the high voltage supply.
On 100 percent negative modulation
peaks, the PDM Amplifiers cease to con-
duct and there will be no voltage differential
across the PA (zero PA volts).
On the highest positive modulation peaks,
the PA voltage may reach the 260 volt level
if the PDM duty cycle reaches 100 percent.
5.3.3.
Polyphase Theory
One inherent drawback to single phase
PDM relates to the amount of filtering that
is required to remove the PDM switching
frequency. Considerable filtering is re-
quired to prevent spurious signals from be-
ing transmitted. With inadequate filtering,
there would be spurious signals above and
below carrier, spaced by the PDM switching
frequency.
However, a large amount of filtering will
also limit the transient response and modu-
lation density of the transmitter. This is
most noticeable when modulating with a
square wave or with the audio heavily proc-
essed. False modulation peaks can occur
due to effects of the filtering.
Polyphase PDM was created as a means
of achieving the benefits of PDM without
the modulation overshoot problems associ-
ated with PDM filtering.
In Polyphase PDM, the frequency of the
PDM is effectively multiplied by the
number of PDM phases. This is very similar
to power supplies with multiple phases of
rectification. A basic half wave supply op-
erating on 60 Hz will have a ripple fre-
quency of 60 Hz. A full wave supply
operating on 60 Hz will have a ripple fre-
quency of 120 Hz. A three phase full wave
supply operating on 60 Hz will have a ripple
frequency of 360 Hz.
As with multi-phase power supplies,
Polyphase PDM requires less filtering to
remove the ripple. In the case of the GATES
Series™, there are four PDM phases of 60
kHz each. The effect of this is a PDM fre-
quency of 240 kHz. Therefore, the PDM
filtering is designed to attenuate 240 kHz
instead of 60 kHz. This degree of filtering
ensures excellent transient response.
5.3.4.
Audio/PDM Signal Flow
Refer to the PDM Flow diagram.
The audio input signal is applied to the
PDM Generator, which generates four
phases of 60 kHz pulse width modulated
signals. These signals are about 13 to 14
volts peak to peak. For further discussion on
the operation of the PDM Generator, refer
to section F.
The outputs of the PDM Generator are
routed to the PDM Amplifiers. The PDM
Amplifiers produce switching of the low
side of the PA modules through the PDM
Rev. T: Jan. 1996
888-2314-001
5-1
WARNING: Disconnect primary power prior to servicing.
Summary of Contents for Gates One 994 9202 002
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