Harris HTlFM - Page 25

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4.1.
Introduction
•
.
..
This section contains circuit descriptions
of
the
FM
BROADCAST TRANS:MITTER.
4.2. FM
Exciter
Refer to the Exciter Technical Manual.
SPLITTER (used
in
HT
lFM
only)
The
FM
Exciter output is connected to the
input
of
the two-way splitter at J9
on
the
back panel
of
the CONTROLLER. The
splitter divides the drive signal into two
equal outputs for the two POWER
AMPLI-
FIERS. The outputs
of
the splitter at
JlO
and
Jl
l
are connected to the input connectors
of
the 500W PA's.
4.3.
500
Watt
Power
Amplifier
(PA)
The Power Amplifier
of
the
FM
Transmit-
ter uses eight transistors in two push-pull
parallel pairs operating class C. All
of
the
impedance matching is done with broad-
band transformers. No tuning is required
The amplifier module includes circuitry
which protects itself from excessive supply
voltage, SWR in excess
of
2.5:
1,
overdrive,
and over temperature operation. Any oper-
ating condition which results in exceeding
the protection thresholds will automatically
disable the amplifier and report out a
"PA
FAULT" condition to the controller.
The power supply for the amplifier is dou-
bly
regulated. The ferroresonant trans-
former provides an almost constant output
voltage regardless
of
the line voltage as
long as the output is loaded. The series pass
regulator section provides low load regula-
tion and ripple reduction, and overcurrent
protection. The regulator consists
of
four
series pass transistors connected in parallel
and controlled by IC
UL
The output
of
the
regulator is variable from 45 to 55 volts at
25 amperes
of
current.
4.4.
250 Watt Power Amplifier
(HT 250FM only)
The 250 Watt Power Amplifier uses four
transistors in two push-pull pairs operating
class
C.
All
of
the impedance matching is
done with broadband transformers.
No
tun-
ing is required. The amplifier module in-
cludes circuitry which protects itself from
excessive supply voltage,
SWR
in excess
of
2.5:1, overdrive, and over temperature op-
eration. Any operating condition which re-
sults in exceeding the protection thresholds
will automatically disable the amplifier and
report out a
PA
FAULT condition to the
controller.
SECTION IV
PRINCIPLES
OF
OPERATION
The power supply for the amplifier is dou-
bly
regulated.
The
ferroresonant trans-
former provides an almost constant output
voltage regardless
of
the line voltage as
long
as
the output is loaded. The series pass
regulator section provides low load regula-
tion and ripple reduction, and overcurrent
protection. The regulator consists
of
four
series pass transistors connected in parallel
and controlled
by
IC
UL
The output
of
the
regulator is variable from 45 to 55 volts at
15
amperes
of
current.
4.5.
Power Combiner
(HT
lFM
ONLY)
The output
of
each
of
the 500W PAs are
combined in a Wilkinson type combiner.
The signals are in phase and, within reason-
able limits, the same power level. Isolation
between the input ports
of
the combiner is
25
dB. This prevents forward power from
one amplifier being sensed as reflected
power by the other amplifier. Any differ-
ence in phase
or
power between the two
PA
output signals is dissipated in the reject
load,
Rl.
If
one
of
the Power Amplifiers
fails, half
of
the remaining power is dissi-
pated in the reject load and half is delivered
to the antenna. At
full
1
KW
power with
APC operating, this fault condition can re-
sult in more than the specified dissipation
in the reject
load
To
prevent this condition and to provide a
fault alarm, the voltage across
Rl
is moni-
tored by transformer
Tl
and associated
components, R2,
CRl
and
Cl.
The voltage
sensed across the reject load is passed to a
comparator in the controller via connector
J8. When the reject load voltage exceeds the
level set by R16, the Reject Load fault latch
is set and the drive power is reduced to
prevent damage to the reject load resistor.
Operation continues at reduced power as
determined by R63.
4.6.
Output Lowpass Filter
Harmonics are attenuated with an Output
Lowpass Filter that passes the carrier fre-
quency but attenuates the carrier harmonics
and any spurious signals. A fixed tuned
elliptical type filter, it passes the complete
Commercial
FM
Broadcast Band (87.5-
108MHz)
and gives the necessary attenua-
tion
to
the
harmonics
to
meet
FCC
requirements. The filter also
cvutai1w
a
sampling coupler which provides a -35 dBc
sample
of
the output signal for monitoring
purposes. No
tuning
or
adjustment is neces-
sary.
888-2312-001
4.7.
Output Directional Coupler
The Output Directional Coupler is a short
section
of
transmission line with coupling
and rectifying elements to give a de voltage
to indicate the forward-going power. The
same procedure is used to give a reflected
power indication. The detector elements are
replaceable and are selected for the partic-
ular transmitter output power. The reflected
power elements are 10 dB more sensitive
than the forward power elements in order to
provide accurate measurement
of
low SWR
values. Both forward and reflected sample
voltages are amplified by the control board
and then used to drive the output meter and
the APC and SWR foldback loop.
4.8.
Controller
The Controller chassis contains the Con-
trol printed-circuit board, Power supply
board, and the Interface board. The circuitry
is in five logically independent sections
which will be discussed separately. The sec-
tions are,
1)
the Power Supply, 2) the Oper-
ating circuits, 3) the Metering circuits
4)
Overload and Alarm circuits, and 5) Power
Control circuits. Portions
of
each
of
these
groups are involved with the AC Restart
facility.
4.8.1.
Power
Supply
(Refer to Figure 4-4.)
The Controller circuits are powered by a
dual regulator system with capacitive stor-
age backup
on
the positive supply. Optional
battery backup is also provided. This sys-
tem begins with a dual 15-volt Power Sup-
ply
on
the
bottom
of
the chassis.
The
negative side is regulated
by
integrated cir-
cuit U2 to -15 volts. The positive side
of
the
power supply is regulated to + 15 Volts by
integrated circuit
UL
The +15 volts is used
to charge capacitors C44, C50 and C51
through isolation diode CR40 to provide the
+B (Battery) line. Diode CR27 assures
equal voltages on
the+
15
and +B line. The
optional battery is isolated from the
+Bline
by
diode CR42 which prevents any dis-
charge
of
the battery until the capacitors
have discharged below the nominal voltage
of
the battery.
During a power failure the +15 and -15
volt regulator circuits are inoperative, but
the +B line maintains a voltage due to the
charge on capacitors C44, C50 and C51.
Since all
of
the circuits powered by the +B
line are low power
CMOS
logic circuits, the
current drain is very low and sufficient volt-
age is available to retain the logic levels for
4-1
WARNING:
Disconnect primary power prior
to
servicing.