1-24
1-110.
PWM DRIVER CIRCUIT.
The PWM control signal from PWM comparator U22A is applied
a PWM driver circuit. The circuit consists of: 1) inverters U21A through U21F, 2) line
drivers U19, U20, and U45, and 3) diodes D6 through D21 and D47 through D50. The
PWM control signal is applied to inverters U21A through U21F. The inverted PWM
signals from U21A through U21F are applied to inverting line drivers U19, U20, and U45.
U19, U20, and U45 operate in parallel to lower the output impedance. The outputs from
U19, U20, and U45 are applied to each RF amplifier module modulator circuit board.
Diodes D6 through D21 and D47 through D50 protect U19/U20/U45 from latch-up
conditions. The PWM circuit is protected from over-modulation conditions by an
over-modulation PWM mute circuit consisting of transistor Q23, capacitor C173, and
diode D65. If the modulation level increases above 150%, the circuit will output a HIGH
to mute line drivers U19, U20, and U45.
1-111.
FREQUENCY SYNTHESIZER.
The exciter circuit board frequency synthesizer is a
phase-locked-loop circuit which generates and maintains the phase and frequency of a
voltage-controlled-oscillator (VCO) to a high level of precision. The circuit is designed
with the ability to synthesize: 1) 119 frequencies within the 522 kHz to 1705 kHz AM
broadcast band in 10 kHz increments or 2) 123 frequencies within the 522 kHz to 1705
kHz AM broadcast band in 9 kHz increments.
1-112.
The synthesizer operates from binary coded carrier frequency information entered into
frequency programming switch S2. The binary formatted frequency is applied to
frequency synthesizer integrated circuit U25. U25 operates from 10.24 MHz reference
oscillator Y1. Capacitor C108 allows the reference to be calibrated. Regulator U24
provides a stable voltage supply for frequency synthesizer U25.
1-113.
Once programmed, U25 will output a series of rectangular-wave pulses to loop filter
U26B and low-pass filter U26A. U26A and U26B function together to generate a stable
dc control voltage for application to a voltage-controlled-oscillator (VCO) module. The
control voltage is used by the VCO module to generate a precision frequency reference. A
constant current source for the VCO module is provided by transistor Q3.
1-114.
Precision alignment of the VCO output is maintained by the phase-locked-loop design.
Feedback samples are monitored by a lock detector circuit. If the VCO frequency shifts
from the programmed operating state, the output of U25 will change to adjust the control
voltage and maintain a stable VCO output.
1-115.
The output of the VCO module is applied through transistor amplifier Q4 to inverter
U30A. The output of U30A is applied to NAND gate U30B. A sample of U30A is applied
to U25 for feedback.
1-116.
The output of U30B is applied to a synchronous divider consisting of integrated circuits
U29, U31A, U31B, U42A, and U42B. Depending on the carrier frequency: 1) the original
synthesizer frequency or a divide-by-2 frequency is used to generate a 4XFc (four times
carrier frequency) signal for application to the stereo circuit board and 2) a divide-by-4 or
a divide-by-8 signal is used to generate the carrier frequency. The carrier frequency is
applied to NAND gate U30C. Jumpers P6A and P6B program the divider as determined
by the carrier frequency.
1-117.
PWM REFERENCE CIRCUIT.
The PWM frequency is generated by oscillator Y2, binary
counter U44, and switch S1. The PWM frequency generator outputs a 115.4 kHz to 138.5
kHz signal to an integrator circuit. The PWM frequency is: 1) generated by one of four
different crystals and 2) determined by several operating parameters. The output of U44
is applied to Integrator U12. U12 is designed to generate a precision triangle-wave signal
for application to PWM comparator U22A. Programming switch S1 is used to program
the PWM frequency generator circuit. The PWM frequency, crystal, and switch
programming are recorded in the final test data sheets.
