Section A
Oscillator (A17)
A.1
Introduction
This section includes a description of the Oscillator and trou-
bleshooting information.
A.2
Location
The Oscillator is located in the Center Control Compartment of
the transmitter, on the inner right wall.
A.3
Principles of Operation
The Oscillator provides an RF signal at the transmitter operating
frequency, and also allows for an external RF input. Refer to
SECTION V, Maintenance, for adjustment and pc board mainte-
nance procedures.
A.4
Circuit Description
Refer to schematic 839-7930-032 in the Drawing Package.
A.4.1
Supply Voltages and Voltage Regulators
Input voltages from the Low Voltage power supply are +22 VDC
and -22 VDC through F1 and F2. Voltage regulator U6 provides
-15 VDC for the crystal oven. All positive voltages are regulated
by zener diodes, and i15 VDC from CR1, +9 VDC from
CR13, and +5 VDC from CR4.
A.4.2
Oscillator Stage
The crystal oscillator stage, Q1, is a standard Pierce circuit,
operating at four or eight times the carrier frequency. The crystal
operates in its parallel resonant mode. A jumper plug, P1, allows
selecting either of two crystals. If one crystal should fail, this
jumper allows quick selection of the backup crystal (the oven
jumper P6 must also be changed). For each crystal, small fre-
quency adjustments can be made with C1 (for crystal Y1) or C3
(for Y2).
For carrier frequencies of 1250 kHz and below, the crystal
frequency is eight times the carrier frequency, and for carrier
frequencies above 1250 kHz, the crystal frequency is four times
the carrier frequency.
The +15 VDC supply voltage for the oscillator is derived from
+22 VDC and is regulated by zener diode CR1. The oscillator
supply voltage can be measured at TP1.
Each crystal is contained in a sleeve type oven, which maintains
temperature at 70°C (+/-3°C, approximately). Oven jumper plug
P6 supplies -15 VDC to either oven. Note that crystal jumper
plug P1 and oven jumper plug P6 must both be in the upper or
lower position.
A.4.3
Buffer/Squaring Amplifier
Buffer amplifier Q2 is coupled to the oscillator output, and
operates as an overdriven amplifier, with a +5 VDC supply
voltage. The output of Q2 is a TTL-level square wave which
drives the frequency divider.
A.4.4
Frequency Divider
Integrated circuits U1 and U2 are dual J-K flip-flops, used as
frequency dividers. Each IC section is connected as a divide-by-
two circuit. The signal at P2-2 is one-fourth the crystal frequency
and the signal at P2-3 is one-eighth the crystal frequency. Jumper
plug P2 routes the selected frequency to buffer/driver U5-2. The
output of U5-7 at TP-5 is a TTL level square wave at the
transmitter operating frequency.
If the transmitter frequency is 1250 kHz or below, P2 is jumpered
between 1 and 2. It the transmitter frequency is above 1250 Khz,
P2 is jumpered between 1 and 3.
A.4.5
External Input
Jumper plug P3 is used to select either the internal oscillator or
an external oscillator. An external input signal can be connected
to BNC jack J2.
Jumper plug P5 selects an external input impedance of 50 Ohms
or 20k Ohms. The high impedance input is for use with TTL level
(4 to 4.5 Volt peak-to-peak square wave) inputs. The 50 Ohm
input impedance is for use with RF input levels from 0 to +25
dbm. Amplifier Q3 and buffer/driver U5 provide a logic-level
signal to P3-2.
A.4.6
Normal or Combined Transmitter Operation
Jumper plug P4 is used to select either normal or combined
transmitter operation. For normal operation, P4-1 and P4-3 are
jumpered, and the RF signal from buffer/driver U5 is fed to U4-4.
For combined operation, P4-1 and P4-2 are connected. The RF
signal is then routed through R29, J4-1, and the External Inter-
face to the combiner control unit. When transmitters are com-
bined, the oscillator from either transmitter can be used. The
combiner control unit will provide two outputs from one oscil-
lator; the selected RF signal is then returned to each transmitter’s
Oscillator at J4-4.
A.4.7
Frequency Monitor Output
Buffer/driver U3-5 provides an output signal for a frequency
monitor or counter. Resistor R17 sets the driver output imped-
ance at 50 Ohms. The frequency monitor output signal, at BNC
connector J5, will be a 4-4.5 Vp-p square wave at the transmitter
operating frequency when the monitor impedance is 50 Ohms.
If the impedance is higher than 50 Ohms, the output signal level
will be higher.
A.4.8
Oscillator Sync
“Oscillator Sync” synchronizes the RF drive phase to any ringing
currents in the combiner/output network during VSWR protec-
tion. The circuit replaces the oscillator signal with a current
888-2297-002
A-1
WARNING: Disconnect primary power prior to servicing.
