TROUBLESHOOTING AND REPAIR
FREQUENCY AND PHASE MODULATION
FM/ØM BLOCK DIAGRAM 6E-2.
Refer to the FM/øM Block Diagram (Figure 6E-2) to identify the major functional
sections and in follow the signal paths of the FM section.
FM/ØM CIRCUIT DESCRIPTION 6E-3.
The FM PCA, (A14), has a phase-locked loop that consists of the following:
• Voltage-controlled 80-MHz oscillator with a modulation port, a control port, and
a presteering section.
• Programmable dividers for reference and variable frequencies.
• Selectable phase detectors, normal and wide range, loop amplifier and filter
circuitry, and logic circuitry.
• Modulation section with both high modulation rate path and low modulation rate
path.
Incorporated in the different sections are logic and controls for achieving frequency
modulation, normal and low rate, and phase modulation, normal and high rate. Also,
the function of DC frequency modulation is included. The FM modulation deviation is
4 MHZ maximum, which is covered in six ranges. Equivalent phase modulation ranges
exist for 400 Radians maximum (40 Radians maximum in high-rate phase modulation).
To attain such a wide deviation the oscillator has a high deviation mode in the top two
deviation ranges. A linearizer is active in the top three ranges to reduce distortion. To
achieve the wide range deviation at low rates, a wide band phase detector is used for the
top deviation ranges, and the phase detector reference frequency is appropriately
selected.
Oscillator Section 6E-4.
The voltage controlled oscillator section is composed of Q1, Q2, L1, CR1-CR8 and
associated components. The adjustable coil L1, adjustable capacitor C9, varactors
(voltage variable capacitors) CR1-CR8, and associated capacitors form the resonant
circuit. Capacitors C2 and C4 couple the resonant circuit to the input of the active
circuit and C10, C11, and C13 couple to the output of the active circuit. Parts C9,
CR15, and L4 are used to switch between the high deviation mode and normal mode.
The circuit of Q5 and Q8 drive the PIN diode CR15 between either conduction or high
impedance. Low impedance at conduction adds C9 in the resonant circuit for a normal
mode high "Q" oscillator circuit. High impedance removes C9 from the oscillator for
the high deviation mode. The varactor voltage must adjust to compensate for the
change of the capacitance of C9 in or out of the circuit. The voltage to the varactors has
both control and modulation functions. Control is applied to the center connection of
the varactors, TP11, and modulation is applied to the ends of the varactors, TP4. VCO
CONTROL, TP11, is the control voltage to keep the oscillator center frequency at 80
MHz. This voltage is about +15V DC for normal high "Q" mode and at about +7V DC
for high deviation mode. FM MODULATION, TP4, is at 0V DC and has the applied
modulation. The components in the control and modulation lines are for isolation and
filtering.
Amplifier U2 is used to buffer the oscillator output to the sum loop. Resistor R45 and
associated resistors adjust and establish the proper level. The circuit of U1 buffers the
80-MHz signal and U3 and its resistors establish ECL levels to the divide-by-four 1C,
U4. The 20-MHz signal from U4 is translated from ECL level to TTL level by the Q12
and Q13 circuit.
6E-2
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