Service manual

July 1, 2002 68P81076C25-C
3-48 Theory of Operation: Voltage Control Oscillator
The VCO output is coupled through C672 to Q645 to amplify the signal and provide load isolation for
the VCO. The collector voltage of Q645 is normally about 5 Vdc.
3.5.1.4 Synthesizer Feedback
The synthesizer locks the VCO on frequency by the VCO feedback to the prescaler IC on the RF
board. The output of the VCO goes into a low-pass filter consisting of C685, L676, and C687. After it
is filtered, the signal splits into three directions - the majority of which passes to the RX buffer
through a 2db attenuator. A smaller portion of the signal passes through C679 to the divider. Finally,
another small portion of the signal is fed back to the RF board through C676 to P601 -1. Although on
a DC connector, P601 -1 is an RF-sensitive node. To measure the synthesizer feedback power, use
a high-impedance probe, or operate the VCO in an external fixture.
3.5.1.5 RX Buffer Circuitry
After the low-pass filtering state, VCO power is attenuated 2dB by R678, R680, and R679. The RX
buffer is a 50-ohm in-and-out stage that uses L681 and C689 for the input match and C691, L678,
C692, and R699 for the output match. The 9.6 Vdc supplies the RX buffer for a gain of about 10db.
Components R677 and C686 help to filter out some of the 9.6-V supply's noise from the RX buffer.
Transistors Q677, Q678, and associated resistors set the bias level of the RX buffer device, Q675.
The collector voltage and current should be near 6.6 V and 29 mA, respectively. Resistor R682 feeds
the base of 0675 while L677 is used as the collector choke; R681, C690, and C688 are added to
increase stability. The cable from the RX frontend is plugged into J642.
3.5.1.6 Frequency Divider and TX Buffer Circuitry
During transmit, the VCO oscillates at twice the transmit frequency. A frequency-divider circuit
following the VCO buffer divides the VCO's output frequency by two. The circuit is known as a
"regenerative frequency divider" in which a mixer and a feedback amplifier are used to divide the
frequency of the input signal. The divider circuit consists of transformers T601 and T602, diodes
CR601, CR602, amplifier Q681, and the associated bias circuitry. The divider action of this circuit
can be understood by tracing the signal through the circuit as follows: The 300 MHz range signal
from the VCO buffer is fed into the primary of T602. Note that T602, T601, and diodes CR601 and
CR602 form a balanced mixer. (CR601 and CR602 are actually two diodes in one SOT-23 package.)
To analyze the frequency division action of the circuit, it must be assumed that the divided output
frequency of 150 MHz already exists at the secondary of T601. This 150 MHz signal passes through
the low-pass filter consisting of L661, L662, and C651. The 150 MHz signal is now at the input of the
amplifier device, Q681. The amplified 150 MHz signal is now applied back into the balanced mixer by
the center tap of T601. The difference frequency of the two applied signals (300 MHz and 150 MHz)
is 150 MHz, which is half the VCO's frequency. The difference frequency is output through the
secondary of T601 where it had been previously assumed to exist. This completes the feedback
loop.
The 150 MHz signal is tapped off of the emitter resistor of Q681 and is amplified by the buffer stage,
Q682. Transistor Q683 amplifies the signal to 10dBm, which is the level required by the power
amplifier. The signal passes through a low-pass filter before exiting the board through J641.
3.5.2 UHF Band
3.5.2.1 General
The VCO is located on an alumina substrate with a metallic cover. The buffer-doubler-buffer section
is located on the PC board and may be repaired using normal repair methods.