Service manual

July 1, 2002 68P81076C25-C
3-50 Theory of Operation: Voltage Control Oscillator
3.5.2.9 Doubler
The first buffer output is coupled to the input of the doubler by C5663. Q5660 doubles the drive
frequency and increases power by approximately 3 dB as a result of the high and low impedances
presented to its collector at the doubled frequency and drive frequency, respectively. The collector
impedances are presented by an elliptical high-pass filter (C5670-C5674, L5670, and L5671). The
filter is terminated in a resistive pad (R5676-R5678) which also serves to terminate one end of the
elliptical low-pass filter (C5675, C5677, and L5672-L5674). In addition to filtering, the low-pass filter
provides part of the impedance match required between the resistive pad and the second buffer. The
remaining impedance match is accomplished with L5680 and C5680, configured to provide
additional high-pass selectivity.
3.5.2.10 Synthesizer Feedback
The base of Q5680 provides the tap location for the synthesizer feedback buffer. C5685-C5686 and
L5681 provide low-pass filtering. R5630, R5631 and R5632 is a resistive pad. Q5630 provides
approximately -5 dBm to the RF board.
3.5.2.11 Second Buffer
The second buffer, Q5680, is a common-emitter amplifier with approximately 12 dB gain. It is biased
to 40 mA. with an active current source, Q5681 and R5580-R5587, which ensures saturated
operation.
3.5.2.12 Receive/Transmit Switch
In the receive mode where K9.4-V is off, Q5640 conducts current to turn on the part of CR5690 (a
dual-common cathode pin diode) that is in series with the receive path, and the part of CR5691 that
is in shunt with the transmit path. The output of Q5680 is then coupled to a resistive pad
R5697-R5699 which sets the power out of J5642 to approximately +12 dBm.
In the transmit mode, K9.4-V applies 9.4 V to the anode of CR5640, thus turning off Q5640. K9.4-V
is also applied to resistors R5688 and R5694 which turn on the parts of CR5690 and CR5691 that
are in series with the transmit path. The output of Q5680 is then coupled to a resistive pad
(R5689-R5691) which sets the power out of J5641 to approximately +16 dBm.
3.5.3 800 MHz Band
3.5.3.1 General
The VCO is located on an alumina substrate with a metal cover. The buffer-doubler-buffer section is
located on the PC board and may be repaired using normal repair methods.
3.5.3.2 Super Filter 8.6 V
Super filter 8.6 V is applied to the VCO carrier board at J601-12. From there, SF8.6 passes to the
drain of Q9641, to the emitters of Q9643 and Q9644, and to the collector of Q9642.
3.5.3.3 VCO
Q9641, the main and transmit/TalkAround transmission lines, and the varactors CR9641 through
CR9644 form the major circuitry of the oscillator. CR9645, C9648, C9647, and R9641 make up an
automatic gain control (AGC) circuit.