Service manual

99R85452E01-O August 30, 2002
VHF Band Information
: Theory of Operation 7-5
RF output from the active VCO is AC coupled to the synthesizer Q323 prescaler input at Pin 8. The
divide counter chain in Q323, consisting of the dual-modulus prescaler, swallow counter and
programmable counter, divides the VCO signal down to a frequency very close to 5.00kHz or
6.25kHz which is applied to the phase detector. The phase comparator compares the phase with the
5.00kHz or 6.25kHz reference signal from the reference divider and drives the external charge pump
(Q314, Q315 and Q316). The synthesizer unlock detector circuit prevents the operation of the
transmitter and receiver, when the phase lock loop (PLL) is unlocked. The following discussion
assumes the unit has been placed in the transmit mode. Q323 lock detector Pin 7 goes high when
the PLL is locked. This high level is applied to Pin 25 of the CPU Q101. A software timing routing
brings the RX/TX line low (Pin 36 of Q101). With the RX/TX line goes low, Q209 is cut off and Q208
is biased on passing +5VTX-B to Q202; it biases on Q201 to pass switched TX-B to the transmitter
amplifier string which enables transmission.
When the PLL is unlocked, the lock detector at Q323 Pin 7 will begin pulsing low. A RC circuit
converts this low pulse to a low level for the CPU. The CPU then changes the RX/TX line to a high,
thus signaling the other transistor switches to drive Q201 into cutoff, which disables transmission.
Therefore, the transmitter remains disabled while the loop remains out of lock, and “PLL ERR” is
displayed.
7.3.5.4 Loop Filter
The Loop Filter, a passive lead-lag filter consisting of R314-R317, R356, C323, C369, and C371,
integrates the charge pump output to produce the DC turning voltage for the VCO. One parasitic
pole, consisting of RF chokes L306/L318, prevent modulation of the VCOs by the 5.00kHz or
6.25kHz reference energy remaining at the output of the loop filter. Direct FM is obtained for
modulating frequencies outside the PLL bandwidth by applying the CTCSS/DCS signals and the pre-
emphasized, limited microphone audio to the VCO modulation circuit. The modulation circuit
consists of R347 and Q329.
7.3.6 Transmitter Section
7.3.6.1 RF Power Amplifier
After the PTT is pressed, the +5VTX-B line switches to approximately 5V. Q309 is turned on,
enabling transmit VCO. The VCO buffer, pre-driver, driver and power amplifier are biased on by
Q201. Q201 is biased on by the +5VTX-B line switching to 5V. RF output from the transmit
VCO(Q325) is applied to the VCO output buffer Q308. Output from Q308 feeds the buffer Q414. The
output signal from Q414 feeds the pre-driver amplifier Q413, and feeds the driver amplifier Q412,
whose output from the driver stage feeds the final RF power amplifier Q411 to produce the rated
output power of 5 watts. The final output is feed to a low-pass filter (C446-C448, C451, C452, L418,
and L419) and then to the transmit/receive switch Q410. RF power is then fed to the antenna via the
output low-pass filter consisting of C440-C445, L415, and L416.
7.3.6.2 Antenna Switching
Switching of the antenna between the transmitter and the receiver is accomplished by the antenna
transmit/receive switch consisting of diodes Q409 and Q410. In the transmit mode, switched TX B+
is applied through R419 and RF choke L417, hard forward biasing the two diodes on Q410, thus
permitting the flow of RF power from the output of the low-pass filter. L414 and C445 isolate the
receiver circuit from the transmitter power input.