Maxon SP-1102, Service Manual, Page: 25 / 35

Service Manual SP - 1402
Page 25 of 35
PLL IC Dual Modules prescaler
The PLL synthesizer of the signal loop PLL circuit with the reference of 12.80 MHz. The IC2 PLL IC
includes all the function such as the reference oscillator, the driver, the phase detector, the lock
detector, and the programmable divider. At the reference oscillator, the 12.80 MHz TCXO of the
TCXO is connected to the pin 12 of the IC2 to oscillate the frequency of 12.80 MHz. The TCXO is
the temperature compensation circuit to maintain the frequency within the allowable error range
even under a low temperature of -20° C. The phase detectors send out the output power to the
loop filter through 16th pin of the IC2. If the oscillation frequency of the VCO is low compared to
the reference frequency, the phase detector sends out output power in positive pulse. If the
oscillation frequency of the VCO is high, phase detector send put can maintain the frequency set.
The programmable divider maintains the desired frequency with control from the CPU. The
dividing ratio, “N” to oscillate the desired frequency is as below: N = VCO oscillation frequency /
reference frequency
Level shifter & charge pump
The charge pump is used for changing output signals at PLL IC from 0-2.8V to 0-5V.The loop
filter a passive lead-lag filter consisting of R4, R84-86,R197,R229,C2,C38,C84,C106 and IC7 this
circuit use IC7 for charge pump which gain is approximate 2 time for through to produce the DC
turning voltage for VCO circuit.
Reference frequency LPF
The loop filter contains R98, C113and C131. LPF settling time is 12mS with 1 KHz frequency. This
also reduces the residual side-band noise for the best signal-to-noise ratio.
VCO
Only one of the VCO runs at a time, which is controlled by Q420 and CPU . When the PTT is
pressed, CPU goes low (approx. 0V) disabling the receive VCO by the Q420 and biases on
Q919 to enable the transmitter VCO.
The receive VCO consists of C116, C126, C147, C150-152, C154 CV901, L13, L24-26,
Q922 and D8. This VCO oscillates at 45.1 MHz below the programmed receive frequency.
The VCO oscillating frequency is tuned by the varicap diode D8 . The tuning voltage is
supplied from the output of the Loop Filter. The output of the VCO is AC coupled (C147 and
R98, C113, C131) to the synthesizer and the output buffer Q22 respectively.
The transmit VCO consists of C97-99, C101-102, C107-108, C127, CV1, L9-11, Q5 and D2.
This VCO oscillates on the programmed transmit frequency. The VCO oscillating frequency is
tuned by the varicap diode D2. The tuning voltage is supplied from the output of the Loop
Filter. The output of the VCO is AC coupled (C107 and R98, C113, C131) to the synthesizer
and the output buffer Q22 respectively.
The transmit voltage controlled oscillator is directly frequency-modulated and operates on the
carrier frequency. In the receive mode, the transmit VCO is disabled and the receive VCO is
enabled, producing receive local oscillator signal at a frequency 45.1MHz below the incoming
receive frequency. The synthesizer is tune in 5.0KHz or 6.25KHz step.