Model No: M760 Plus
Customer: INTEK Rev No: A
16
VCO, VARICAP DOIDE (D402,D403)
IC5 is a cmos LSI that includes most of PLL block and driver, the Q403-Q406,Q408, C412, C421, C416, C417
and L17, Varicap diode D402,D403 are clapp oscillator circuit to operate as a VCO. Q406 is a switching
transistor to connect or disconnect the tuning capacitor in the VCO oscillator tank circuit for transmitter or
receiver. Q411 works as a buffer AMP for RX local frequencies (38MHz) and TX generating frequencies (26
MHz).
B. Reference frequency
The crystal, X2 (4.000 MHz) and other components at IC4 can make a reference frequency oscillator with internal
amplifier.
C. VCO
Q408 and surrounding parts are consisting a clapp oscillator works as a VCO . the VCO can be oscillate over the
required of 25.610MHz to 41.115 MHz
D. Phase detector and VCO control
The detector is a digital phase comparator which compares the phase of the reference signal with programmable
divider output square waves and develops a series of pulses whose DC level depends on the phase error of each
signal.
E. Transmitter/Receiver buffer AMP
Output signal of Q408 is fed into buffer AMP Q411,
F. Switching of tuning capacitor in VCO
The VCO circuit must tune with a wide rang of frequencies 25.610MHz to 30.15MHz for transmitter and
36.305~41.115 MHz for receiver. To comply above rang of VCO, the tuning capacitance should switched for
transmission or reception.
G. Receiver local oscillator outputs
First Mixer:
The secondary output signals is injected to the sources of 1
st
mixer Q2,Q3 in the 1
st
IF mixer
section
Second Mixer:
The output of 10.24 MHz oscillator circuit with XF1 is injected into the IF IC internally. Incoming IF
signal and 10.24 MHz are mixed inside the IF IC to extract 2
nd
IF signal 455 KHz. FM,AM audio
signals are recovered with the way of quadrature detector, AM signals are recovered with envelope
