Hytera MD652, Service Manual

Page: 135 / 207

VHF1 (136
‐
174 MHz)
Circuit Description
27
The 19.2 MHz frequency generated by reference crystal oscillator (X8001) goes into PLL IC (U8001) for
division, generating the reference frequency. Meanwhile, the frequency generated by VCO goes into
PLL IC (U8001) for frequency division. The resulting frequency will be compared with reference
frequency in terms of phase difference in the phase detector. After comparison, the resulting frequency
is converted to CV voltage via the loop filter, to control and lock the frequency.
VCO
The VCO is composed of transistors (Q8002 and Q007), varactors and four Colpitts oscillators. There
are two VCOs in all. One VCO (Q8002) is used to transmit excitation signals and the other VCO (Q8007)
is used to receive LO signals. U1002 controls the operating frequency of the VCO. Q8004, Q8005 and
Q8006 constitute the buffer amplifiers for both the transmitter circuit and receiver circuit. The
digital-to-analog converter IC (U8004) modulates the TX oscillator signal.