User's Manual

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5. DESCRIPTION OF RADIO CIRCUIT
5-1. Frequency synthesizer
Frequency synthesizerconsists of VCO, loop filter and RFIC. RFIC includes
MIX,IF, Compander and PLL. Frequency synthesizer uses the PLL of thte RFIC.
a) VCO
VCO is composed of ONE VCO
Oscillation circuit takes colpitts circuit using variable Diode. The VCO
generates FRS frequencies and consists of L17, D5, C65, L19, C69, C70, R45,
R44, R46, C71, C72, Q14, R47, C74, R48, Q15, L29, C147, C151.
VCO control voltage through loop filter adjusts frequency and Microphone
Signal through Modulation terminal makes modulation.
N = VCO oscillation frequency / reference frequency
If the desired frequency is 462.5625 MHz
1) TX : N = 462.5625 MHz / 0.00625 MHz = 74010
2) RX : N = [462.5625 MHz – 21.7 MHz] / 0.00625 MHz = 70538
b) PLL IC
RFIC(PLL) is adjustable IC to produce the wished frequency which VCO
provides through loop filter. It has internal counter using 21.25Mhz
reference frequency to make 6.25kHz as reference Signal. VCO frequency
from prescaled input is divided signal is compared with Reference signal
phase in phase comparator. Built-in charger pump changes voltage (until two
signals are in phase) and charged voltage supplies VCO through loop filter
to produce the desired frequency.
Frequency data associated with channel goes to PLL IC by CPU through
CLOCK , DATA. PLL IC enables by strobe line of CPU.
c) Loop Filter
Loop filter is composed of C98, R77, C37,R78, C102, R79 and changes
pulse from U2.46 to DC . And eliminates harmonic component in pulse.
It helps VCO oscillate clearly as DC voltage is supplied into Varicap.
5-2. Receiver
This is composed of Dual Conversion Super Heterodyne. First IF is 21.7Mhz.
Local oscillator frequency is lower in 1’st IF than Rx frequency.
It is called low side injection. Second IF is 450kHz . 2’nd local oscillator
Frequency comes to 21.25MHz.
a) Rx / Tx Conversion Circuit
Rx signal goes to Rx / Tx conversion circuit through FIXED antenna
connector, low pass filter (L1,L2,L3,C6,C1,C2,C3,C4,C5) and receiver
resonance circuit composed of L4,C24. When transmitting,
Voltage through R1,L5,D1 supplies,D2 of receive input is short and
Tx is on condition. When PIN diode is off in condition of Rx, C25,C86