ⓓ
SF1 SAW filter fail.
ⓔ
VCO problem.
④
Squelch does not work
ⓐ
U12 problem.
ⓑ
Control logic problem.
5. DESCRIPTION OF RADIO CIRCUIT
5.1 Frequency synthesizer
Frequency synthesizer consists of VCO, PLL IC(built in PRESCALER) and
loop filter.
a) VCO
VCO is composed of ONE VCO. Oscillation circuit takes colpitts circuit using
variable Diode. And VCO is composed of D1,Q8,Q9,C81,C75,VC1,L1,C74,C76.
VCO control voltage through loop filter adjusts frequency and microphone
signal through modulation terminal generates FM modulation.
b) PLL IC
PLL IC is adjustable IC to produce the desired frequency which VCO
provides through loop filter. It has internal counter using 21.25MHz
reference frequency to generate 6.25kHz as reference Signal. VCO frequency
from prescaled input is divided signal and 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 R48,R49,C84,C85 and forms pulses from pin14
Into to DC and eliminates harmonic components in pulses.
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. 2nd
local oscillator frequency comes to 21.25MHz.
