ALAN Electronics Tectalk FM PMR 446, Руководство по обслуживанию

Страница: 18 / 34

2) RECEIVER
Rx does not work
Speaker line open problem or connector problem.
Receiver power circuit problem.
Audio amplifier Base band IC U4 problem.
Only noise sound
U12 problem.
VCO problem.
Rx sensitivity is weak
Antenna mounting problem.
Front-End circuit problem.
Local oscillation frequency deviation.
SF1 saw filter fail.
VCO problem.
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, L11, C74, C76.
VCO control voltage through loop filter adjusts frequency and
Microphone signal through Modulation terminal makes modulation.
b) PLL IC
PLL IC 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 R48,R49,C84,C85 and changes pulse from
pin14 to DC. and eliminates harmonic component in pulse.
It helps VCO oscillate clearly as DC voltage is supplied into
Varicap.