TK-2310R
13
2-3. IF Amplifier Circuit
The first IF signal is passed through a four-pole mono-
lithic crystal filter (XF401) to remove the adjacent channel
signal. The filtered first IF signal is amplified by the first IF
amplifier (Q405) and is then applied to the IF system IC
(IC401).
The IF system IC provides a second mixer, limiting
amplifier, quadrature detector and RSSI (Received Signal
Strength Indicator). The second mixer mixes the first IF sig-
nal with the 50.4MHz signal of the second local oscillator
output (Q403) and produces the second IF signal of 450kHz.
The second IF signal is passed through the ceramic filter
(CF402) to remove the adjacent channel signal. The filtered
second IF signal is amplified by the limiting amplifier and
demodulated by the quadrature detector with the ceramic
discriminator (CD401). The demodulated signal is routed to
the audio circuit.
2-4. Audio Amplifier Circuit
The demodulated signal from IC401 is sent to an audio
power amplifier (IC815) through baseband IC (IC812), where
it is amplified and output to the speaker.
2-5. Squelch Circuit
Part of the AF signal from the IC401 enters the FM sys-
tem IC (IC401) again, and the noise component is amplified
and rectified by a filter and an amplifier to produce a DC
voltage corresponding to the noise level.
The DC signal from the FM IC goes to the analog port of
the MCU (IC820). IC820 determines whether or not to output
sounds from the speaker by checking if the input voltage is
higher or lower than the preset value.
To output sounds from the speaker, IC820 sends a high
signal to the AF_CONT line and turns IC815 on through
Q813, Q814, Q818 and Q819.
CIRCUIT DESCRIPTION
3. Transmitter System
3-1. Microphone Amplifier Circuit
The signal from the microphone is limited by the AGC
circuit, which is composed of D807, D808, Q810, Q811 and
the mute switch (Q809). IC812 is composed of a high-pass
filter, low-pass filter and pre-emphasis/IDC circuit.
The signal from the microphone and the low speed data
from the MCU (IC820) enter the baseband IC (IC812) and
pass through each path and are mixed inside the IC.
The output signal from the baseband IC goes to the VCO
modulation input. The other output signal passes through the
amplifier (IC3) and goes to the TCXO modulation input.
AMP
IC3
AGC
SW
D807,808
Q810,811
MIC_MT
X1
TCXO
VCO
LSD
OUT
IC812
Q809
MIC
Baseband
IC
Fig. 4 Microphone amplifier circuit