TK-3201
7
4) Audio Amplifier Circuit
The demodulated signal from IC201 goes to AF amplifier
through IC301.
The signal then goes through an AF volume control, and is
routed to an audio power amplifier (IC302) where it is amplified
and output to the speaker.
5) Squelch
Part of the AF signal from the IC enters the FM IC (IC201)
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 microprocessor (IC405). IC405 determines whether to
output sounds from the speaker by checking whether the
input voltage is higher or lower than the preset value.
To output sounds from the speaker, IC405 sends a high
signal to the SP MUTE line and turns IC302 on through
Q303,Q304,Q305,Q306 and Q316. (See Fig. 4)
6) Receive Signalling
(1) QT/DQT
The output signal from FM IC (IC201) enters the
microprocessor (IC405) through IC301. IC405 determines
whether the QT or DQT matches the preset value, and
controls the SP MUTE and the speaker output sounds
according to the squelch results.
Fig. 3 AF amplifier and squelch
RECEIVE SIGNALLING
RECEIVE SIGNALLING
SP
Q306,316
SW
IF Amp
FM IF IC201
IC301
IC302
AF PA
IC405
Q303,304,305
SW
QT/DQT
DTMF
CLK,DATA,
STD,LOADN
SIGNAL
AF CONT
QT/DQT IN
BUSY
CPU
AK2346
CIRCUIT DESCRIPTION
1. Frequency Configuration
The receiver utilizes double conversion. The first IF is 38.85
MHz and the second IF is 450 kHz. The first local oscillator
signal is supplied from the PLL circuit.
The PLL circuit in the transmitter generates the necessary
frequencies. Fig. 1 shows the frequencies.
1) Front End (RF AMP)
The signal coming from the antenna passes through the
transmit/receive switching diode circuit, (D103,D104,D106
and D122) passes through a BPF (L229 and L228), and is
amplified by the RF amplifier (Q205).
The resulting signal passes through a BPF (L214,L212) and
goes to the mixer. These BPFs are fixed.
2) First Mixer
The signal from the front end is mixed with the first local
oscillator signal generated in the PLL circuit by Q1 to
produce a first IF frequency of 38.85 MHz.
The resulting signal passes through the XF201 MCF to cut
the adjacent spurious and provide the opitimun
characteristics, such as adjacent frequency selectivity.
3) IF Amplifier Circuit
The first IF signal is passed through a four-pole monolithic
crystal filter (XF201) to remove the adjacent channel signal.
The filtered first IF signal is amplified by the first IF amplifier
Fig. 2 Receiver section
2. Receiver
The frequency configuration of the receiver is shown in Fig. 2.
(Q203) and then applied to the lF system IC (IC201). The IF
system IC provides a second mixer, second local oscillator,
limiting amplifier, quadrature detector and RSSI (Received
Signal Strength Indicator). The second mixer mixes the first
IF signal with the 38.4MHz of the second local oscillator
output (TCXO X1) and produces the second IF signal of
450kHz.
The second IF signal is passed through the ceramic filter
(CF201) 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 (CD201). The demodulated signal is routed to
the audio circuit.
SP
BPF
IC301
ANT
AK2346
CF201
TCXO
ANT SW
BPF
1st Local
MIXER
Q204
RF AMP
Q205
MCF
XF201
IC201
IF,MIX,DET
Q1
X3 multiply
IC302
AF PA
IF AMP
Q203
AF VOL
X1
12.8MHz
2nd Local
Fig. 1 Frequency configuration
SP
TX:
446.00625~446.09375MHz
RX:
407.15625~407.24375MHz
PLL
VCO
IF SYSTEM
ANT
TCXO
X3 multiply
38.85MHz
MCF
AF
AMP
MIC
MIC
AMP
TX
AMP
RF
AMP
38.4MHz
CF
450kHz
TX/RX: 446.00625~446.09375MHz
12.8MHz
RF
AMP
ANT SW
