TK-3301
7
1. Frequency Confi guration
The receiver utilizes double conversion. The first IF is
38.85MHz and the second IF is 450kHz. The fi rst local oscil-
lator signal is supplied from the PLL circuit.
The PLL circuit in the transmitter generates the neces-
sary frequencies. Fig. 1 shows the frequencies.
ANT
TX/RX : 446.00625~
446.09375MHz
ANT
SW
RF
AMP
TX
AMP
RF
AMP
1st MIX
MCF
38.85MHz
CF
450kHz
IF
SYSTEM
AF
AMP
SP
RX:
407.15625~
407.24375
MHz
PLL
VCO
MIC
AMP
MIC
TX: 446.00625~
446.09375MHz
38.4MHz
X3
multiply
TCXO 12.8MHz
Fig. 1 Frequency confi guration
2. Receiver
The receiver is double conversion super-heterodyne, de-
signed to operate in the frequency range 446MHz.
The frequency confi guration of the receiver is shown in
Fig. 2.
ANT
ANT
SW
BPF
RF AMP
Q204
MIXER
Q203
IF AMP
Q202
BPF
1st Local
MCF
XF201
IC201
IF, MIX, DET
IC308
AQUA-L
Q2
X3 multiply
X1
TCXO
CF201
2nd Local
12.8MHz
AF VOL
IC309
AF PA
SP
Fig. 2 Receiver section
■
Front End (RF AMP)
The signal coming from the antenna passes through the
transmit/receive switching diode circuit, (D103, D104 and
D106) passes through a BPF (L213 and L212), and is ampli-
fi ed by the RF amplifi er (Q204).
The resulting signal passes through a BPF (L209, L207
and L206) and goes to the mixer. These BPFs are non-vari-
able type.
CIRCUIT DESCRIPTION
■
First Mixer
The signal from the front end is mixed with the fi rst local
oscillator signal generated in the PLL circuit by Q203 to pro-
duce a fi rst IF frequency of 38.85MHz.
The resulting signal passes through the XF201 MCF to
cut the adjacent spurious and provide the opitimun charac-
teristics, such as adjacent frequency selectivity.
■
IF Amplifi er Circuit
The fi rst IF signal is passed through a four-pole monolithic
crystal fi lter (XF201) to remove the adjacent channel signal.
The fi ltered fi rst IF signal is amplifi ed by the fi rst IF ampli-
fier (Q202) and then applied to the lF system IC (IC201).
The IF system IC provides a second mixer, limiting amplifi er,
quadrature detector and RSSI (Received Signal Strength In-
dicator). The second mixer mixes the fi rst IF signal with the
38.4MHz of the second local oscillator output (TCXO X1 and
Q2) and produces the second IF signal of 450kHz.
The second IF signal is passed through the ceramic fi lter
(CF201) to remove the adjacent channel signal. The fi ltered
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.
■
Audio Amplifi er Circuit
The demodulated signal from IC201 goes to AF amplifi er
through IC308.
The signal then goes through an AF volume control , and
is routed to an audio power amplifi er (IC309) where it is am-
plifi ed and output to the speaker.
■
Squelch
Part of the AF signal from the IC enters the FM IC (IC201)
again, and the noise component is amplifi ed and rectifi ed by
a fi lter and an amplifi er to produce a DC voltage correspond-
ing to the noise level.
The DC signal from the FM IC goes to the analog port
of the MCU (IC306). IC306 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, IC306 sends a high
signal to the AF_CONT line and turns IC309 on through
Q312, Q313, Q314, Q315 and Q316. (See Fig. 4)
IC201: FM IF
IF amp
SIGNAL
DTMF
IC308
AQUA-L
QT/DQT
BUSY QT/DQT IN
IC306
MCU
AF_CONT
SP
Q315,316
IC309
AF PA
Q312,313,314
CLK, DATA,
STD, LOADN
SW
SW
Fig. 3 AF amplifi er and squelch
