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TK-5400
5-3. IF Amplifier
The 1st IF signal is amplified (Q304) and fed into IC303 in
the MIX, IF AMP IC. The IF signal is then mixed with the
2nd local oscillator frequency of 44.395MHz to generate the
2nd IF of 455kHz. The 455kHz signal is then passed through
a ceramic filter (CF301) and fed back into IC303 for addi-
tional amplification. Again the 455kHz signal is then passed
through a ceramic filter (CF302).
5-4. Digital Signal Processor (DSP) Demodulation
Processing
The 455kHz signal that passes through the ceramic filter
is amplified to the appropriate value by the 2nd IF amplifier
and fed to ADC (pin 6 of IC609). The base band signal A/D-
converted at the ADC is processed by the DSP (IC611).
The DSP performs FM demodulation for FM signals and
C4FM demodulation for C4FM signals. Then, the base band
signal is D/A-converted and an AF signal is output from
CODEC (pin 15 of IC608).
The D/A-converted noise component enters the SW (pin
5 of IC706) through CODEC (pin 16 of IC608) and is output
as a squelch noise signal.
IC302
IC609
2nd IF
ADC
AMP
Fig. 5 DSP demodulation processing
5-5. Squelch Circuit
The output from the SW (IC706 pin 6), goes through a
low-pass filter (IC601 2/2). The noise component from
IC601 (2/2) is amplified by Q602 and rectified by D602 to
produce a DC voltage corresponding to the noise level. The
DC voltage is fed to the CPU (IC507 pin 88).
IC601
NOISE
LPF AMP
Fig. 6 Squelch circuit
10
CIRCUIT DESCRIPTION
MOD
IC706
NOISE
IC611
IC608
DSP
CODEC
AF
Q602
D602
IC507
DET
CPU
AMP
5-6. Audio Amplifier Circuit
• TX-RX unit
The converted D/A signal from IC608 is amplified by AF
amplifier IC606 (2/2). The signal then goes through an elec-
tronic volume control (IC605), an AF amplifier IC607 (2/2),
and an AF switch (Q8 is on and Q7 is on/off the control unit),
and is routed to audio power amplifier (IC1 of the control
unit), where the signal is amplified and output to the internal
speaker.
• Control unit
The audio mute signal (AMP SW) from the microproces-
sor becomes Low in the standby mode and Q5 and Q6 in
the power supply circuit for IC1 are turned off. When the
audio outputs, AMP SW becomes High to turn Q5 and Q6
on, and the DC is supplied to power terminal VP of IC1.
Speaker switching is performed by the IC701 (TX-RX unit)
using INT AFC or EXT AFC. First, the logic level at the
speakers switching terminal (SSW) on the universal connec-
tor is fed to the microprocessor (IC507 TX-RX unit). The mi-
croprocessor then outputs data to IC701 based on this in-
put.
When there is no SP-MIC installed, this logic level be-
comes high. When the INT AFC is high, the EXT AFC goes
low, so the AF signal is only fed to the amplifier for the inter-
SW
nal speaker (INT SP) of IC1.
However, when a SP-MIC has been connected, this logic
level becomes low, so the INT AFC goes low and the EXT
AFC goes high. In this case, the AF signal is fed only to
amplifier for the external speaker (EXT SP) of IC1.
AF
IC606
AF AMP
Fig. 7 Audio amplifier circuit
SB
Q5
AMP SW
IC1
Q8
Q7
SW
SW
INT AFC
Q15
SW
EXT AFC
IC605
IC607
DAC
AF AMP
VOL
Q6
INT.SP
EXT.SP
AF
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