Icom IC-F70DT Service Manual page 8

The quadrature detector is a detection method which uses a
ceramic discriminator (X2).
The demodulated AF signals are output from pin 9, and
applied to the AF cricuits.
4-1-5 AF CIRCUITS
The demodulated AF signals from the FM IF IC are ampli-
fied and filtered at AF circuit. This transceiver employs the
base band IC for audio signal processing for both transmit
and receive. The base band IC is an audio processor and
composed of pre-amplifier, compressor, expander, scram-
bler, etc. in its package.
The AF signals from FM IF IC (IC3, pin 9) are applied to the
base band IC (IC301, pin 23) via the digital/analog switch
(IC302, pins 12, 14).
The applied AF signals are amplified at the amplifier section
and level adjusted at the volume control section, and then
suppressed unwanted 3 kHz and higher audio signals at
LPF section. The filtered AF signals are applied or bypassed
the TX/RX HPF, scrambler, de-emphasis, sections in
sequence, then applied to another volume controller.
The TX/RX HPF filters out 250 Hz and lower audio signals,
and the de-emphasis obtains –6 dB/oct of audio character-
istics. The expander expands the compressed audio signals
and also noise reduction function is provided.
The AF signals are level adjusted at the volume controller
and amplified at the amplifier section. The amplified AF sig-
nals are output from pin 20 and applied to the D/A coverter
(IC303, pin 16) to be adjusted its level, and then applied to
the FRONT UNIT via J3 (pin 28).
The level controlled AF signals from the MAIN UNIT are
passed through the mute switch (FRONT UNIT; IC205,
pins 1, 3) and applied to the AF power amplifier (IC201,
pin 4: FRONT UNIT) to obtain 500 mW of AF output power.
The power amplified AF signals are applied to the internal
speaker (CHASSIS UNIT; SP1).
4-1-6 SQUELCH CIRCUITS
• NOISE SQUELCH
Noise squelch circuit mutes AF output signals when no RF
signals are received. By detecting noise components in the
• BASE BAND IC BLOCK DIAGRAM
VR1
TXIN 3
(HPF)
TXA1
VR3
RXIN 23
(HPF)
RXA1
SDEC 21
MTDT 10
MTCK 9
MDIO 11
MRDF 12
Control
MSCK 13
Register
MDIR 14
Com- Pre-
pressor emphasis
TX/RX
HPF
RX
LPF
MSK
Modulator
MSK MSK
BPF Demodulator
demodulated AF signals, the squelch circuit switches the
AF mute swithch and AF power amplifier controller ON and
OFF.
A portion of the demodulated AF signals from the FM IF IC
(IC3, pin 9) are applied to the converter (IC303, pin 1) to
be adjusted its level. The level controlled signals are output
from pin 2 and applied to the active filter (IC3, pins 7, 8;
R74, R75, R77 C137–C139). The filtered signals are applied
to the filter amplifier section to amplify the noise components
only.
The amplified noise components are converted into the
pulse-type signal at the noise detector section, and output
from pin 13 as the "NOIS" signal and applied to the CPU
(IC307, pin 37). Then the CPU outputs "AFON" signal from
pin 18 according to the "NOIS" signal level to toggle the AF
mute circuit (FRONT UNIT; IC205) and AF amplifier control-
ler (FRONT UNIT; Q202, Q203) ON/OFF.
• CTCSS AND DTCS
The tone squelch circuit detects tone signals and opens the
squelch only when receiving a signal containing a matched
sub audible tone (CTCSS or DTCS). When the tone squelch
is in use, and a signal with a mismatched or no sub audible
tone is received, the tone squelch circuit mutes the AF sig-
nals even when the noise squelch is open.
A portion of the demodulated AF signals are passed through
the LPF (IC12, pins 12, 14) to filters CTCSS/DTCS signal.
The filtered signal is applied to the CPU (IC307, pin 46)
after being amplified at the buffer amplifier (IC2, pins 1, 3).
The CPU compares the applied signal and the set CTCSS/
DTCS, then output the AF mute switch (IC205) AF amplifier
controller (Q202, Q203) control signal from pin 18.
Limiter
Scrambler/
De-scrambler
De-
emphasis
4 - 2
IC301 AK2346
7 MOD
Splatter VR2 SMF
18
Expander
VR4
19
20 SIGNAL
RXA2