Kenwood TH-22A Service Manual page 6

TH-22A/AT/E
CIRCUIT DESCRIPTION
e IF amplifier
The first IF signal is amplified by Q11 and enters IC4 (FM
signal processing IC), where it is mixed with the second local
oscillator signal and so converted into the second IF signal.
The unwanted frequency band of the second IF signal is
eliminated by ceramic filter CF1. The resulting signal is then
amplified and detected.
Item Rating
Center frequency of 6¢B} Within 455 kHz+1.5 kHz
bandwidth (fo)
6dB bandwidth +7.5 kHz or more
40dB bandwidth +15 kHz or less
Passband ripple 1.5 dB or less (within 455+1.5 kHz)
Guaranteed attenuation | 27 dB or more (within +100 kHz)
e AF amplifier
The frequency characteristics of the audio signal output
by the FM detector are corrected by the Q12 active high-
pass filter and deemphasis circuit consisting of R67 and
C71. The audio signal is then passed through an AF variable
resistor and amplified by power amplifier C208 to obtain the
desired output.
e Squelch and mute circuits
The output detected by |C4 is amplified by Q10, wave-
form shaped by D16 and C80, and the direct current is input
to the microprocesser analog port (pin 5). The voltage input
to the microprocesser is digitized, and the microprocesser
controls the MUTE, AFCO, and AFC1, thus controlling the
audio signal. This port has hysterisis. The microprocesser
also controls the MUTE, AFCO, and AFC1 during the T. ALT,
Insertion loss 6 OB or less CTCSS and DTSS operations, thus controlling the audio sig-
Input/output impedance | 1.5 kQ nal.
Tabie 3 Ceramic filter (L72-0362-05) characteristics
(TX-RX unit CF1)
noe Q221
Ic 4
4 sp
11 9
- ASB
aro -_Die ney | L< 3c
5 -
, 8 * J rh
Qi3 7
5 /
BUSY BEEP
Mute f= MUTE
AFCO 40
IC206 AFC1 31
Fig. 3 AF amplifier, squelch, and mute circuits
Condition MUTE | AFCO | AFC1
Transmission L H L
Reception } Normal Squelch on L H L
operation | squeich off H | L | oH
Bell Standby L H L
operation Receive (Bell operation) L L H
MUTE: Muted when low
AFCO: Muted when high
AFC 1: Muted when low
Table 4 Muting conditions