Kenwood TH-D74E Service Manual page 6

2.1.3 Receiver System
2.1.3.1 VHF receiver circuit
A band VHF receiver circuit
The reception signal from the antenna passes through the filter circuit and enters the RF amplifier (Q179). The signal amplified by the
RF amplifier (Q179) enters the distribution circuit for signal distribution with the B-band. The distributed signal is amplified by the
second RF amplifier (Q178), passes through the band-pass filter, and enters the mixer (Q281). The signal is converted to a first IF of
57.150 MHz after the signal undergoes upper heterodyne conversion by the first local oscillation frequency. Then the signal is
amplified by the IF amplifier (Q282) after the signal passes through the MCF (XF281), and applied to the IF IC (IC281).
The signal undergoes upper heterodyne conversion in the IF IC (IC281) by the second local oscillation frequency (57.600 MHz) tripled
from the reference frequency (19.200 MHz). Then the signal is converted to a second IF of 450 kHz and detected to generate an audio
signal.
B band VHF receiver circuit
The reception signal from the antenna passes through the filter circuit and enters the RF amplifier (Q179). The signal amplified by the
RF amplifier (Q179) enters the distribution circuit for signal distribution with the A-band. The distributed signal is amplified by the
second RF amplifier (Q193), passes through the band-pass filter, and enters the mixer (IC283). The signal is converted to a first IF of
58.050 MHz after the signal undergoes upper heterodyne conversion by the first local oscillation frequency. Then the signal is applied
to the IF IC (IC284) through the MCF (XF282).
The signal undergoes lower heterodyne conversion in the IF IC (IC284) by the second local oscillation frequency (57.600 MHz) tripled
from the reference frequency (19.200 MHz). Then the signal is converted to a second IF of 450 kHz and detected to generate an audio
signal. While the transceiver is in AM/SSB/CW mode, the signal is converted to a third IF of 10.8 kHz after the signal undergoes upper
heterodyne conversion by the 3rd local oscillation frequency (460.8 kHz). Then the signal is input into the CODEC IC (IC707).
VHF accompanying circuit
Tuning voltage output from the D/A converter (IC671) under the control of the main MPU (IC702) is applied to each band-pass filter
through a discrete variable-capacitance diode so that the respective band-pass filters will be tuned to target frequencies.
ANT
DUPLEXER
LPF LPF LPF
HPF
2.1.3.2 UHF receiver circuit
A band UHF receiver circuit
The reception signal from the antenna passes through the filter circuit and enters the RF amplifier (Q172). The signal amplified by the
RF amplifier (Q172) enters the distribution circuit for signal distribution with the B-band. The distributed signal is amplified by the
second RF amplifier (Q171), passes through the bandpass filter, and enters the mixer (Q281). The signal is converted to a first IF of
57.150 MHz after the signal undergoes lower heterodyne conversion by the first local oscillation frequency. Then the signal is amplified
by the IF amplifier (Q282) after the signal passes through the MCF (XF281), and applied to the IF IC (IC281).
The signal undergoes upper heterodyne conversion in the IF IC (IC281) by the second local oscillation frequency (57.600 MHz) tripled
from the reference frequency (19.200 MHz). Then the signal is converted to a second IF of 450 kHz and detected to generate an audio
signal.
1-6 (No.RA044<Rev.001>)
st
BAND A 1 IF 57.15M
st
1 MIXER
Q281
BAND A 136~260M
RF AMP
D180 HPF Q179
RF SW
ABVS
D181
BAND B 108~310M
RF SW
AB220S
st
BAND B1 IF 58.05M
Fig.2 VHF receiver circuit
IC281
MCF
IF AMP
nd
BAND A 2 IF 450k
XF281 nd
Q282
2 MIXER AGC1/2 BPF0/1/2/3
+/-7.5kHz
57.6M
BPF(VCAP)
RF AMP
D182/184 D185
Q178
RF SW
OAGCA OBPFA AVS
RF AMP BPF(VCAP)
D192
Q193 D188-191
RF SW
OAGCB OBPFB/APC BVS
IFBUF
MCF
nd
BAND B 2 IF 450k
XF282
nd
PGA0 2 MIXER AGC1/2 IFBPF
+/-7.5kHz
57.6M
IC284
(To MAIN MPU)
DISCRI LPF
(To CODEC IC)
LIMITER
NOISE RECTIFIER
(To MAIN MPU)
NOISE AMP
rd
BAND B 3 IF 10.8k
rd
3 MIXER
IFBUF(LPF)
IC285
ATT IC286
(To CODEC IC)
IFOUT
460.8k
(To MAIN MPU)
DISCRI
(To CODEC IC)
LIMITER
NOISE RECTIFIER
(To MAIN MPU)
NOISE AMP
SMA
AFA
SQA
IFB
SMB
AFB
SQB