Kenwood UBZ-LJ8 Service Manual page 3

■ IF Amplifier
The IF circuit consists of FM IC (IC2). This IC contains
local line input buffer, Mixer, FM DET and audio frequency
amplifier.
The First IF signal that comes from IF amplifier (Q11) then
goes into the FM IC mixer. It is mixed with second local fre-
quency (21.25MHz) that comes from the PLL IC reference
oscillation crystal so as to produce a second IF signal.
(21.7MHz – 21.25MHz = 2nd IF = 450kHz)
The 2nd IF signal then enters to ceramic filter (CF1) in or-
der to eliminate unwanted components. The filtered signal is
amplified by a limiter-amplifier, which consists of 6 differen-
tial amplifiers, to eliminate AM signal from 2nd IF.
The FM detector acts as a quadrature discriminator. The
phase of the output signal from the limiter is shifted by 90
degrees by a quad coil (L36). This output signal is compared
with the original non-shifted waveform to produce FM detec-
tion output.
BUSY
RSSI
IC208
MPU
SP AMP
■ AF Amplifier
The FM detection output signal passes through the base
band IC (IC209) which consists of band pass filter
(300Hz~3kHz), de-emphasis circuit and amplifier. Next, the
signal is passed to the volume control potential meter VR201
and then goes into the audio power amplifier (IC203) which
drives the speaker to produce audio output.
■ Loudness ON/OFF Circuit
Loud On/Off switching circuit is controlled by AF switch
(Q217,Q219,Q218). When Loud on, Q217 is On Q218 is Off.
When Loud off, Q217 is off Q218 is on.
When the SP/MIC option is inserted, Loud circuit be-
comes Off automatically.
CIRCUIT DESCRIPTION
1st IF input
3R
D11
1
Q13
2
Batt
Q216
IC203
AF AMP
AF IN SP
10
2nd local OSC
Q220
Fig. 4 IF amplifier and Squelch circuit
■ Squelch Circuit
A squelch circuit is provided to prevent interference from
noise or weak signal being output to the speaker during non-
transmitting state.
This circuit contains an amplifier (Q13) and a rectifier
(D11). The amplifier amplifies noise signals from FM detec-
tion output and passes them to a rectifier.
When there is no signal or weak signal being received, the
FM IC detection will output high frequency noise and the rec-
tifier will output high voltage level. When strong signal is
received, the FM IC detection noise is suppressed and the
rectifier output voltage level will be low.
MPU compares this rectifier output level to a predeter-
mined level so as to determine when to turn on the speaker.
Noise amplifier (Q13) and rectifier (D11) are used here in-
stead of FM IC internal circuit to prevent temperature toler-
ance.
(21.7MHz)
L36
Quad
coil
16 15 14 13 12
RSSI
MIX
Buff
IF AMP
OSC
1 2 3 4 5
21.25MHz
CF1
PLL IC
450kHz
IC209 : Base band IC
De-emphasis
20
RXAF
AMP
VR201
IC203
AF AMP
BTL
SP AMP
SP MUT
Fig. 5 AF amplifier and Loudness on/off circuit
UBZ-LJ8
3R AF OUT
11 10 9
LPF
QUAD DET
Noise
AMP
INV
6 7 8
IC2
FM IC
AMP
23
AFOUT
RXIN
BPF
Q217
C206
+
Q219
Q218
SP
3