
5.
CIRCUITDESCRIPTIONS
FM Front End
A unitized variable capacitor type front end
unit is used consisting of an FET RF single stage
amp, local oscillator/mixer IC and an IF trans-
former.
FM lF Amp, DetectorandMPX Circuit
A 2-transistor IF amp with ceramic filter is used
and the next stage
hasan IC (M51533L) contain-
ing the FM IF detdctor andMPX circuit. The block
diagram
of IC M51533L is shownin Fig. 5-1.
This
IC uses a peak detection method which can be
adjusted with a single coil. The PLL method is
employed to reconstitute the 38kHz sub-carrier
for the MPX circuit.
The IF signalis input at pin 1 andthe detection
output is obtained from pin 5. The detection out-
put passes
through the muting Q4, goes
to pin 9,
passes through the MPX circuit and the stereo
signals are obtained frorn pins 11 and 1'2. In
addition, this unit detectsthe presence
or absence
IL OUT ROUT
of a pilot signal to automatically switch between
stereo and mono reception by R14 which is con-
nected to pin 14. When a stereosignalis received,
pin 13 drops to the low level to light the stereo
indicator. When pin 2 is grounded, the PLL VCO
and FM IF operation stop (for AM).
FM Mutingand LED Driver
With this unit, muting is automatically activated
when the antenna input drops below 10pV. As the
antenna input decreases, the voltage at pin 3
drops, Q5 is turned off and Q6 is turned on. As a
result, the gate voltage of Q4 drops, Q4 is turned
off and the detector output circuit is blocked
(Fig. 5-1). When an FM or AM station is tuned
in, the base
potential of Q5 is raised,Q5 isturned
on and the tuning indicator lights.
nrtr F?? |
OETECTOR
INPUT
BUFFER
LEVEL
METER
CIRCUIT
S
TER
EO
5W
rTC
H
Fig.5-1 FMlF,Detectorand
MPX
circuit
w
w
w
.
x
i
a
o
y
u
1
6
3
.
c
o
m
Q
Q
3
7
6
3
1
5
1
5
0
9
9
2
8
9
4
2
9
8
T
E
L
1
3
9
4
2
2
9
6
5
1
3
9
9
2
8
9
4
2
9
8
0
5
1
5
1
3
6
7
3
Q
Q
TEL 13942296513 QQ 376315150 892498299
TEL 13942296513 QQ 376315150 892498299
http://www.xiaoyu163.com
http://www.xiaoyu163.com