Philips LX3950W/01 Service Manual page 74

LNA and mixer
+8V
F710
F710
MIXER
+8V
10.7 MHz
5615
3 4
LNA
2
1 6
3612 2607
7602
47R BFR92A
1p5
7601
BFC520 2602
5614
8n2
2603 2641
33p 12p
Figure 1-21 Mixer circuit
The LNA is a cascode configuration around the 7601. This
circuit provides gain with a low noise figure while providing
isolation from the oscillator to antenna. This isolation is
necessary to comply with the legal requirements for radiated
interference. The output of the LNA and the LO signal is mixed
in 7602. This is a one transistor mixer that provides high
conversion gain with low LO input signal. This level is only
around 20 mV. The reason for this high conversion gain is that
there is a 10.7 MHz trap at the base, 5605-2610.
The output of the mixer is a tuned circuit at 10.7 MHz. The coil
is pre-aligned at the factory.
The FM IF radio circuit
RECEIVER/BACKEND
2709
1n5
2708
3712
10K
47n
2707
2710
10u 16V 470p
3713
2K2
7710
3741 LA1836M
3742
100R
4k7
AM AM
OSC RF-AMP AGC
MUTE
ALC
BUFF
AM AM AM
IF DET
LEVEL MIX
F708
S-METER
F708
AM
FM
IF
BUFF COMP
S-CURVE TUNING
DRIVE
I.F. RADIO LEVEL
DET
FM
FM IF DET REG GND
Figure 1-23 Receiver / backend circuit
FM Detector
The IF signal is demodulated in the 7710, LA1836M the IF
radio IC. This IC has a coil FM detector that is aligned. In the
alignment instructions this adjustment has been given. The
stereodecoder is integrated and needs no adjustment. The
demodulated signal is filtered with 1730-1740. These filters are
pre-aligned and are reducing the pilot- and subcarrier fre-
quency. This is necessary to prevent wrong operation of the
expander circuit.
11-24
IF amplifier
+8V
IF
F709
F709 IF AMP
CL36532008_081.eps
220403
1706
IF
3709
7707
I O
BFS20
180R
GND
SFELA
10M7
Figure 1-22 IF amp circuit
The mixer output signal is filtered by two ceramic IF filters. An
IF amplifier 7707 is reducing the loss introduced by these
filters.
1730 1740
6 3 3745 6
O I O
4 1 4
NC GND NC GND
5 2 5
FXM2 FXM2
16V 10u
2720
16V 10u
2721
DECODER
ANTI BIRDIE
STEREO
SWITCH
STEREO
DRIVE
MUTE
F715 F714
TRIG
VCO FF FF FF
F715 F714
AM
FM
PHASE PILOT
SW
DET DET
AM
FM
VCC
+8V
1u
+8V
2725
CL36532008_083.eps
+8V
P20
BF545A
7705
P21
1707
I O
GND
SFELA
10M7
CL36532008_082.eps
290403
Figure 1-24 Receiver / backend circuit
There is a sliding stereo circuit for reducing the stereo noise at
F715 F714
3 3746
I
1
2
50V
The audio compander uses a device from SA572 (7770) that is
intended for high-end applications. It is to be used together with
290403
a low noise opamp (7780).
There is an expanding factor of 2 in dB. There is a variable gain
cell (variable resistor) in the negative input of the opamp
(NJM4565M). There is a rectifier cell that detects the input
voltage and translates into a current send to the gain cell .The
rectifier has an attack time constant and a decay timeconstant,
which is optimised to give the best auditive result.
If the input Vin rises with e.g. 2 times then the output rise 4
times. This is because the negative input resistor formed by the
gain cell is decreased with a factor of 2.
Remember for an opamp Vout = Vin* ((Rf+Rin)/Rin) where Rf
is constant . (Rin= internal gain cell resistor)
11-24
low signal fieldstrengths. 7705 is short circuiting Audio left and
Audio Right at low fieldstrengths. The level where the crossover
F747
happens is aligned. In the alignment instructions this adjust-
F747
ment has been given.
3714
22K There is also a MUTE LEVEL alignment, which is set at 10 µV
7706
BC857B
RF input level. At this signal level the wireless audio link is
6711
muted.
BAS316
R S
CL36532008_084.eps
290403
The audio expander circuit
3794 3795
LO
+8V
RO
F707 3755
3796 3797
F707
3779
1K0
7770
SA572D 16
VCC
50V 2u2
7 GIA GOA 5
2u2 G
9 GIB GOB 11
2772
2780
2 RCA THTA 6
2776
10u 16V 2786
14 RCB THTB 10
BUFFER
2774 10u 16V
4 ACA
TRTA 1
1u 2778
12 ACB
TRTB 15
3772 50V 2u2 1u 3 RIA
3K3 RECT
3776 50V 2u2 13 RIB 3778
2773
3K3 1K0
GND
2777
8
Figure 1-25 Expander circuit
Microcontroller functions
The microcontroller is an OTP Microchip PIC12C508A type with
512-bit EPROM and 6 I/O lines. The controller has to be
programmed on the production line according the required
version of 864, 914 MHz and 433 MHz.
The main functions of the uP are reading the setting of the 4
position slide switch within every 250 milliseconds and read the
Tuned info continuously and start the powersave mode if
necessary.
2782
4n7
RO
3781 3782
18K 47K
F712
F712
4
6
50V 2u2 3783
7 1K8
2783
NJM4565M
5 8
7780-B
+8V BC847B
2788
4n7
LO
3787 3788
18K 47K
F713
F713
4
2
50V 2u2 3789
1
1K8
NJM4565M 2789
3 8
7780-A
+8V
CL36532008_085.eps
290403