Philips LX 3700D Service Manual Download

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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.

IF amplifier

The mixer output signal is filtered by two ceramic IF filters. An
IF amplifier 7707 is reducing the loss introduced by these
filters.

The FM IF radio 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.

There is a sliding stereo circuit for reducing the stereo noise at

low signal fieldstrengths. 7705 is short circuiting Audio left and
Audio Right at low fieldstrengths. The level where the crossover
happens is aligned. In the alignment instructions this adjust-
ment has been given.
There is also a MUTE LEVEL alignment, which is set at 10 µV
RF input level. At this signal level the wireless audio link is
muted.

The audio expander circuit

The audio compander uses a device from SA572 (7770) that is
intended for high-end applications. It is to be used together with
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)

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.

Figure 1-21 Mixer circuit

Figure 1-22 IF amp circuit

Figure 1-24 Receiver / backend circuit

Figure 1-25 Expander circuit

Figure 1-23 Receiver / backend circuit

LNA and mixer

BUFFER

RECT

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2782

2788

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