Description Of Major Ics - JVC AX-Z1010TN Service Manual

AX-Z1010TI
Description of Major ICs
@ TORX172 (J101): Optical Receiving Module
(1) Circuit Configuration
Optical
Digital
Input
) a SND (4) no
—|> 3) Vec
Amplification circuit
GND
Comparator
Output
—_
Reference voltage
generation circuit
Peak hold circuit
(2) Circuit Description
When an optical is input to the Si-PIN photodiode, a current
flows with a sensitivity of 0.3 A/W (xp= 650 [nm])
or less. This current is impedance-converted and amplified by
the amplifier circuit, and the resulting signal voltage
is input to the comparator.
On the other hand, the reference voltage of the comparator
is given by the ATC (Automatic Threshold Control) circuit.
The ATC circuit is made up of a peak hold circuit which
detects the peak value of the input voltage and holds
this peak value for a certain period. The period during which
the peak value is held is known as the ''time cons-
tant'. It is se to 1—3 usec in case of 'Toslink'.
The signal voltage from the amplifier circuit is divided
in two by a resistor and is input to the peak hold circuit.
Thus, the comparator performs a comparison between
the output voltage of the amplifier circuit and the peak
value that is 1/2 the output voltage.
By virtue of this, the comparator output can accurately reproduce
the signal transmitted from the optical transmis-
sion module of the transmitter at any time, even when
the optical! input varies.
Moreover, since the reference voltage generation circuit
is provided to keep the output voltage at the same
level
as the voltage output of the amplifier circuit when there
is no optical input, so that the reference voltage varies
according to the temperature drift in the amplifier circuit
to minimize the change in property due to the tempera-
ture variation.
In addition, a constant current power supply is provided and
the reference voltage of the comparator is set slightly
higher than the output voltage of the reference voltage
generation circuit so that the transmission is made ac-
curately even under the condition that there is no optical
input for a long period.
(No, 20115} 1-23