Extension Call Circuit - Sony IT-M708 Operation Manual

6-6. Extension Call
Extension call is realized by using a carrier out of sound/voice frequency range and AM
modulation is applied.
Specifications
1 CARRIER frequency
2 Modulation system
3 EXPANDER/COMPRESSER
4 Receiving Intermediate Frequency 40KHz
5 Receiving AGC
Transmitting circuit
Transmitting carrier is 281.25KHz and receiving carrier is 321.43KHz. These carriers are made by
CPU and AM modulation is made by OUTPUT TR(Q801) .AF signal passed through
COMPRESSER is amplified by OP AMP(IC802 1/4) and added to collector of OUTPUT
TR(Q801), then the carriers are made AM modulation.
Receiving circuit
Extension signal which is received from telephone line is mixed with transmission career at
transmitting/receiving circuit, and difference between transmission CARRIER and reception
CARRIER, 40KHz is taken out by filter and then inputted to AGC circuit which consists of
MC14007 (IC801-2/3,3/3) and go through AMP circuit that consists of MC14007 (IC801-1/3) of
26dB and then inputted to detector circuit that consists of OP-AMP (IC802-4/4) and DETECTOR
DIODE (D802).
Detector circuit output is divided into two routes. One which DC level is sifted by SHOTTOKEY
DIODE(D806) pass through LPF(fc=3Hz) then DC component is picked out and amplified by
OP-AMP(IC802-3/4), and then it turns direct current which corresponds to CARRIER level and
then AGC control signal.
Other detector circuit output is passed through LPF AMP(IC802-2/4, fc=3.4KHz) and CARRIER
component is removed, and AF component is taken out and inputted to EXPANDER
circuit(IC803).
Transmitting : 281.25KHz, Receiving : 321.43KHz
Made of CPU TIMER frequency.
AM modulation
Standard modulation ratio:30%
equipped
equipped
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