The Ericsson Group 1968 - History of Ericsson - History of Ericsson

Fig. 2
Insertion of tone generator in telephone set
circuit
52
The two groups of frequencies are produced by two oscillators which together
form the tone generator (fig. 2). The frequencies for group I are generated
by an oscillator consisting of IC2, TL, Rrl, V\, CL, CI. The frequencies
for group II are generated by IC 3, Tn, Rell, V2, Cu. C2. The components
Ri, D\, D2, D3, Rz and VI are common to the two oscillators.
The resonant circuits are connected so as normally to store energy from the
telephone line, and this energy is released only when a push-button is actuated
(shock excitation).
When a button is pushed by the subscriber, the tuning capacitors C, and Cu
are connected via contacts K\ and K2 to the desired outputs from the
resonant circuits so as to obtain the correct frequency combination. This is
followed by a make-before-break contact action (K3). The microphone is
disconnected and the oscillator connected to the speech transformer. Since the
resonant circuits are loaded with diodes D2 and £>3, the oscillators do not
oscillate until A'3 disconnects these diodes. Energy stored in the resonant circuits
is then released and the oscillators start near full amplitude. This shortens
the starting time. The amplitude is limited by means of the two voltagedependent
resistors V\ and V2 (silicon varistors).
The oscillations are then extended via K3, D2 and D3 to the speech transformer
and out on the line.
The purpose of the components V3 and Rz is to limit the telephone set D.C.
voltage and at the same time regulate the output level from the oscillator
according to a desired characteristic as function of the line current. V3 is composed
of two silicon varistors in series, each being shunted by a resistance.
The diode D\ and resistor Ri keep the emitter current for the respective
transistors constant irrespective of the telephone set D.C. voltage.