electronic telephone sets coaxial cable systems - The history of ...

6
Fig. 5
Frequency generation for pushbutton dialling
AD 10-bit counter
BD 9-bit counter
FFA Flip-flop filter, frequency group A
FFB Flip-flop filler, frequency group B
LPA Low-pass filter, frequency group A
LPB Low-pass filter, frequency group B
each counter. In this way a frequency
stability of 0.15 % is obtainable for the
necessary pushbutton dialling frequencies.
The filtering of the output can be achieved
in different ways, for example with
a low-pass filter on each oscillator output.
With an active low-pass filter, three
capacitors and three resistors, the
overall distortion can be kept below
5 °/o. The output level for the oscillator
is then determined by the amplifier
which connects the two oscillators to
the line.
The introduction of audio frequency
pushbutton dialling systems, except in
the USA, has hitherto taken place only
on a limited scale. In parallel with field
trials of audio frequency pushbutton
dialling systems, decadic electronic
pushbutton dialling systems for resistance
diode dialling have been tried out
on a large scale by, among others, the
Swedish Telecommunications Administration.
"Dial impulses" generated
electronically
The earlier mentioned development of
the MOS technique has opened the way
to electronic generation of DC pulses
within the frequency interval 10—20 Hz,
i.e. "dial pulses". The first generation
of decadic electronic pushbutton dialling
units for generation of dial pulses
were fed from a chargeable battery.
The battery either received a continuous
charge, about 2 mA, during ringing
(parallel charge) or, during conversation,
from the line current of the telephone
set (series charge). The power
requirement for the available binary
storage and counting circuits has successively
diminished, so that pushbutton
units for dial pulses can be constructed
today for feed from the line
current with auxiliary current from a
buffer capacitor (1,000—300 /(F). A
block schematic of L M Ericsson's batteryless
electronic pulsing unit is
shown in fig. 6. The frequency selective
element of the oscillator unit consists
of a 3-component RC network. The
oscillator generates a square wave
with frequency 20 kHz. The generated
voltage pulses serve both as clock signal
generator and feed voltage for the
storage and logic unit. The feed voltage
(approx. 15 V) is obtained through the
fact that theoscillatorgenerator passes
a 3-step cascade generator of diodes
and capacitors.
The storage and logic unit consists of
code converters for conversion of
pushbutton code to 4-bit binary code,
four parallel working 20-bit shift registers
for storage of up to 20 digits, a
count-down chain for division of 20
FFB