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

Fig. 4
RC oscillator, circuit diagram
Fi oscillator amplifier, frequency group 1
Fi oscillator amplifier, frequency group 2
acts the rise of voltage across the feed
circuit of the oscillator. From the DC
point of view the oscillator exhibits a
variable resistance which on a short
line draws more current than would a
constant resistance. In this way one
obtains a DC characteristic for the
oscillator unit which is practically identical
to that of the transmission circuit,
so that the risk of transients owing to
sudden change of a voltage drop on
switching between telephone set circuit
and dialling circuit is eliminated.
The output impedance of the oscillator
and the levels for the two frequency
groups are selected individually and
are entirely independent of the current
regulating circuit.
Frequency generation by
digital technique
The development of the MOS (Metal
Oxide Silicon Gate) technique, result­
5
ing in digital counters and storage circuits
of LSI type with extremely small
power requirement for their function,
opens the way to the solution of frequency
generation for pushbutton dialling
by using digital technique.
The principle is shown in fig. 5. A crystal
oscillator, with a frequency of 1 MHz
passing through the two binary counters
AD and BD, of which AD is a 10-bit
counter and BD a 9-bit counter, gives
the larger counter a division factor up
1024. This results in a minimum frequency
of 976 Hz. For a 9-bit counter
the corresponding division factor is
512, giving a minimum frequency of
1953 Hz. By adding an extra flip-flop
in each output one obtains a division
factor of 2, which results in minimum
frequencies of 488 and 976 respectively.
Through the gates GA and GB and
pushbutton contacts each counter can
be zeroed after four division factors
within the range of the bit speed for