Method and circuit for generation of digitally frequency-shiftable electric signals

Claim 1

A frequency-switchable wave generator suitable for use in transmitting coded information by frequency shift including an oscillator, a frequency-switchable frequency divider for dividing the frequency of the output of the oscillator by a switchable ratio, a repetitive function generator circuit for converting the output of the frequency divider to a wave of a predetermined shape and an output stage responsive to the output of the repetitive function generator circuit in which the improvement is embodied that: the repetitive function generator circuit (55) contains a counter (87) having its stepping input connected for response to a signal derived from the output of said frequency divider and connected by digital combinational logic means (90 to 108) to an array of resistors (R 1 to R 5 ) energized in parallel through an additional resistor (R 6 ) on which a d.c. voltage of predetermined value is applied, so as to produce at a first output (109), at the connection between said additional resistor (R 6 ) and said resistors energized in parallel (R 1 to R 5 ), a stepped periodic voltage wave of which the periodicity depends upon the stepping frequency of the counter (87), and which consists of an alternately rising and falling succession of intervals of a set of non-zero voltage levels of the same polarity of which the lowest and the highest are maintained for two-count intervals, approximating a succession of unipolar halves of a bipolar sinusoidal wave that passes through zero in the middle of the two-count interval of said lowest voltage level; said ouput stage includes a coupling amplifier circuit (110,111) to the input of which said first output of said function generator circuit is connected and to the output of which is connected an output transformer having two mutually coupled energizing winding halves (112,113) across which the stepped periodic voltage wave produced by the repetitive function generator circuit (55) is caused to appear, and said output transformer is interconnected with low pass filtering means (114, 115) and with gating means (116,117) respectively connected to said winding halves (112,113) and with an alternating gating wave generator (120) responsive to said second output of said function generator so as to provide alternating polarity reversal in such a way that for the duration of a first period of the stepped periodic voltage wave a current is caused to flow through a first one of said winding halves (112) and for the duration of the next period of the stepped periodic voltage a current is caused to flow through the second of said winding halves (113), and so on in alternation.

Claim 19

A method of generating frequency-shift code signals for a system for automatic exchange of coded information, with an oscillator shiftable between predetermined frequencies, a repetitive function generator circuit and an output stage, comprising the steps of: generating electrical digital signals by means of said oscillator at at least two predetermined frequencies in a sequence of single-frequency emissions; producing, by using current from a direct current source, in said function generator circuit, in response to said digital signals, a repeated stepped wave formed of a succession of intervals of a constant non-zero voltage level of the same polarity of which the highest and the lowest levels are maintained for twice as long as each of the others and the different voltage levels succeed each other in steps alternately rising from the lowest to the highest and descending from the highest to the lowest; reversing the polarity of the wave produced in the preceding step at the middle of each interval of said lowest voltage level to produce an output wave; filtering the wave produced in the preceding step to smooth out the stepped structure, and shifting the frequency at which said electrical digitalsignals are generated from time to time but only at times coinciding with a polarity reversal which reverses the polarity of said output wave in a predetermined sense.

Claim 20

A method of generating frequency-shift code signals for a system for automatic exchange of coded information, with an oscillator shiftable between predetermined frequencies, a repetitive function generator circuit and an output stage, comprising the steps of: generating electrical digital signals by means of said oscillator at at least two predetermined frequencies in a sequence of single-frequency emissions; producing, by using current from a direct current source, in said function generator circuit, in response to said digital signals, a repeated stepped wave formed of a succession of intervals of a constant non-zero voltage level of the same polarity of which the highest and the lowest levels are maintained for twice as long as each of the others and the different voltage levels succeed each other in steps alternately rising from the lowest to the highest and descending from the highest to the lowest; reversing the polarity of the wave produced in the preceding step at the middle of each interval of said lowest voltage level to produce an output wave; filtering the wave produced in the preceding step to smooth out the stepped structure, and shifting the frequency at which said electrical digitalsignals are generated from time to time but only at times coinciding with a polarity reversal which reverses the polarity of said output wave in a predetermined sense.