Generation of electric signals having approximately sinusoidal waveforms

Claim 2

In a method of generating an electric signal having an approximately sinusoidal waveform, said method being of the kind comprising the steps of (A) frequency dividing a first train of pulses by a factor to derive a second train of pulses having a lower pulse repetition frequency, (B) utilizing the second train of pulses to address in turn locations of a digital store which stores in digital form a plurality of quantized amplitudes of a sine wave, and (C) deriving from the signals supplied by the digital store an output signal having an approximately sinusoidal waveform, the improvement comprising: (D) making a plurality of progressive alterations to the factor by which the frequency of the first train of pulses is divided to form the second train of pulses so as to change the fundamental frequency of the output signal.

Claim 3

In a generator for generating an electric signal having an approximately sinusoidal waveform and selectively one or other of two fundamental frequencies, said generator being of the kind comprising (A) a source for supplying a first train of pulses having a repetition frequency F, (B) a divider circuit which is arranged to derive from the first train of pulses a second train of pulses having a repetition frequency F/N, where N is an integer, (C) a digital store which stores in digital form a plurality of quantized amplitudes of a sine wave, (D) a counter which is arranged to respond to said second train of pulses and which supplies signals to address in turn the locations of said digital store at which are stored the quantized sine wave amplitudes, and (E) a digital to analogue converter which is arranged to respond to the digitized sine wave amplitudes supplied by the digital store so that when, during use, the digital store is addressed as aforesaid the converter supplies an output signal having an approximately sinusoidal waveform, the improvement comprising: (F) means operable when the fundamental frequency of the output signal is to be changed to control said divider so that the divisor N is progressively altered in a plurality of steps.

Claim 4

In a generator for generating an electric signal having an approximately sinusoidal waveform and selectively one or other of two fundamental frequencies, said generator being of the kind comprising (A) a source for supplying a first train of pulses having a repetition frequency F, (B) a divider circuit which is arranged to derive from the first train of pulses a second train of pulses having a repetition frequency F/N, where N is an integer, (C) a digital store which stores in digital form a plurality of quantized amplitudes of a sine wave, (D) a counter which is arranged to respond to said second train of pulses and which supplies signals to address in turn the locations of said digital store at which are stored the quantized sine wave amplitudes, and (E) a digital to analogue converter which is arranged to respond to the digitized sine wave amplitudes supplied by the digital store so that when, during use, the digital store is addressed as aforesaid the converter supplies an output signal having an approximately sinusoidal waveform, the improvement comprising: (F) means operable when the fundamental frequency of the output signal is to be changed to control said divider so that the divisor N is progressively altered in a plurality of steps.

Claim 5

The improvement in a generator as claimed in claim 2 in which the time over which the divisor N is altered is not less than the period of one cycle of the output signal at the lower frequency thereof.

Claim 6

The improvement in a generator as claimed in claim 2 in which the counter is formed of two interconnected four-stage binary counters.

Claim 7

The improvement in a generator as claimed in claim 2 in which the divisor N is identified by a group of binary signals.

Claim 8

The improvement in a generator as claimed in claim 5 wherein eight binary signals identify the value of the divisor N at any time.

Claim 9

The improvement in a generator as claimed in claim 2 in which the digital store is a read only memory.

Claim 10

The improvement in a generator as claimed in claim 2 in which the digital store stores sixteen quantized amplitudes of a sine wave.

Claim 11

The improvement in a generator as claimed in claim 2 in which the divisor N is progressively altered in nine steps.

Claim 12

The improvement in a generator as claimed in claim 2 in which the repetition frequency F is 921.6 kiloherz while the lowest value of N is 128 and the highest value of N is 148.

Claim 13

The improvement in a generator as claimed in claim 2 in which the changes in the value of N follow an approximately raised cosine law.

Claim 14

The improvement in a generator as claimed in claim 2 in which the changes in the value of N follow an approximately linear law.