Generation of electric signals having approximately sinusoidal waveforms

• US 4,346,448 A
• Filed: 11/23/1979
• Issued: 08/24/1982
• Est. Priority Date: 11/23/1978
• Status: Expired due to Term

• Current Assignee: GEC Plessey Telecommunications Limited
• Original Assignee: General Electric (China) Co., Ltd. (GE Aerospace)
• Inventors: Insam, Howard F., Laurenson, Stephen R.
• Primary Examiner(s): Ruggiero, Joseph F.
• Application Number:
  US06/096,841
• Time in Patent Office: 1,005 Days
• Field of Search: 364/721, 364/703, 364/607, 364/608, 328/14, 328/15
• US Class Current: 708/8
• CPC Class Codes: G06F 1/0321 Waveform generators, i.e. devices for generating periodical functions of time, e.g. direct digital synthesizers G06F1/0314, G06F1/035 take precedence
• CPC Class Codes: G06F 2101/04 Trigonometric functions
• CPC Class Codes: H04L 27/122 using digital generation of carrier signals digital function generators G06F1/02, H04L17/10; generating pulses having stepped portions using digital techniques H03K4/026

Abstract

For the purpose of generating an output signal having a sinusoidal waveform, a digital store stores at different locations binary-coded amplitudes at equally spaced instants of time throughout a complete cycle of a sine wave. Those locations are addressed in turn by means of a pulse counter and the signals supplied by the memory are passed to a digital/analogue converter from which the required output signal is derived. The pulse signal supplied to the counter is obtained from a train of clock pulses by a programmable divider. To effect a frequency change in the output signal (for the purpose of signalling binary data), the divisor of the divider is changed in a sequence of steps so that the overall change in output frequency is less abrupt than would otherwise be the case.

The arrangement described is suitable for use in a modem working with the Viewdata system currently adopted by the British Post Office.

Claims

1. 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.

2. 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.

3. 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.

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

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

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

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

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

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

10. 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.

11. 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.

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