COHERENT DIGITAL DEMODULATOR

• US 3,761,829 A
• Filed: 06/01/1972
• Issued: 09/25/1973
• Est. Priority Date: 06/01/1972
• Status: Expired due to Term

• Current Assignee: David Adams Spaulding
• Original Assignee: David Adams Spaulding
• Inventors: Spaulding, David Adams
• Primary Examiner(s): Brody, Alfred L.
• Application Number:
  US05/258,609
• Time in Patent Office: 481 Days
• Field of Search: 329/50,104,109,122 328/133,115,117,151 325/329,330,331,320,324
• US Class Current: 329/311
• CPC Class Codes: H03K 9/02 of amplitude-modulated pulses
• CPC Class Codes: H04B 14/023 using pulse amplitude modulation

Abstract

A digital coherent demodulator for signals pulse-amplitude modulated in the passband of data transmission systems multiplies sampled values of quadrature-related components of received datachannel signals by the respective sampled values of sines and cosines of the demodulating carrier wave and combines these two products to form a sampled baseband signal from which the transmitted data is recoverable. All samples are taken at twice the highest baseband frequency rather than at twice the highest passband frequency. The usual requirement for a low-pass filter to eliminate double-frequency components generated in the conventional demodulation process is also avoided.

Claims

1. A coherent digital demodulator for a signal wave whose modulating baseband signal is bandlimited to a fixed maximum frequency comprising a Hilbert transformation circuit for rotating all frequency components through 90 electrical degrees, a demodulating carrier-wave source providing cosine and sine output components simultaneously, first means for multiplying said signal wave by said cosine output component, second means for multiplying the Hilbert transformation of said signal wave from said transformation circuit by said sine output component, means for combining signals from said first and second multiplying means to form an output for said demodulator, and means for sampling said signal wave at a rate not exceeding twice said maximum frequency to constitute said demodulator output signal in digital form.

2. The coherent digital demodulator defined in claim 1 in which said Hilbert transformation circuit is A wideband 90* phase shifter.

3. The coherent digital demodulator of claim 1 in which said sampling means operates on said signal wave prior to demodulation in said first and second means.

4. The coherent demodulator of claim 1 in which said sampling means operates on the output from said combining means.

5. The combination defined in claim 1 in which said multiplying means comprises a nonerasable memory storing selected sine and cosine values in digital form, an address register for providing access to the respective sine and cosine values stored in said memory, an analog-to-digital converter for transforming periodic samples of signal waves from said signal wave source into serial binary bit streams, a serial-parallel multiplier for operating on serial bit streams from said converter with selected sine and cosine values from said memory to form products, and an accumulator for combining pairs of consecutive products from said multipliers to form digital representations of said information signal.

6. In combination, a signal wave source supplying an information signal of limited frequency bandwidth modulated into a passband associated with a sinusoidal carrier wave whose frequency is at least equal to the maximum frequency in said information signal, a demodulating carrier-wave source associated with a receiver for said information signal providing cosine and sine output components simultaneously, means for multiplying said signal wave directly and after a quadrature phase shift of all frequency components by respective cosines and sines of a demodulating wave at said carrier frequency from said demodulating carrier-wave source, means for combining the products of said signal waves obtained in said multiplying means, and means in tandem with said multiplying and combining means for sampling said signal wave to form a digital output at a rate substantially equal to twice the maximum frequency in said information signal.

7. A coherent digital demodulator for a passband signal wave whose modulating baseband data signal is bandlimited to a fixed maximum frequency comprising means for delaying said passband signal by half the effective sampling interval substantially equal to the reciprocal of twice the highest frequency in said baseband data signal, means for forming the Hilbert transformation of said passband signal, an analog-to-digital converter, a timing source poviding a gating wave at twice the frequency of the effective sampling rate, means responsive to said gating wave for alternately applying the delayed passband signal from said delaying means and the Hilbert transform of said passband signal to said converter to form bit streams representing passband signal samples, a demodulating carrier wave source, a nonerasable memory for storing values of the sine and cosine trigonometric functions in digital form, an address register under the joint control of said timing and carrier-wave sources providing access to selected sine and cosine values in said memory, a serial-parallel multiplier having as one input passband signal samples from said converter and as another input selected sine and cosine values from said memory for products of said signal samples and respective sine and cosine values, and an accumulator for consecutive pairs of products from said multiplier for forming digital representations of said baseband signal wave.