Modulation and demodulation of plural channels using analog and digital components
First Claim
1. Apparatus for demodulating a carrier signal having a plurality of channels, occupying separate frequency bands, the apparatus including a constant frequency analogue oscillator for producing an oscillation, analogue combining means for combining the oscillation with the carrier signal to produce an intermediate frequency signal carrying the channels in separate frequency bands of lower frequencies than in the carrier signal, analogue to digital conversion means for converting the intermediate frequency signal into a first digital signal, and a channel processing unit comprisinga digital oscillator, responsive to a channel select signal, for producing samples of a substantially periodic signal of the frequency of the selected channel in the first digital signal or a sub-multiple of that frequency,and digital combining means for combining samples of the substantially periodic signal with samples of the first digital signal so as to produce a demodulated signal.
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Accused Products
Abstract
In the modulation and demodulation of a plurality of frequency separated channels on a radio frequency carrier by digitally coded speech or data, the speech or data is modulated on a digitally generated sub-carrier by quadrature phase shift keying and after conversion to analogue form the modulated sub-carrier is mixed with an RF carrier of fixed frequency to produce the signal for transmission. Reception and demodulation of the transmitted signal are effected by the reverse processes. Frequency multiplication is effected after the digital to analogue conversion by producing analogue samples of very short duration and applying them to a suitable filter. Frequency division during the analogue to digital conversion is effected by sub-sampling.
134 Citations
25 Claims
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1. Apparatus for demodulating a carrier signal having a plurality of channels, occupying separate frequency bands, the apparatus including a constant frequency analogue oscillator for producing an oscillation, analogue combining means for combining the oscillation with the carrier signal to produce an intermediate frequency signal carrying the channels in separate frequency bands of lower frequencies than in the carrier signal, analogue to digital conversion means for converting the intermediate frequency signal into a first digital signal, and a channel processing unit comprising
a digital oscillator, responsive to a channel select signal, for producing samples of a substantially periodic signal of the frequency of the selected channel in the first digital signal or a sub-multiple of that frequency, and digital combining means for combining samples of the substantially periodic signal with samples of the first digital signal so as to produce a demodulated signal.
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11. Apparatus for demodulating a carrier signal having a plurality of channels, occupying separate frequency bands, the apparatus including an analogue oscillator for producing an oscillation, combining means for combining the oscillation with the carrier signal to produce an intermediate frequency signal carrying the channels in separate frequency bands of lower frequencies than in the carrier signal, analogue to digital conversion means for converting the intermediate frequency signal into a first digital signal, the analogue to digital conversion means having a sampling rate such that the first digital signal carries the channels in separate frequency bands of lower frequencies than in the intermediate frequency signal, and a channel processing unit comprising
a digital oscillator for producing samples of a substantially periodic digital signal, and digital combining means for combining samples of the substantially periodic digital signal with samples of the first digital signal so as to produce a demodulated signal.
- 12. A method of demodulating a carrier signal having a plurality of channels, occupying separate frequency bands, the method including combining the carrier signal with a local oscillation so as to produce an intermediate frequency signal carrying the channels in separate frequency bands of lower frequencies than in the carrier signal, sampling the intermediate frequency signal and converting the samples into digital form so as to form a first digital signal and combining the first digital signal, with a substantially periodic signal in digital form, of the frequency of a selected channel in the first digital signal, or a sub-multiple of that frequency, so as to produce a demodulated signal.
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21. An oscillator which, in use, produces alternately, at times determined by a clock signal, samples of two different phases of a periodic waveform, the oscillator comprising a memory containing a plurality of addresses at which are stored values representing the amplitude of the waveform at points distributed over at least a part of a period of the waveform during which the waveform assumes all possible values of the amplitude of the waveform and addressing means for reading selected ones of the stored values from the memory, the means including register means for storing an address signal for application to the memory, an adder connected to add the value of the address signal in the register means to the value of an increment signal to produce the value of the next address signal, and to store the next address signal in the register means, and increment means connected to provide as the increment signal alternately a first value representing the phase shift between the two phases and a second value representing the phase shift between the two phases in the opposite sense plus a value representing the phase increment between consecutive samples of each phase of the waveform produced by the oscillator.
- 22. Apparatus for producing a modulated carrier signal, including a digital oscillator for producing samples of a periodic subcarrier waveform having a frequency determined by a channel select signal, digital combining means for combining the samples of the subcarrier waveform with a modulation signal in digital form to produce a first digital signal carrying the modulation signal in a band at the frequency of the subcarrier frequency, digital to analogue conversion means for converting the first digital signal into a first analogue signal, a constant frequency analogue oscillator for producing a first carrier signal, and mixing means for combining the first analogue signal with the carrier signal to form the modulated carrier signal.
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23. Apparatus for producing a modulated carrier signal including, a plurality of digital oscillators each arranged to produce samples of a subcarrier waveform, the subcarrier waveforms produced by the oscillators having different frequencies determined by respective channel select signals, a plurality of digital combining means for combining the individual subcarrier waveforms with respective modulation signals in digital form to produce respective modulated subcarrier waveforms, addition means for adding together all the modulated subcarrier waveforms to form a first digital signal, digital to analogue conversion means for converting the first digital signal to a first analogue signal, a constant frequency analogue oscillator for producing a carrier signal, and mixing means for combining the first analogue signal with the carrier signal to form the modulated carrier signal.
Specification