Frequency synthesis circuit tuned by digital words
First Claim
1. A frequency synthesis circuit whose output frequency is determined by input digital words comprising,an input line receiving the input digital words for tuning the output frequency of the frequency synthesis circuit,a local oscillator producing a local oscillator signal having a local oscillator frequency,a delay line connected to the local oscillator having a plurality of selectable delays corresponding to a plurality of incremental phase shifts relative to the local oscillator signal, spanning one cycle of the local oscillator frequency,a multiplexer connected to receive the plurality of selectable delays from the delay line to produce an output signal corresponding to the local oscillator frequency with a selected phase delay as indicated by a digital word,a modulo accumulator connected to the input line to receive the input digital words determinative of desired output frequency and to transmit corresponding accumulator overflow signals to a second accumulator producing second digital words transmitted to the multiplexer, whereby the multiplexer selects a delay corresponding to the accumulator overflow signals which depends on the input digital words,whereby the local oscillator frequency, repeatedly shifted by unit phase delay corresponding to said input digital words, forms a synthesized output waveform whose output frequency depends on the input digital words.
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Abstract
A direct digital frequency synthesizer featuring an accumulator having a modulo overflow signal addressing a multiplexer. The multiplexer receives a series of delay signals generated from digital circuits. The delay signals establish the phase of a reference oscillator. The number of units of delay are sufficient to resolve expected jitter. The accumulator is a digital counter which increments by only a single digit for each count, such as a Gray code counter. In one embodiment, the delay signals are generated by a charge pump feeding individual logic circuits driving integrated capacitors in a loop. Feedback to the charge pump establishes that the total delay will subdivide a single clock cycle of the reference clock. In a second embodiment, a single shifter or several shifters, with output in phase reversal relation, subdivide a single clock cycle. A clock multiplier and divider are used to assure the synchronism of each clock cycle with the total number of units of delay. The output of the multiplexer is the reference oscillator signal, adjusted by the phase delay, forming a synthesized output frequency.
57 Citations
31 Claims
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1. A frequency synthesis circuit whose output frequency is determined by input digital words comprising,
an input line receiving the input digital words for tuning the output frequency of the frequency synthesis circuit, a local oscillator producing a local oscillator signal having a local oscillator frequency, a delay line connected to the local oscillator having a plurality of selectable delays corresponding to a plurality of incremental phase shifts relative to the local oscillator signal, spanning one cycle of the local oscillator frequency, a multiplexer connected to receive the plurality of selectable delays from the delay line to produce an output signal corresponding to the local oscillator frequency with a selected phase delay as indicated by a digital word, a modulo accumulator connected to the input line to receive the input digital words determinative of desired output frequency and to transmit corresponding accumulator overflow signals to a second accumulator producing second digital words transmitted to the multiplexer, whereby the multiplexer selects a delay corresponding to the accumulator overflow signals which depends on the input digital words, whereby the local oscillator frequency, repeatedly shifted by unit phase delay corresponding to said input digital words, forms a synthesized output waveform whose output frequency depends on the input digital words.
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13. A frequency synthesis circuit whose output frequency is determined by input digital words comprising,
an input line receiving the input digital words, each word being K bits wide and establishing control of the phase of a local oscillator, a modulo-2L accumulator connected to the input line to receive the input digital words therefrom, where L is the size in bits of said accumulator, and whose output is connected to a threshold circuit wherein the threshold circuit establishes an absolute threshold value of the accumulator and provides an accumulator overflow signals whenever the output of the accumulator exceeds the absolute threshold value, the accumulator also having a clock terminal, a delay line connected to the local oscillator having a plurality of selectable delays corresponding to a plurality of incremental phase shifts relative to the output frequency of the local oscillator, spanning one cycle of the local oscillator frequency, a multiplexer connected to receive the plurality of selectable delays from the delay line and to the output of the threshold circuit to produce an output signal corresponding to the local oscillator frequency with a selected phase delay as incremented by the accumulator overflow signal, the output of the multiplexer fed back to the clock terminal of the accumulator, whereby the local oscillator frequency, repeatedly shifted by selected amounts of delay corresponding to the input digital words, forms a synthesized output waveform whose frequency depends on said input digital words.
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21. A frequency synthesis circuit whose output frequency is determined by input digital words, comprising:
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an input line receiving said input digital words determinative of the output frequency; a modulo accumulator coupled to the input line for integrating the values of the input digital words, the accumulator providing successive phase selection values on an output thereof; a local oscillator producing an oscillator signal having a reference frequency; a delay line connected to the local oscillator to receive the oscillator signal therefrom, the delay line producing a plurality of versions of the oscillator signal with different delays, the delays corresponding to a plurality of incremental phase shifts of the received oscillator signal at the reference frequency, the plurality of incremental phase shifts spanning one cycle of the oscillator signal at the reference frequency; and a multiplexer connected to the delay line to receive the plurality of phase shifted versions of the oscillator signal therefrom, the multiplexer having a control input connected to the output of the accumulator to receive successive phase selection values therefrom, the multiplexer repeatedly selecting one of said plurality of phase shifted versions of the oscillator signal as an output of the circuit in accord with the successive phase selection values, whereby the repeatedly phase shifted selections form a synthesized output frequency. - View Dependent Claims (22, 23, 24, 25, 26, 27, 28, 29, 30, 31)
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Specification