Direct digital frequency synthesis enabling spur elimination
First Claim
1. A method of synthesizing an analog signal, comprising:
- using digital logic to produce a first digital bit stream in response to a numeric frequency;
sampling an analog frequency signal to produce a second digital bit stream; and
combining the first and second bits streams to produce a digital signal representing at least one of a frequency difference and a phase difference between the numeric frequency and the analog frequency.
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Accused Products
Abstract
The present invention, generally speaking, provides improved methods of generating clean, precisely-modulated waveforms, at least partly using digital techniques. In accordance with one aspect of the invention, a "difference engine" is provided that produces a digital signal representing the frequency error between a numeric frequency and an analog frequency. The frequency error may be digitally integrated to produce a digital signal representing the phase error. The difference engine may be incorporated into a PLL, where the analog frequency is that of an output signal of a VCO of the PLL. Direct modulation of the PLL output signal may be performed numerically. By further providing an auxiliary modulation path and performing calibration between the direct modulation path and the auxiliary modulation path, modulation characteristics may be separated from loop bandwidth constraints. In particular, the loop bandwidth of the PLL may be made so low as to reduce spurs (usually associated with DDS techniques) to an arbitrarily low level. A loop filter of the PLL may be realized in digital form. Using a digital loop filter would ordinarily require use of a high-resolution DAC. Various techniques are described for reducing the resolution requirements of the DAC.
82 Citations
33 Claims
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1. A method of synthesizing an analog signal, comprising:
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using digital logic to produce a first digital bit stream in response to a numeric frequency; sampling an analog frequency signal to produce a second digital bit stream; and combining the first and second bits streams to produce a digital signal representing at least one of a frequency difference and a phase difference between the numeric frequency and the analog frequency. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
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12. A frequency synthesis circuit comprising:
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digital logic for producing a first digital bit stream in response to a numeric frequency; means for sampling an analog frequency signal to produce a second digital bit stream; and means for combining the first and second bits streams to produce a signal representing at least one of a frequency difference and a phase difference between the numeric frequency and the analog frequency. - View Dependent Claims (13, 14, 15, 16, 17, 18, 19, 20, 21, 22)
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23. A phase lock loop, comprising:
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means responsive to an analog waveform having a frequency attribute and an input bitstream representing a desired waveform for producing an output bitstream representing a difference quantity between the analog waveform and the desired waveform; a filter and a digital to analog converter cooperating to produce a filtered analog difference signal; a controlled oscillator that produces the analog waveform; and a first capacitor coupling the filtered analog difference signal and an input terminal of the controlled oscillator. - View Dependent Claims (24, 25, 26, 27, 28, 29, 30, 31, 32, 33)
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Specification