Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Synthesizer
Patent
1993-09-13
1995-05-02
Callahan, Timothy P.
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Synthesizer
364721, 327129, G06F 102
Patent
active
054123380
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a frequency synthesizer for generating a sinusoidal analogue signal, and in particular direct digital synthesizer or numerically controlled oscillators.
BACKGROUND OF THE INVENTION
Conventional direct digital synthesizers (DDS) comprise a digital triangle wave generator whose output signal is fed to a look-up table in the form of a PROM to shape or smooth the digital triangle wave, the output from the look-up table being fed to a digital-to-analogue converter (DAC) having a linear transfer function which then generates an analogue output sine wave. The problem with these known synthesizer is their complexity and expense.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a frequency synthesizer for generating a sinusoidal analogue signal comprises means for generating a substantially triangular digital signal; and a digital-to-analogue converter which receives the digital signal and which has a non-linear transfer function shaped such that a sinusoidal analogue signal is generated.
We have developed a new approach in which the previously used PROM and DAC with a linear transfer function are replaced by a single DAC with a non-linear transfer function. This has the advantage that the fast PROM required in the conventional approach is no longer needed so saving chip area and cost.
Preferably, the non-linear transfer function generates a piece-wise linear approximation to a sine wave. The piece-wise linear approximation can be analysed as the sum of triangular pulse trains. For four segments there are four equations in cos (wt) so harmonics can be cancelled up to ninth order above which they will be small. Furthermore, as a sinewave moves from the zero crossing to the peak its slew rate decreases and the accuracy required reduces. This allows the complexity of the DAC to be reduced compared to a linear DAC improving yield and reducing chip area.
Preferably, the DAC comprises a number of subsidiary DACs; and control means responsive to the digital signal to activate different groups of the subsidiary DACs dependent on the segment of the sine wave being generated. This has the advantage that the individual segments or pieces can be trimmed by adjusting only the reference resistors of the subsidiary DACs, which is not interactive and should be easy to do in production. Furthermore, once the segments have been trimmed, monotonicity is guaranteed. In addition, it is very simple to interconnect DACs of differing resolution which is what is required for waveform synthesis.
In the preferred example, each subsidiary DAC comprises a conventional DAC with an additional switch so that the normally dumped current can be utilized. This is particular advantageous since only standard, binary DACs are required.
A further problem which can arise in conventional DDS systems is that since its amplitude is quantised (it can only change amplitude at integer multiples of the clock period) it effectively comprises a true sinewave plus an error signal. For practical reasons, the DAC resolution of both conventional DDS systems and systems according to the first aspect of the invention is limited and the digital sinewave (which can be very finely quantised and so have very little error) is truncated. The conventional truncation process completely loses the electrical information present in the least significant bits (LSB) and introduces errors in the form of non-harmonic, spurious tones which limit the dynamic range.
In accordance with a second aspect of the present invention, a frequency synthesizer for generating a sinusoidal analogue signal comprises means for generating a digital signal, each value of which has M bits; and a digital-to-analogue converter which receives the N most significant bits (MSBs) of the digital signal and generates in response an analogue sinewave signal characterised in that the synthesizer further comprises an adder between the digital signal generating means and the DAC, M-N least significant bits (LSBs) from the adder be
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patent: 4998072 (1991-03-01), Sheffer
patent: 5258937 (1993-11-01), Blackmon
patent: 5274373 (1993-12-01), Kanoh
Collier James D. Y.
Richards Matthew J.
Callahan Timothy P.
Cambridge Consultants Limited
Le Dinh
Smith Albert C.
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