Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Synthesizer
Reexamination Certificate
2000-08-31
2001-12-25
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Synthesizer
C106S157900
Reexamination Certificate
active
06333649
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to direct digital synthesizers (DDSS) and more particularly to efficient implementations of DDSs in programmable logic devices.
BACKGROUND
Direct digital synthesizers (DDSs), or numerically controlled oscillators (NCOs), are a functional requirement of many digital communications systems, including modems and software defined radios. Frequency synthesis is commonly realized using application specific parts (ASSPs) or software executing on a digital signal processing (DSP) processor. With the increasing capabilities of PLDs, for example, field programmable gate arrays (FPGAs), DSP functions are increasingly being implemented on PLDs. PLDs have the advantages of speed that approaches the speed of ASIC implementations along with the programmability associated with DSP processors.
Quadrature oscillators (a type of DDS) are used for constructing digital down converters, demodulators, and various types of modulation schemes, such as phase shift keying, frequency shift keying, and minimum shift keying. With careful design, a DDS can also be used in systems that require frequency hopping and chirp waveform synthesis. One method for digitally generating a complex or real valued sinusoid employs a look-up table. The look-up table stores samples of a sinusoid, and a digital integrator is used to generate a suitable phase argument that is mapped by the look-up table to the desired output waveform.
The fidelity of a signal formed by recalling samples of a sinusoid from a look-up table is affected by both the phase and amplitude quantization of the process. The depth and width of the look-up table affect the signal's phase angle resolution and the signal's amplitude resolution respectively. The depth of the look-up table is the number of sample points stored therein, and the width of the look-up table is the number of bits used to represent each sample. These resolution limits are equivalent to time base jitter and to amplitude quantization of the signal. The resolution limits add spectral modulation lines and a white broad-band noise floor to the signal's spectrum.
It would therefore be desirable to implement a circuit arrangement that controls these undesired spectral terms, and which minimizes usage of logic resources.
SUMMARY OF THE INVENTION
A circuit arrangement and method for direct digital synthesis (DDS) are provided in various embodiments of the invention. The phase error introduced by a quantizer in a DDS system is fed forward and used to correct sine and cosine values output from a lookup table. Correction of the sine and cosine values based on the fed-forward error values results in a significant reduction in spectral artifacts.
In one embodiment, the circuit arrangement includes a phase accumulator, a quantizer, and a lookup table. The phase accumulator accumulates phase increment values, and the quantizer quantizes accumulated values. The quantized values are then used to address sine and cosine values in the lookup table. A feed-forward error-correction circuit determines a value for the error introduced by the quantizer and adjusts the sine and cosine values from the table as a function of the error value and sine and cosine values from the lookup table.
It will be appreciated that various other embodiments are set forth in the Detailed Description and Claims which follow.
REFERENCES:
patent: 4652832 (1987-03-01), Jasper
patent: 4893316 (1990-01-01), Janc et al.
patent: 5864492 (1999-01-01), Sadot
patent: 5999581 (1999-12-01), Bellaouar et al.
Dick Christopher H.
Harris Frederic J.
Lam Tuan T.
Maunu LeRoy D.
Nguyen Hiep
Xilinx , Inc.
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