196:'s phase accumulator step size. Because the NCO operates in the discrete-time domain, it changes frequency instantaneously at the clock edge coincident with a change in the value stored in the FCR. The DDS output frequency settling time is determined mainly by the phase response of the reconstruction filter. An ideal reconstruction filter with a linear phase response (meaning the output is simply a delayed version of the input signal) would allow instantaneous frequency response at its output because a linear system can not create frequencies not present at its input.
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in the feedback path acts to multiply the phase noise of the reference oscillator and, within the PLL loop bandwidth, impresses this excess noise onto the VCO output. A DDS, on the other hand, reduces the reference clock phase noise by the ratio
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translates directly to the output, but this jitter is a smaller percentage of the output period (by the ratio above). Since the maximum output frequency is limited to
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While some authors use the terms DDS and NCO interchangeably, by convention an NCO refers to the digital (i.e. the discrete-time, discrete amplitude) portion of a DDS
145:, and precise control of the output phase across frequency switching transitions. Disadvantages include spurious responses mainly due to truncation effects in the
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The reference oscillator provides a stable time base for the system and determines the frequency accuracy of the DDS. It provides the clock to the
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The output frequency of a DDS is determined by the value stored in the frequency control register (FCR) (see Fig.1), which in turn controls the
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is the reference clock frequency). In order to reject these undesired images, a DDS is generally used in conjunction with an analog
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Jane Radatz, The IEEE Standard
Dictionary of Electrical and Electronics Terms, IEEE Standards Office, New York, NY, 1997
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At offsets far removed from the carrier, the phase-noise floor of a DDS is determined by the power sum of the DAC
310:, the output phase noise at close-in offsets is always at least 6 dB below the reference clock phase noise.
567:. The Kluwer international series in Engineering and Computer Science. Boston, MA: Kluwer Academic Publishers.
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performance of a DDS stems from the fact that it is a feed-forward system. In a traditional
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from a single, fixed-frequency reference clock. DDS is used in applications such as
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because the fractional division of the clock derives its output. Reference clock
125:. The output reconstruction filter rejects the spectral replicas produced by the
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63:
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A basic Direct
Digital Synthesizer consists of a frequency reference (often a
114:
59:
121:. The sampled, digital waveform is converted to an analog waveform by the
43:
544:"Direct Digital Synthesis: A Tool for Periodic Wave Generation (Part 1)"
532:
404:
554:
Direct
Digital Synthesis: A Tool for Periodic Wave Generation (Part 2)
266:
117:) whose period is controlled by the digital word contained in the
379:"DDS Controls Waveforms in Test, Measurement, and Communications"
557:
IEEE Signal
Processing Magazine, DSP Tips & Tricks column
547:
IEEE Signal
Processing Magazine, DSP Tips & Tricks column
565:
Direct
Digital Synthesizers: Theory, Design and Applications
160:, in addition to the desired waveform at output frequency F
137:
A DDS has many advantages over its analog counterpart, the
464:"Single-Chip Direct Digital Synthesis vs. the Analog PLL"
141:(PLL), including much better frequency agility, improved
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noise floor and the reference clock phase noise floor.
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223:
82:
Figure 1 - Direct
Digital Synthesizer block diagram
502:
302:
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113:version of the desired output waveform (often a
109:, which produces at its output a discrete-time,
398:"Direct digital synthesis enables digital PLLs"
559:, pp. 110–117, Vol. 21, No. 5, Sep. 2004.
563:Jouko Vankka & Kari A.I. Halonen (2010).
168:are also generated (the primary image is at F
8:
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549:, pp. 50–54, Vol. 21, No. 4 July 2004.
129:inherent in the analog conversion process.
431:. Lattice Semiconductor Corporation. 2009.
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482:Direct Digital Frequency Synthesizers
7:
533:Tutorial on Digital Signal Synthesis
505:Introduction to Linear System Theory
509:. Holt, Rinehart and Winston, Inc.
429:"Numerically Controlled Oscillator"
527:External links and further reading
14:
342:Numerically controlled oscillator
96:numerically controlled oscillator
1:
258:{\displaystyle f_{clk}/f_{o}}
182:reconstruction lowpass filter
102:(DAC) as shown in Figure 1.
16:Method for creating waveforms
42:used for creating arbitrary
337:Digital-to-analog converter
151:digital-to-analog converter
100:digital-to-analog converter
613:
403:. RFDesign. Archived from
119:Frequency Control Register
54:in communication systems,
38:) is a method employed by
592:Digital signal processing
303:{\displaystyle f_{clk}/2}
66:and as part of a digital
32:Direct digital synthesis
396:Paul Kern (July 2007).
597:Electronic oscillators
352:Table-lookup synthesis
304:
259:
204:The superior close-in
200:Phase noise and jitter
184:as shown in Figure 1.
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40:frequency synthesizers
28:
480:Kroupa, Venceslav F.,
347:Reconstruction filter
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260:
81:
22:
484:, IEEE Press, 1999,
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221:
501:Chen, C.T. (1970).
358:wavetable synthesis
332:Digital synthesizer
156:Because a DDS is a
56:function generators
327:Crystal oscillator
300:
255:
84:
64:sound synthesizers
29:
25:function generator
574:978-1-4419-4895-3
516:978-0-03-077155-2
214:frequency divider
210:phase locked loop
188:Frequency agility
139:phase-locked loop
68:phase-locked loop
52:local oscillators
48:signal generation
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552:L. Cordesses,
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537:Analog Devices
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468:Analog Devices
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412:. Retrieved
405:the original
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385:Corporation.
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315:quantization
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98:(NCO) and a
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35:
31:
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212:(PLL), the
206:phase noise
143:phase noise
133:Performance
586:Categories
414:2010-01-15
366:References
60:modulators
58:, mixers,
356:Multiple
176:, where F
111:quantized
44:waveforms
321:See also
115:sinusoid
74:Overview
88:crystal
571:
535:(From
513:
488:
267:jitter
23:A DDS
408:(PDF)
401:(PDF)
569:ISBN
511:ISBN
486:ISBN
194:NCO
178:clk
174:out
170:clk
162:out
147:NCO
123:DAC
107:NCO
92:SAW
90:or
36:DDS
588::
466:.
455:^
381:.
172:-F
164:,
153:.
70:.
62:,
50:,
577:.
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519:.
417:.
298:2
294:/
288:k
285:l
282:c
278:f
251:o
247:f
242:/
236:k
233:l
230:c
226:f
34:(
27:.
Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
