Numerically controlled oscillator: Difference between revisions

Revision as of 15:12, 20 June 2021

Revision as of 19:36, 21 April 2023

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{{short description|Digital signal generator}}

A '''numerically- controlled oscillator''' ('''NCO''') is a digital [[signal generator]] which creates a [[Synchronous circuit|synchronous]] (i.e., clocked), discrete-time, discrete-valued representation of a [[waveform]], usually [[sinusoidal]].<ref name="IEEE">{{cite book |last=Radatz |first=J. |title=The IEEE Standard Dictionary of Electrical and Electronics Terms |publisher=IEEE Standards Office |___location=New York, NY |year=1997 }}</ref> NCOs are often used in conjunction with a [[digital-to-analog converter]] (DAC) at the output to create a [[direct digital synthesizer]] (DDS).{{#tag:ref|While some authors use the terms DDS and NCO interchangeably,<ref name="latticeSC" /> by convention an NCO refers to the digital (i.e. the discrete-time, discrete amplitude) portion of a DDS<ref name="IEEE"/>}}

Numerically- controlled oscillators offer several advantages over other types of oscillators in terms of agility, accuracy, stability and reliability.<ref name="latticeSC">{{ cite web | title = Numerically Controlled Oscillator | url = http://www.latticesemi.com/Products/DesignSoftwareAndIP/IntellectualProperty/IPCore/IPCores02/NumericallyControlledOscillator.aspx | publisher = Lattice Semiconductor Corporation | year = 2009 }}</ref> NCOs are used in many communications systems including digital up/down converters used in 3G wireless and software radio systems, digital [[phase-locked loop]]s, radar systems, drivers for optical or acoustic transmissions, and multilevel [[Frequency-shift keying|FSK]]/[[Phase-shift keying|PSK]] modulators/demodulators.<ref name="latticeSC"/>

==Operation==

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*A ''phase-to-amplitude converter'' (PAC), which uses the phase accumulator output word (phase word) usually as an index into a waveform [[look-up table]] (LUT) to provide a corresponding amplitude sample. Sometimes [[linear interpolation|interpolation]] is used with the look-up table to provide better accuracy and reduce phase error noise. Other methods of converting phase to amplitude, including mathematical algorithms such as [[power series]] can be used, particularly in a software NCO.

[[Image:Generic NCO.png|frame|Figure 1: Numerically- controlled oscillator with optional quadrature output]]

When clocked, the phase accumulator (PA) creates a [[modulo operation|modulo]]-2^N [[sawtooth wave]]form which is then converted by the phase-to-amplitude converter (PAC) to a sampled sinusoid, where N is the number of bits carried in the phase accumulator. N sets the NCO frequency resolution and is normally much larger than the number of bits defining the memory space of the PAC [[look-up table]]. If the PAC capacity is 2^M, the PA output word must be truncated to M bits as shown in Figure 1. However, the truncated bits can be used for interpolation. The truncation of the phase output word does not affect the frequency accuracy but produces a time-varying periodic phase error which is a primary source of spurious products. Another spurious product generation mechanism is finite word length effects of the PAC output (amplitude) word.<ref name="kroupa">{{cite book |last=Kroupa |first=V. F. |title=Direct Digital Frequency Synthesizers |publisher=IEEE Press |year=1999 |isbn=0-7803-3438-8}}</ref>

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{{good article}}

{{DEFAULTSORT:Numerically- controlled oscillator}}

[[Category:Digital signal processing]]

[[Category:Synthesizers]]