Terminology » RF Basics » NCO vs VCO: Understanding the Key Differences

NCO vs VCO: Understanding the Key Differences

oscillator nco vco signal processing rf basics

Numerically Controlled Oscillators (NCOs) and Voltage Controlled Oscillators (VCOs) share the fundamental purpose of generating periodic waveforms with controllable frequencies, yet they differ in their distinct underlying control mechanisms. Both oscillators allow for dynamic frequency adjustments, facilitating their applications in various communication and signal processing systems.

VCOs are analog devices where the output frequency is controlled by an input voltage, allowing for continuous tuning across a range. In contrast, NCOs operate in the digital domain, utilizing numerical values (tuning words) to precisely control the frequency of the generated waveform.

DDS internal functional modules

NCO is a digital oscillator whose output frequency changes in direct proportion to the tuning word. The output frequency of NCO depends on the following formula:

Output_Frequency = [ (Tuning_word * NCO_Clock_frequency) / 2 Bit_width_of_tuning_word ]

The figure above depicts NCO as part of a DDS synthesizer. A D/A converter is used to convert the digitized sine wave into an analog sine wave as shown.

VCO in PLL

The frequency of a VCO is controlled by an input voltage. Changes in the voltage level applied to the control input result in corresponding changes in the oscillator’s output frequency. It is commonly used in analog circuits and systems. Voltage control allows for smooth and continuous tuning of the frequency. The figure above depicts a VCO as part of a PLL circuit used in a frequency synthesizer.

Difference between NCO and VCO

The following table compares both oscillator types and derives the difference between NCO and VCO.

Parameters	NCO	VCO
Full form	Numerically Controlled Oscillator	Voltage Controlled Oscillator
Control Mechanism	Controlled digitally through a tuning word	Controlled by an input voltage
Signal nature	Digital	Analog
Accuracy and stability	High precision due to digital nature	Due to analog characteristics, it may be influenced by drift and temperature variations
Flexibility	Can generate various waveforms of different shapes digitally	Commonly used for generating analog waveforms
Integration	Easily integrated into DSP (Digital Signal Processing) systems	Primarily used in analog circuits and systems
Frequency control	Achieves frequency control through numerical values	Achieved by varying an input voltage
Nature of output signal	Digital waveform representation	Continuous analog waveform
Applications	DSP, SDR, digital communication systems	Analog synthesizers, communication systems

Conclusion

In summary, while both NCOs and VCOs serve the purpose of generating oscillatory signals, their control mechanisms and applications differ significantly. NCOs are digital and offer precise numerical control, making them suitable for digital signal processing applications. On the other hand, VCOs operate in the analog domain and are often used in analog synthesizers and communication systems where analog frequency control is required.