Understanding Phase Noise in a Frequency Synthesizer

Key Takeaways

• All oscillators exhibit some phase noise, which appears as some timing jitter on the output signal.

• Phase noise arises due to intrinsic quantization of charge carriers and poor power integrity.

• Just like in digital systems, phase noise creates some jitter, but this leads to desynchronization of signals in analog systems.

Frequency synthesis creates some phase noise generated via multiple mechanisms

Phase noise is an undesired effect in an analog signal generation that mimics the effects of jitter in digital systems. All oscillators exhibit a small amount of phase noise naturally, both due to intrinsic (quantization) effects, the physical structure of the oscillator, and extrinsic effects. Frequency synthesis circuits will always have some nonlinearity, leading to additional noise problems, particularly harmonic generation and nonlinearly related phase noise in higher harmonics.

If you have excessive phase noise in your frequency synthesizer circuit, you can take some design steps to reduce phase noise. However, there will be some limits in reducing phase noise that you can achieve in a practical system. Eventually, you will want to switch to an alternative type of oscillator. In systems requiring precise signal synthesis, sampling, and reproduction, it’s critical to understand phase noise and select the best oscillator.

Understanding Phase Noise in Frequency Synthesizers

A frequency synthesizer generates a sinusoidal signal at a specific frequency and amplitude. These circuits can be as simple as oscillators with a filter, but they typically involve multiple stages in a signal chain with a frequency generator, filters, amplifiers, or other circuit blocks. Frequency synthesizers can be built as integrated circuits in standard packaging or discrete components, particularly when the synthesizer is used in a higher power system.

Any RF system that will operate with a harmonic signal needs at least one signal source, and this will typically be some type of frequency synthesizer. Most RF systems that use a frequency synthesizer require the circuit to comply with a set of specifications to ensure signal quality and reproducibility. Phase noise is one of those specifications, along with noise immunity (in amplitude), temperature stability, and frequency drift.

Frequency Synthesis

There are several methods used to generate harmonic signals for use in RF systems. Some of these methods include:

• Direct synthesis of an analog waveform with a VCO, NCO, or DAC, where the desired waveform is coded as a digital bitstream and converted into an equivalent analog signal.
• Use a relaxation oscillator circuit with a standard topology to generate a particular waveform, then shape or filter the waveform to the desired sinusoid.
• Use a positive feedback amplifier (comparator) to generate a square wave, and select the desired harmonic with a filter.
• Use a reference oscillator (e.g., a crystal) with a PLL to produce a stepped-up or stepped-down sinusoid.
• Generate harmonics from a reference oscillator using a nonlinear component and pass the desired harmonic through a bandpass (notch) filter to select the desired frequency.

Among these options, a PLL can generate a sinusoidal signal with very low phase noise. If integrating a PLL into an IC design, you can choose from integer and fractional PLL circuits to generate waveforms. This is the principle type of circuit used to generate high-precision clocks in digital ICs.

Phase Noise vs. Ideal Behavior

Phase noise appears as unexpected jitter in the timing of a signal and is typically at a very small level. The exact mechanism of phase noise generation depends on the circuit being used to generate a sinusoid. If you look at a graph of the generated signal in the frequency domain, phase noise will be clearly visible when looking near your signal’s carrier frequency. The example graph below shows the power spectrum for an ideal sine wave with its real power spectrum. The concentration of power around the desired central frequency is due to phase noise.

Example graph comparing the power spectrum of an ideal sinusoidal wave generated by an oscillator (left) with the real power spectrum (right)

How can we interpret this? In some ways, phase noise is just a manifestation of random variations in the frequency of a sine wave, translating into minor variations in timing and some distortion. In fact, if we consider the phase of an arbitrary sine wave as a random quantity, this can be rewritten as some variation in frequency as follows:

In the linear limit, phase noise translates into a frequency and timing variation in the time domain

The circuit or system designer’s goal is to ensure stability in the oscillator circuit that generates your initial sine wave and all other sections in the signal chain where noise can be superimposed onto the desired signal.

Causes of Phase Noise and Solutions

There are several causes of phase noise in different systems; three common causes that appear in most frequency synthesizers include:

• Nonlinearity: The nonlinear components (diodes, transistors, etc.) used in frequency synthesizer circuits can increase phase noise via intermodulation and harmonic generation. In particular, transistor amplifier-based frequency synthesizers should be run at lower input levels to reduce phase noise.
• Random noise: Any of the intrinsic random noise sources associated with discrete charge carriers will contribute to phase noise and will define the system’s noise floors. This includes 1/f noise, Brownian noise, and shot noise.
• Power bus noise: Amplifier-based oscillators will experience phase noise whenever there is a transient response on the power bus. This effect is equivalent to that in digital ICs when they draw bursts of current during switching.

Other randomly received noises, such as via crosstalk or inductively coupled noise, can generate a signal that appears as phase noise on an interconnect. With the proper modeling and simulation tools, you can examine how these effects produce phase noise in circuits and propose steps to solve these problems.

Cadence’s systems analysis software can help you build models to describe your oscillator circuits and examine the phase noise in frequency synthesizers. The PSpice Simulator application can help you prepare simulations and build electrical models for subcircuits to use in larger systems-level simulations. You’ll also have access to a range of simulation features you can use in power and signal integrity analysis, giving you everything needed to evaluate a system’s functionality.

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