## Choosing a Crystal for High Frequency Applications: The Concept of Phase Noise, You Must Know It! Many high-frequency applications, such as phase-locked loops and 5G applications, are inseparable from clock signals. And the phase noise of these high-frequency applications is often a critical factor in the success or failure of a project. This article explains the concept of phase noise by comparing the ideal clock signal with the actual clock signal; then introduces two key indicators of phase noise – error vector magnitude (EVM) and VCO blocking; and finally introduces how to choose a low phase noise crystal oscillator.

Many high-frequency applications, such as phase-locked loops and 5G applications, are inseparable from clock signals. And the phase noise of these high-frequency applications is often a critical factor in the success or failure of a project. This article explains the concept of phase noise by comparing the ideal clock signal with the actual clock signal; then introduces two key indicators of phase noise – error vector magnitude (EVM) and VCO blocking; and finally introduces how to choose a low phase noise crystal oscillator.

Comparison of ideal clock signal with actual clock signal

Many high-frequency applications are inseparable from the clock signal. The figure below is a typical falling edge of the clock signal. The ideal clock signal is shown by the red line in the figure below, while the actual clock signal is shown by the purple line in the figure below. Figure 1: The falling edge of a typical clock signal (Image credit: ADI)

Also for a sine wave, an ideal sine wave corresponds to a frequency signal at a frequency, as shown by the red line in the figure below. In fact, due to the existence of noise and spurious frequencies, the sine wave in the frequency domain image is shown as the blue line on the right side of the figure below. Figure 2: Typical sine wave signal (Image credit: ADI)

Usually in order to correctly measure phase noise, we often use two kinds of instruments:

• Spectrum Analyzer: used to measure the frequency domain plot of the signal
• Oscilloscope: Time domain plot for measuring signals

The concept of phase noise

Let’s take the LO (local oscillator frequency) output of a phase-locked loop PLL as an example to see the concept of phase noise. Phase noise is the noise due to unexpected lead or lag when the signal arrives at the receiver of the system.

The ideal LO output is free of noise or extra spurious frequencies. Figure 3: Ideal LO output (Image credit: ADI)

But in practice, the phase noise appears like a skirt at the carrier edge, as shown in the figure below. Figure 4: Actual LO output (Image credit: ADI) We can plot the phase noise data into the frequency deviation from the RF carrier. Figure 5: Phase noise plotting method (Image credit: ADI)

In addition to SSB phase noise, for communication systems, error vector magnitude (EVM) and VCO blocking are also often used from a PLL perspective.

Error Vector Magnitude (EVM)

Error vector magnitude (EVM), defined as the ratio of the rms value of the average power of the error vector signal to the rms value of the average power of the ideal signal, and expressed as a percentage. The smaller the EVM, the better the signal quality. This indicator can comprehensively measure the amplitude error and phase error of the modulated signal. EVM can be thought of as the percent degradation in performance of an ideal modulated signal relative to the ideal point. Figure 6: Visualizing phase error (Image credit: ADI)

EVM, integrated phase noise, rms phase error, jitter, and more can be effectively measured using a signal analyzer.

VCO blocking

For 5G applications, VCO blocking specifications are important in cellular systems where strong emissions are a concern. If the receiver signal is weak, and the VCO noise is too high, the nearby transmitter signal may downmix and overwhelm the target signal. Figure 7: VCO noise blocking (Image credit: ADI)

The figure above demonstrates how a nearby transmitter (800kHz apart) can overwhelm the -101dBm target signal when transmitting at -25dBm if the receiver VCO is very noisy. These specifications form part of wireless communication standards. The blocking specification directly affects the performance requirements of the VCO.

Low Phase Noise Crystal

In the high-frequency design, the crystal oscillators we use are mainly quartz crystal oscillators. Some external factors (such as temperature) will affect the stability of the crystal oscillator. The Digi-Key website provides detailed crystal oscillator parameters for everyone to screen (see Digi-Key crystal oscillator for details). For the screening of the performance index of phase noise, you can pay attention to the “frequency stability” parameter. Figure 8: The “Frequency Stability” parameter on the Digi-Key crystal selection page

In addition, the type of crystal oscillator is also a very important factor, as shown in the following figure: Figure 9: The “Type” parameter on the Digi-Key crystal selection page

Below is our summary of the characteristics of common crystal types:

 Crystal type Features application XO: Standard Crystal Oscillator The most basic type of crystal oscillator Wide range of applications TCXO: Temperature Compensated Oscillator Perform temperature compensation to make the output frequency more stable Suitable for occasions where the ambient temperature changes greatly, applications in dynamic environments, such as cellular phones, smart phones, etc. VCXO: Voltage Controlled Oscillator The output frequency is controlled by the input voltage Commonly used in phase-locked loops, mixers, signal generators, etc. OCXO: Oven Controlled Crystal Oscillator Keeps the crystal oscillator temperature constant, resulting in a more stable output frequency Commonly used in occasions with high frequency stability requirements

If you want to know about crystal oscillators, you can click to view the following post – Electronic components Selection Basics – Crystal oscillators.