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Inside Frequency Control

Searching for the Perfect Crystal Oscillator? Ask These 4 Key Questions

Posted by Rob Rutkowski on Feb 27, 2019 9:36:00 AM

4 Key Questions to Select The Perfect Oscillator

The harsh truth is, selecting the wrong quartz crystal oscillator can quickly kill any design. With the wide variety of options and specs available in the market today, selecting the perfect crystal oscillator for your design can be a difficult and time consuming task.

However, we're about to give you 4 simple questions to ask yourself that will make finding the perfect crystal oscillator for your design quick and easy!

4 Key Questions That'll Help You Find the Perfect Crystal Oscillator for your Design

1. Do You Need a Crystal or an Oscillator?

This is a simple mistake that many engineers make. There is actually a big difference between quartz crystals and quartz crystal oscillators.

A packaged quartz crystal is simply a quartz crystal finely cut and polished to resonate at a very specific frequency when an electrical input is applied. A quartz crystal provides clock synthesis and precise timing when connected to a design with integrated oscillator circuitry. The quartz crystal alone does not provide a clock output.

Most consumer and battery powered applications use a standard quartz crystal (along with a system-on-chip (SoC) device) because of cost, size, and power restrictions.

Difference between a crystal and a crystal oscillator

A quartz crystal oscillator (XO) is a complete device that contains the quartz crystal, oscillator circuit, output driver, and potentially a phase-locked loop (PLL). An oscillator does provide a clock output unlike a quartz crystal alone.

Crystal oscillators are more commonly used in higher-end applications such as data centers, telecom, satcom, defense operations & PNT, etc. This is because oscillators are better at maintaining their frequency and low phase noise even in extreme temperature, vibration, and g-sensitive environments. 

Another benefit is that oscillators incorporate integrated power supply noise rejection to minimize the impact of board-level phase noise.

2. What Phase Noise Performance is Required?

Speaking of phase noise, it's important to know how critical maintaining low phase noise is for your application.

Phase noise, on a very basic level, is the amount of distortion or added noise in the clock's frequency signal. Since an oscillator is usually the main heartbeat of a timing system, maintaining low phase noise is typically desirable.

Low phase noise oscillators with <250 fs-RMS are crucial in higher-performance applications. This is because high phase noise levels can lead to high bit-error rates (BER) or even total loss of system communication. Therefore, it's always safer to start with a low phase noise clock source, especially in higher-end applications.

Related: Low Phase Noise Solutions for RADAR and Communications Systems

A hardware designer will not typically have a comprehensive set of phase noise requirements for all key components of the system. Reference designs are helpful in this case because the oscillator for the design has already been vetted.

minimizing-oscillator-phase-noise-in-high-vibration-environments-7-638

Choosing a trusted and proven oscillator supplier with many low noise options is also a great way to help you find the best fit. It may be worth incurring a slightly higher cost for better phase noise requirements at first, then relax them down the road if needed.

3. Will Your Frequency Change?

There are a few available options when it comes to an oscillator's desired frequency.

  1. An oscillator that maintains a single, specific frequency. For example a 10 MHz oscillator.
  2. The frequency provided by the crystal oscillator might need to change. For example, a video framer may need to toggle between two different video frame rates of 297 MHz and 297/1.001 MHz
  3. Intentionally add a small frequency deviation to the oscillator as part of margin testing to stress-test the system-level setup and hold times

There are many times when design engineers might not know exactly which frequency the final design will require, but they know they will need an oscillator to provide the reference. In this case, there are 2 main types of oscillators that can provide a solution:

  1. Dual and quad oscillators that can provide multiple, pre-stored frequencies
  2. I2C-programmable XO that provide consistent low phase noise capabilities over a wide frequency range. This option offers the most frequency flexibility and can be reprogrammed to a nearly infinite number of frequency possibilities. 

4. How Important is Frequency Stability?

In a nutshell, frequency stability is how much an oscillator's frequency drifts away from the desired frequency output. Frequency drift can occur based on a number of factors including temperature, external vibrations, and g-forces. Timing errors or complete loss of communication is likely to occur if the frequency drifts beyond what the application demands.

Frequency stability is expressed in parts per million (ppm) and sometimes even parts per billion (ppb) for critical applications such as precise military and defense technology.

Quartz crystals slowly age over long periods of time, which causes the output frequency to drift slowly over time. A good crystal oscillator supplier will show aging levels at different time frames such as 1 year, 5 years, and 10 years at a given temperature (typically 25oC). This provides a reliable guarantee for long-term stability. 

When in doubt, it is safer to use a timing device with guaranteed specifications over more stringent conditions to provide more design margin.

Where to Find The Perfect Crystal Oscillator

Bliley Technologies has over 85 years of experience manufacturing quartz crystals and oscillators. We offer a wide variety of OCXOs, TCXOs, VCXOs, and clocks for many different high-performance and low phase noise solutions. Contact us to see how we can help take your timing application further!

Download FREE RF Design Guidelines

 

Topics: crystal oscillators