The crystal oscillator purchasing process can be a headache if you don't know what to expect. However, learning the basics of procuring frequency control devices can help you to plan accordingly and decrease your frustration. We'll keep this one short and to the point.
Quartz crystal oscillators are the high and mighty option for low phase noise and added frequency stability in circuit design. Yes, simple oscillators like those made with resistor-capacitor (RC) or inductor-capacitor (IC) resonators are fine for some circuits. But if you're dealing with higher performance applications in aerospace, military, and space industries... you're going to want a higher performance crystal oscillator that can maintain low phase noise & strong stability. Otherwise, you'll risk deviating from the very specific (and many times critical) center frequency selected for your design.
Maintaining strong frequency stability in electronic RF circuits by eliminating phase noise is important in many high-end communication applications. This is especially true for precise targeting in radar systems and spectral purity in other communication systems.
Let's take a deep dive into exactly what phase noise and jitter mean. This will help give you a better idea as to why reducing a system's phase noise is significant.
What Is Phase Noise?
In the simplest terms, phase noise describes the stability of an oscillator in the Frequency Domain, while jitter describes stability in the Time Domain.
A Simple 5-Step Path to Understanding Phase Noise
Step 1: Spectral Density
Step 2: When plotting each spectral density point at varied frequency intervals of your choosing (In this case every 1Hz), you're left with a graph that looks like this:
Step 2: Signal Power Density
You are now looking at what is known as the Signal Power Density of the noise.
Now focus only on the upper sideband of the graph from fstart to fstop, this is called the "Single Side Band".
Step 3: Noise Power Density
We can now refer to the plotted part of the single side band as noise (Anything above the nominal oscillator frequency (Fosc) and not harmonically related can be considered phase noise). The technical term for this part of our graph is Noise Power Density (Step 3). We measure noise power density in dBW (LOG(Watts)) at this point because of the large range which we are looking.
Step 4: SSB Noise Density
Step 5: Phase Noise
What Causes Phase Noise?
- High vibrations
- Micro vibrations
- g forces and acceleration sensitivity
What's Next? Learn How to Achieve Low Phase Noise...
RF engineers would love to get their hands on ideal crystal oscillator circuits. That is, a quartz crystal oscillator that transmits at the designated frequency for the entire life of the device without any frequency deviation. Unfortunately, that ideal circuit world is a mathematical fantasy.
There are many factors that contribute to quartz oscillator stability and frequency drift issues. To prevent these problems as much as possible, having a firm understanding of precise frequency stability will help give you the tools to keep your applications performing optimally. But there's another problem...
What is a Swept Quartz Crystal?
Like many other things, radiation can impact a crystal oscillator's frequency. This is because radiation physically alters the quartz crystal inside the oscillator (changes position of weakly bound compensators that change the elastic constants of quartz). In some cases, radiation can even impact the crystal series resonance. The increase of resonance can be serious enough to cause the oscillator to stop oscillating if the crystal isn't radiation resistant. When a quartz crystal is manufactured to be radiation resistant, the crystal is referred to as a swept quartz crystal.
There are 7 key factors of understanding successful crystal oscillator circuit design. These include
- Series circuit
- Load Capacitance
- Parallel Circuit
- Drive Level
- Frequency vs mode
- Design Considerations
- Negative Resistance
Topics: crystal oscillators
What if I told you that specifying more Electronic Frequency Control (EFC) than you need could actually be hurting your pocketbook? Well, it very well may be! Paying attention to whether your supplier is using AT cut vs SC cut crystals will help you save money in the long run with OCXOs.
Ahh the mysterious OCXO. What the heck is an OCXO anyway? What's it good for? When should you use one in your electronic system design? These questions and more will all be answered in this post.
Want to know what's better than a crystal oscillator? A crystal oscillator combined with Electronic Frequency Control (EFC). Of course, it really comes down to your specific application and what requirements you're looking for to determine if EFC would be a good addition to your crystal oscillator circuit design, and if so, which method is best for you.
There are 4 options to choose from when selecting an Electronic Frequency Control method for your crystal oscillator. These 4 options are
- Pulse Width Modulation & Low Pass Filter
- Reference RF Signal & Phase Locked Loop (PLL)
- Voltage Divide
- Digital-to-Analog Converter (DAC)
In this post, let's take a closer look at each option and compare the pros and cons between them.