From table salt to jewelry, there are different types of crystals all around us. Crystals are used regularly for therapy and religious practices, and they also play heavily into genres of literature and entertainment. And let’s face it, a crystal looks pretty good when set in a necklace as well. Most people don’t realize that crystals also play an essential part in electronics and can be found in several of the devices we use regularly.
The resonance of a vibrating quartz crystal is often used in electronics to determine the oscillation frequency for timing signals in a circuit board. While there are different ways to create this timing, quartz crystals are well known for their effectiveness and stability. Like many other high-speed circuit board design aspects, the oscillator circuitry needs to be laid out with care to avoid signal integrity issues. Here are some crystal oscillator PCB layout guidelines that can help.
Details About Crystal Oscillators
The ability to synchronize the signals of digital circuitry is essential in most electronic systems and devices. Signal synchronization is done with an oscillating circuit, and either a ceramic or crystal resonator generates a stable and precise resonant frequency. Both the ceramic and the crystal resonator have piezoelectric properties that cause it to distort when a voltage is applied physically. When the voltage is removed, the resonator will return to its previous shape generating an electric field that oscillates at a specific frequency. The use of crystal resonators in oscillator circuits has led to them being called “crystal oscillators.”
Crystal oscillators are considered superior to ceramic resonators due to their higher precision, stability, and quality. They are also lower in cost, smaller, and packaged in hermetically sealed components making them more durable when installed on a circuit board. Quartz crystals are used in these parts, and at one time, were processed from natural quartz mined mainly in Brazil. Now synthetic quartz crystals are used, resulting in near-perfect crystalline structures with better production yields and more consistent performance.
The oscillating frequency generated by the crystal generates stable pulses to provide clock signals that regulate digital circuitry. Some of the electronic applications that use crystal oscillators include consumer electronics such as computers and cell phones, automotive engine control and navigation systems, industrial sensor and telecommunications, and military and aerospace systems. Recently there has been growth in the use of microelectromechanical system (MEMS) oscillators, which are smaller, more reliable, and are superior in all but the extreme high end of the performance range. Quartz crystal oscillators, however, are still the most widely used components for clock generation. Next, we’ll look at some of the PCB layout guidelines associated with crystal oscillators.
Crystal Oscillator PCB Layout Guidelines for Design
When using a crystal oscillator in your PCB design, remember that it is very EMI sensitive and must be incorporated into the layout carefully. Starting with the component footprint, keep the pad patterns as small as possible to reduce any chance of noise coupling from adjacent signals. Here are some layout considerations for placing and routing the crystal oscillator.
- Place the crystal oscillator as close as possible to the corresponding input and output pins of the chip.
- Avoid placing the oscillator near high-frequency devices and traces to avoid capacitive coupling.
- Keep the associated capacitors close to the crystal pins.
- Isolate the crystal oscillator for other devices as much as possible.
- Keep the traces connecting the crystal, capacitors, and IC oscillator pins as short and as wide as possible. Not only will this reduce the possibility of noise coupling, but parasitic inductance and resistance as well.
- Avoid right angle bends on these traces, which could change their characteristic impedance leading to reflections.
- Keep differential pair output traces as close together as possible, as well as the same length.
- Keep clock lines and switching signal lines away from the crystal and its connections as far as possible.
- Keep the area under the crystal well-grounded.
These crystal oscillator layout guidelines are similar to the place and route recommendations for analog and power supply circuitry. In general, keep the components connected as close together as possible so that their routed connections can be short, wide, and direct. It is also wise to keep these components on the same layer and use as few vias as possible when routing them together. In addition to the crystal oscillator, there are many other fine points of PCB layout to explore.
For Additional PCB Layout Tips
Although crystals and their associated oscillator circuitry do not occupy much of a circuit board layout, you could experience a wealth of problems if they are misdesigned. There are many aspects of laying out a PCB that require extra care, and it pays to have a resource available for technical help. Fortunately, the engineering team at your PCB contract manufacturer has the expertise that you need.
At VSE, we have years of working with customers like you—bringing their circuit board designs from concept to physical hardware. Our engineers can help through all aspects of PCB layout, from component selection to DFM enhancements. If you have questions concerning crystal oscillator layout options or any other part of your design, give us a call. We can help.