Choosing optimal frequency control

When developing a typical electronic system, the designer initially focuses on defining the system architecture and choosing the best microcontroller, memory, and I/O components. Next comes how to supply the various clocks required by the system. Complex systems often require ten or more clocks. There is broad choice of discrete resonators, crystal oscillators (XOs), fanout buffers, clock generators and other timing device options available today. The engineer must determine the best way to provide all of the frequencies. Consolidating the timing needs into silicon clock generator components can reduce system cost and board real estate. But does this approach always make sense, and what are the system tradeoffs in terms of board area, cost, system timing margin, noise immunity and EMI?

A critical step in finalizing your board layout is selecting the frequency references for all the components in a typical system design, as shown in Fig. 1 . Let's say, for example, you need eight frequency sources: one for each of the A/D, D/A, MCU, memory, LAN, and WLAN components and two for the processor. If you could generate all of the frequencies from a single clock generator and a single crystal and route them to the various components, you could save a lot of area and component cost, as well as improve reliability. But will the system still work? Can the clock generator provide the frequency and signal quality needed for each component, and what other advantages or disadvantages might occur?

Fig. 1: Typical system design requiring frequency-sourcing selection.

If you've ever experienced that uncertainty, you are not alone. While each system is different, consider the following guidelines for making that decision.

Frequency generator basics

To understand the trade-offs in consolidating frequency sources into clock generators, let's consider the benefits and limitations of alternative sources, as shown in Fig. 2 .

Fig. 2: Types of frequency sources.

Discrete resonators

Discrete resonators are designed to work with a semiconductor gain circuit connected to both terminals of the resonator. The output of the gain circuit is initially the amplified noise at its input. The piezoelectric and physical properties of the resonator material allow the vibrating resonator to act as an electronic filter, passing the frequency components in its passband back to the input of the amplifier. At the passband frequency where the loop gain is >1 and the phase is 360º, the resonator begins to oscillate, producing a stable frequency source at the amplifier output.

The two most common discrete resonators are ceramic (typically made of lead-zirconium-titanium or PZT) resonators and quartz crystal (made of silicon dioxide or SiO₂ ). Ceramic resonators are lower cost and much less precise with initial accuracy of ±5,000 ppm, and they drift significantly with temperature and age. Crystal resonators are much more precise, with accuracy of ±50 ppm inclusive of temperature and aging for AT cut crystals.

A major drawback of discrete resonators is the effort required to ensure proper matching of the gain circuit, resonator and board layout. The analysis includes verification of reliable startup and accuracy over temperature, process and voltage, as well as ensuring that the crystal is not overdriven, which would accelerate aging. The low amplitude and sinusoidal waveform of the external signal results in slow signal edges, making discrete resonators more sensitive to external noise. The advantages of discrete resonators include excellent close-in phase noise, noise within KHz of the resonant frequency, and low power.

Discrete oscillators

A discrete oscillator combines a resonator with a semiconductor amplifier in the same package. A crystal resonator is the most common resonator type, although surface acoustic wave (SAW) resonators and microelectromechanical system (MEMS) resonators are sometimes used. SAW resonators operate at higher frequencies (>400 MHz), and MEMS resonators provide performance similar to that of a crystal with the advantage of being smaller and more shock resistant.

A key advantage of discrete oscillators is that the amplifier, resonator, and connection capacitance can be matched in the factory to ensure reliable startup and frequency accuracy independent of board layout. However, this comes at increased component cost, area, and power consumption compared to a discrete resonator. Since only one frequency is generated from most oscillators, systems requiring multiple frequencies are often better served by consolidating frequencies into one or two clock generators.

Clock generators

Clock generators combine the oscillator with one or more PLLs, output dividers, and output buffers. In most cases, the resonators are external, but there is a trend to include the resonators within the clock generator package. However, even if the resonator is external, the effort required to match resonators, amplifiers and board layout is greatly reduced since the clock generator only needs one reference to generate all other frequencies.

There are many advantages of consolidating frequencies into a clock generator. The output frequency can be changed in real time, which is useful when systems must adapt to various standards around the world, to accommodate system variations by end user, or to accommodate BOM changes used to ensure supply. System clock frequencies also can be varied slightly during system validation or production testing to ensure sufficient timing margin, and spread-spectrum clocking can be employed to reduce costs of EMI suppression.

There are many different types of clock generators, and each is optimized for different performance and cost targets. These differences include:

• PLLs based on ring oscillators and LC oscillators (ring oscillator PLLs provide lower cost, power, and performance).
• Single-ended CMOS outputs vs. differential outputs such as LVPECL, LVDS, and HCSL that minimize coupled noise, at the expense of higher power consumption.
• Incorporation of automatic gain control on the crystal oscillator to reduce gain after startup to minimize crystal power dissipation and associated aging vs. a lower-cost inverter-based oscillator with an internal or external power-limiting resistor.
• Availability of a programming serial interface vs. pre-programmed frequencies and pin-selectable functionality.
• A small number of outputs in smaller packages.
• Allowing mixed-voltage supplies to drive different output voltage levels vs. a single supply.

Selecting the right frequency sources

The following five paragraphs will provide information to help narrow the frequency source choices and minimize the frequency sourcing components and associated cost in your system.

1. If your system only requires one or two frequencies of 50 MHz, or multiple copies or special control of a frequency is needed, a fanout buffer or clock generator is needed.

2. If there are components in your system that will pull the frequency of a discrete crystal, a discrete crystal is your only choice. Make sure you use one recommended by the ASIC vendor or one that matches the detailed crystal parameters they specify.

3. If components in your system require extremely accurate clocks (

 Fig. 3: A clock generator reference output and a CMOS ring oscillator PLL output.

4. Components needing frequencies with specific phase noise requirements (typically for wireless communication references) often need to be sourced from a crystal oscillator or an LC-based frequency generator. Since lower-cost ring-based clock generators often use a crystal oscillator for the reference, most clock generators output that frequency directly (without using a PLL) to provide a low phase noise signal as shown in the reference output of Fig. 3 .  Note that the reference output has much lower phase noise than the PLL output.  If multiple outputs with different frequencies are being generated by the same clock generator, be sure to check the spur content in the reference frequency output spectrum to ensure it does not interfere with or alias to adjacent wireless channels. The spur locations will change depending on the combination of generated frequencies. If the spur levels or locations are incompatible with the application, moving some of the clock generation into a second clock generator may resolve the problem. Otherwise, use a discrete resonator or a discrete oscillator.

5. Systems needing reference frequencies having stringent rms jitter specifications, such as high-speed digital communication systems, can also use a clock generator. Jitter is the uncertainty or error of a clock edge in time relative to a “perfect” clock signal, and rms phase jitter is the integral of phase noise over a specific frequency band.

There are obviously a number of frequency control solutions available. An excellent example of a very small, stable, and power-efficient alternative to crystal oscillators is the Si50x CMEMS oscillator family from Silicon Labs. These MEMS-based oscillators provide a highly integrated, reliable pin-compatible replacement solution for traditional XOs used in high-volume industrial, embedded, and consumer electronics applications. They are available today and priced at $0.44 each in lots of 10,000.