The Built-in Reference Crystal Oscillator

All PLL synthesizers need a stable frequency reference source. This source, along with the PLL chip, is used to phase-lock a free running Voltage Controlled Oscillator (VCO) to a specific frequency. Some PLL ICs have only a single-frequency reference input pin, and require the use of an external oscillator. Others have the option of using either an external oscillator or built-in oscillator circuitry. The built-in oscillator may not have as many features of an external oscillator, but it can be used in many applications where tight frequency stability is not a requirement.

The Fujitsu MB15E03SL, MB15E05SL, MB15E07SL are good examples of PLL ICs that offer built-in oscillator circuitry.

Fig. 1: A typical PLL block diagram using a crystal Reference Oscillator

The most common PLL oscillator circuit is a form of the Pierce oscillator. An inverting logic gate is used as the active element. This circuit is referred to as a logic inverter oscillator, or digital inverter, and is basically a logic inverter with a resistor connected between the input and output, forcing the circuit operating point to about half Vcc. The resistor is normally built-in to the IC and is not user supplied.

Fig. 2: Schematic of the basic oscillator

The crystal operates in the parallel mode, and therefore represents high impedance at its resonant frequency and it must be operated with a specific "load" capacitance connected to it. The crystal manufacturer adjusts the frequency with this load capacitance in place.

Care must be taken when the crystal is in the final circuit that the design load capacitance is present, otherwise the oscillator will not run at the correct frequency.

C1 and C2 (in Fig. 2) plus any stray capacitance from the leads, or PC board traces, determine the load capacitance, determined by:

Usually C1 and C2 are the same value and the load capacitance is therefore one-half the value of C1/C2. The typical crystal load capacitance specification can vary from 10 to 75 pF, however Fujitsu recommends a load of 15 to 30 pF; this translates to having C1 and C2 between 30 and 60pF. R1 (Fig. 2) is used to limit the drive to the crystal although at frequencies above 4 MHz it may not be required.

An advantage of operating the crystal in the parallel mode is a trimmer capacitor can be used to adjust or "pull" the crystal to the exact frequency. This can be implemented by making C2 a fixed capacitor in parallel with a small trimmer capacitor.

Operating the crystal in the parallel mode also offers another advantage: One of the "load" capacitors (C1 or C2) can be selected to have a temperature coefficient that will help correct for any temperature-related frequency changes in the crystal. The effects of temperature Vs. frequency depend on how the crystal is "cut". The chart below shows the effects of temperature on the frequency of crystals.

Your exact oscillator requirements should be discussed with the crystal manufacturer to determine the temperature range, the "cut" of the crystal and other specifications.

Note that any stray signal pick-up on the oscillator traces can affect the level of spurious signals and noise, so the layout of the PC board must be give careful consideration. There are three very important things to do when laying out the PC board to reduce such noise and spurs:

1. Provide the shortest possible path between the PLL IC and the crystal circuitry. If a packaged oscillator is being used, it should be mounted close to the IC.
2. Bypass the Vcc lines that feed the PLL chip with a smaller value capacitor (0.1 m F) and a larger one (10 m F.) These capacitors should be placed as close as possible to the Vcc pins.
3. Provide good ground connections for all the components.

In conclusion, the use of the built-in oscillator should be considered when designing any new PLL application, simplifying the PLL design and saving space. It can also be cost-effective by eliminating the need for an expensive external "packaged" oscillator.

References:

Fujitsu Customer Response center: 1-800-866-8608
MB15ExxSL Series data booklet: Fujitsu TC-DS-20787-2/99
"Simulation of Pierce Oscillators with Digital Inverters Using the Negative-Resistance Model" by Detlef Göhri & Jörg Haffelder, Microwave Journal, September 1998, p. 156
"Fundamentals of Quartz Oscillators", Hewlett-Packard, Application Note 200-2
International Crystal Manufacturing: 1-800-725-1426

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