Crystal Oscillators - CML Microcircuits

APPLICATION NOTE

Crystal Oscillator Circuit Design

4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA All trademarks and service marks are held by their respective companies.

In this application note we shall discuss our recommended crystal oscillator circuit, explain each component in the circuit

and provide some guidelines on selecting values for these components. Finally, we shall give a few precautions to take in

order to avoid in-stability and start-up problems.

RLC

Figure 1. Crystal equivalent Circuit.

Anti-Resonance

Series

Resonance fs

Parallel

Resonance

Impedence

Figure 2. Reactance Vs Frequency plot of a crystal.

Figure 1. shows the crystal equivalent circuit. R is the effective series resistance, L and C are the motional inductance and

capacitance of the crystal. CP is the shunt capacitance due to the crystal electrodes. Figure 2. shows the reactance-

frequency plot of the crystal. When a crystal is operating at series resonance it looks purely resistive and the reactances

of the inductor and the capacitor are equal (XL = XC). The series resonance frequency is given by the equation

fs LC

When the crystal is operating in parallel resonant mode it looks inductive. The frequency of operation in this mode is

defined by the load on the crystal. The crystal manufacturer should specify the load capacitance CL for parallel resonant

crystals. In this mode the frequency of oscillation is given by the equation.

fLCC

aLP

Crystal Oscillator Circuit Design 2 Application Note

4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA All trademarks and service marks are held by their respective companies.

In parallel resonance mode the crystal can be made to oscillate anywhere on the fs - fa slope of the reactance plot, shown

in Figure 2, by varying the load of the crystal. All of MX-COM’s crystal oscillator circuits recommend using parallel

resonant mode crystals.

Figure 3. shows the recommended Crystal oscillator circuit diagram. In this type of setup the crystal is expected to

oscillate in parallel resonant mode. The inverter which is internal to the chip acts as class AB amplifier and provides

approximately 180° phase shift from input to the output and the π network formed by the crystal, R1, C1 and C2 provides

additional 180° phase shift. So the total phase shift around the loop is 360°. This satisfies one of the conditions required to

sustain oscillation. The other condition, for proper startup and sustaining oscillation is the closed loop gain should be ≥1.

The resistor Rf around the inverter provides negative feedback and sets the bias point of the inverter near mid-supply

operating the inverter in the high gain linear region. The value of this resistor is high, usually in the range of a 500KΩ ~

2MΩ. Some of MXCOM’s ICs have this resistor internal, refer to the external component specifications in the data sheet of

a particular chip.

Internal to IC

Inverter

C2 Crystal

Figure 3. Crystal oscillator circuit.

The capacitors C1 and C2 form the load capacitance for the crystal. The optimum load capacitance (CL) for a given crystal

is specified by the crystal manufacturer. The equation to calculate the values of C1 and C2 is

CCC

CC C

Where CS is the stray capacitance on the printed circuit board, typically a value of 5pf can be used for calculation

purposes. Now C1 and C2 can be selected to satisfy the above equation. Usually C1 and C2 are selected such that they

are approximately equal. Large values of C1 and/or C2 increases frequency stability but decreases loop gain and may

cause start-up problems.

R1 is the drive limiting resistor, the primary function of this resistor is to limit the output of the inverter so that the crystal is

not over driven. R1 and C1 form a voltage dividing circuit, the values of these components are chosen in such a way that

the output of the inverter goes close to rail-to-rail and the input to the crystal is 60% of rail-to-rail, usual practice is to make

resistance of R1 and reactance of C1 equal at the operating frequency, i.e. R1 ≈ XC1. This makes the input to the crystal

half that of the inverter output. Always make sure that the power dissipated by the crystal is with-in the crystal

manufacturer’s specifications. Over-driving the crystal may damage the crystal. Please refer to the crystal manufacturer’s

recommendations.

Ideally the inverter provides 180° phase shift, but the inherent delay of the inverter provides additional phase shift

proportional to the delay. In order to ensure the total phase shift of

360° around the loop, the π network should provide

180° less the phase shift due to the inverter delay. R1 can be varied to accomplish this. With fixed C1 and C2, the closed

loop gain and phase can be altered by varying R1. In some applications R1 can be ignored if the above two conditions are

met.

Crystal Oscillator Circuit Design 3 Application Note

4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA All trademarks and service marks are held by their respective companies.

Some ICs have all the external components (Rf, R1, C1, and C2) internal to the chip, thus eliminating worries to the circuit

designer. In this case simply connect the crystal across the XTAL and XTAL pins.

Hints:

Select a crystal with low effective series resistance (ESR), which helps with crystal start-up problems. Lower ESR

increases the loop gain.

Reduce the stray capacitance on the board layout by shortening the traces. This would help with startup problem and as

well as the frequency of oscillation.

Always test the circuit in applicable temperature and voltage ranges to ensure the crystal starts and sustains oscillations

and tweak the component values if necessary.

For best results, a crystal oscillator design should drive the clock inverter input with signal levels of at least 40% of Vdd,

peak to peak. Tuning fork crystals generally cannot meet this requirement. To obtain further crystal oscillator design

assistance, consult your crystal manufacturer.

The recommended way to optimize R1 is first calculate C1 and C2 as explained earlier and connect a potentiometer in

place of R1, set its initial setting at approximately equal to XC1, then vary the potentiometer setting if required until the

crystal starts under all conditions and sustains oscillation under steady state condition.