AN2867 datasheet(30/41 Pages) STMICROELECTRONICS

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Tips for improving oscillator stability

AN2867

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DocID15287 Rev 9

Tips for improving oscillator stability

7.1

PCB design guidelines

The 32 kHz crystal oscillator is an ultra-low-power oscillator (transconductance of a few

μA/V). The low oscillator transconductance affects the output dynamics since smaller

transconductance values generates a smaller oscillating current. This results in a lower

peak-to-peak voltage on the oscillator outputs (from a few dozen to a few hundred mV).

Keeping the signal-to-noise ratio (SNR) below acceptable limits for a perfect operation of the

oscillator means more severe constraints on the oscillator PCB design in order to reduce its

sensitivity to noise.

Therefore, great care must be taken when designing the PCB to reduce as much as

possible the SNR. A non-exhaustive list of precautions that should be taken when designing

the oscillator PCB is provided below:

•

High values of stray capacitance and inductances should be avoided as they might

lead to uncontrollable oscillation (e.g. the oscillator might resonate at overtones or

harmonics frequencies). Reducing the stray capacitance also decreases startup time

and improves oscillation frequency stability.

•

To reduce high frequency noise propagation across the board, the microcontroller

should have a stable power supply source to ensure noiseless crystal oscillations. This

means that well-sized decoupling capacitor should be used for powering the

microcontroller.

•

The crystal should be mounted as close as possible to the microcontroller to keep short

tracks and to reduce inductive and capacitive effects. A guard ring around these

connections, connected to the ground, is essential to avoid capturing unwanted noise

which might affect oscillation stability.

Long tracks/paths might behave as antennas for a given frequency spectrum thus

generating oscillation issues when passing EMI certification tests. Refer to Figure 11:

PCB with separated GND plane and guard ring around the oscillator and Figure 13:

Signals around the oscillator.

•

Any path conveying high-frequency signals should be routed away from the oscillator

paths and components. Refer to Figure 11: PCB with separated GND plane and guard

ring around the oscillator.

•

The oscillator PCB should be underlined with a dedicated underneath ground plane,

distinct from the application PCB ground plane. The oscillator ground plane should be

connected to the nearest microcontroller ground. It prevents interferences between the

oscillator components and other application components (e.g. crosstalk between

paths). Note that if a crystal in a metallic package is used, it should not been connected

to the oscillator ground. Refer to Figure 10: Recommended layout for an oscillator

circuit, Figure 11: PCB with separated GND plane and guard ring around the oscillator

and Figure 12: GND plane.

•

Leakage current might increase startup time and even prevent the oscillator startup. If

the microcontroller is intended to operate in a severe environment (high

moisture/humidity ratio) an external coating is recommended.

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