AN2441

Pierce oscillator design

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Pierce oscillator design

Pierce oscillators are commonly used in applications because of their low consumption, low

cost and their stability. They are based on the principle in the Figure 5.

Figure 5.

Pierce oscillator circuitry

3.1

Amplifier element: 74V1GU04 ST inverter + feedback resistor

The amplifier is the active element in the oscillator design.

Basically, an inverter operating with a VDD power supply has the following response:

Figure 6.

An Inverter Response

The transfer function of an inverter is non-linear. However, it acts as an amplifier only in the

shaded area.

By biasing and operating the inverter near the maximum gain point, amplification is

achieved. For this reason, a resistor is connected between the input and the output of the

inverter.

For CMOS technology, typical values are between 1 M

Ω and 10 MΩ. However, for TTL

technology, typical values are around several hundred

Ω.

In our example, the STMicroelectronics unbuffered inverter 74V1GU04 and a 2M

Ω feedback

resistor have been selected for the oscillator design.

The inverter creates 180° phase shift. In order to satisfy the Barkhausen’s criteria phase

condition for oscillation, a suitable feedback element should create the additional 180°

phase delay. This is ensured by using a crystal operating at parallel resonance.

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