AN2441 datasheet(5/14 Pages) STMICROELECTRONICS

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AN2441

Quartz crystal characteristics

5/14

Two resonant frequencies can be calculated.

●

The series-resonance frequency Fs is attained when the impedance Z approaches 0.

The phase shift at the series resonance frequency is zero. At the series resonance, the

impedance is minimal and the current flow is maximal.

●

The anti resonance frequency Fa is attained when the impedance Z approaches the

infinity.”

The area between Fs and Fa is called the “usual parallel resonance” or simply “parallel

resonance”

When the crystal operates in parallel resonance, its frequency Fp is between Fs and Fa and

has the following expression

Parallel resonance means that a small capacitance called load capacitance CL should be

placed across the crystal terminals to obtain the desired operating frequency.

Figure 4 shows load capacitance with the crystal equivalent circuitry.

Figure 4.

Load capacitance across a parallel resonant quartz

2.1

Frequency vs. mode

A crystal resonance can occur:

●

at the Fa or Fp frequencies (formulas given earlier). This is the fundamental mode

which is used for frequencies smaller than 30 MHz.

●

at an odd multiple of the Fa or Fp (harmonics of Fa or Fp). These are the third, fifth, (...)

overtone frequencies. This mode is used for frequencies above 30 MHz.

The crystal must be specified to operate at the desired frequency and on the desired

overtone. One should never attempt to order a crystal operating at its fundamental

frequency and operate it at an overtone frequency.

π LC

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