AN2441 Datasheet, PDF (5/14 Pages) STMicroelectronics – Low cost effective oscillator for STR71x MCUs

AN2441
Quartz crystal characteristics
Two resonant frequencies can be calculated.
● The series-resonance frequency F_s is attained when the impedance Z approaches 0.
F_s
=
---------¹---------
2π LC
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 F_a is attained when the impedance Z approaches the
infinity.”
F_a = F_s
1 + --^C----
C₀
The area between F_s and F_a is called the “usual parallel resonance” or simply “parallel
resonance”
When the crystal operates in parallel resonance, its frequency F_p is between F_s and F_a and
has the following expression
F_p = F_s
1 + ---------^C-----------
C_L + C₀
Parallel resonance means that a small capacitance called load capacitance C_L 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
R
L
C
C₀
C_L
2.1
Frequency vs. mode
A crystal resonance can occur:
● at the F_a or F_p frequencies (formulas given earlier). This is the fundamental mode
which is used for frequencies smaller than 30 MHz.
● at an odd multiple of the F_a or F_p (harmonics of F_a or F_p). 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.
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