Pierce circuit

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The Pierce circuit , also known as the Pierce oscillator, is an oscillator circuit that is named after its inventor, George W. Pierce . It is a modified form of the Colpitts circuit , which can be constructed with a minimum of discrete components and therefore very inexpensively. That is why it is a form of the crystal oscillator that is often used in digital technology . It can be built with a quartz oscillator or, alternatively, with an inexpensive ceramic resonator .

function

General Pierce Circuit

The resistor R ₁ ensures the linear operation of the first inverter U ₁ (e.g. the component 74HC04) and can also be omitted in some applications, since the ohmic component of the oscillating crystal X is already sufficient. R ₂ limits the peak current and, if the output resistance of the inverter is high enough, it can also be replaced by a direct connection (0 Ohm). The quartz crystal X is located in the feedback branch . The two _draw capacitors C ₁ , C ₂ are used for phase shifting in order to be able to reliably meet the phase condition of 180 °. Together with the quartz oscillator, they represent a π filter . Due to the small capacitance values, even here parasitic capacitances in the area of ​​the circuit board are sufficient in some structures . The quartz oscillator works below the resonance frequency as an inductance with a very high circular quality .

The Schmitt trigger U ₂ serves as an amplifier stage and supplies at its output a rectangular voltage curve required for digital circuits , indicated in the sketch in the color blue.

Pull capacitor

Since the quartz works as a component with an inductive character, the remaining circuitry at the oscillation frequency must have a capacitive character. Therefore, a so-called pull-up capacitor with a size of a few picofarads can pull the resonance frequency of the entire circuit back to the nominal value in series or parallel to the quartz oscillator.

Load capacity

The capacity which the quartz crystal at its terminals by the remainder of the circuit "sees" is, as the load capacitance C _L , respectively. It is listed by the manufacturer of the quartz crystal in the respective data sheet . With commercially available crystals it is around 20 pF and, in the case of parallel resonance, is determined by the series connection of the capacitors. In addition to the discrete components C ₁ and C ₂ , the values ​​of which can be easily selected when designing a circuit, there are other capacitances that have to be accepted as given: the input and output capacities C _i and C _{o of} the inverter stage ( Schmitt trigger) as well as the stray capacitance C _S , which is primarily determined by the structure of the circuit board and the housing of the quartz crystal. The stray capacitance in typical structures is in the range from 3 pF to 9 pF.

The load capacity C _L is given as:

${\displaystyle C_{\mathrm {L} }={(C_{1}+C_{i})\cdot (C_{2}+C_{o}) \over (C_{1}+C_{i}+C_{2}+C_{o})}+C_{\mathrm {S} }}$

Increasing the load capacitance slightly reduces the frequency of the oscillator.

Web links

• Pierce-gate oscillator crystal load calculation (PDF; 27 kB), application note from the trade journal RF Design , July 2004 (engl.)
• HCMOS Crystal Oscillators (PDF; 46 kB), Fairchild Semiconductor, Application Note 340 (engl.)

Individual evidence

1. ↑ Quartz crystal glossary of terms ( PDF ; 914 kB) Abracon Corporation. Retrieved June 6, 2007.
2. ↑ CX miniature crystals ( PDF ) Euroquartz. Retrieved June 6, 2007.
3. ↑ Pierce-gate oscillator crystal load calculation ( PDF ; 27 kB) Crystek Crystals Corp .. Accessed August 26, 2008.