Pierce oscillator

The Pierce circuit, also called Pierce oscillator is an oscillator circuit which is named after its inventor, George W. Pierce. It is a modified form of the Colpitts circuit that may be built very economically with a minimum expenditure of discrete devices and therefore. Therefore, it is in a form of digital technology often used the quartz crystal. It may alternatively be constructed with a low-cost ceramic resonator with a vibrating quartz, or.

Function

The resistor R1 represents the linear operation of the first inverter U1 (for example, the block 74HC04 ) safely and in some applications can be omitted, because the resistive component of the quartz oscillator X is already sufficient. R2 limits the peak current and, if the output resistance of the Schmitt trigger is high enough, even through a direct connection (0 ohms) to be replaced. In Rückkopplungsszweig the quartz crystal is X. The two capacitors C1, C2 (so-called pull- capacitors (see below) ) are used to phase shift in order to satisfy the phase condition of 180 ° safely. Make with the quartz crystal, a π - filter dar. Due to the small capacitance values ​​range here in some structures already parasitic capacitances in the circuit board out. The quartz resonator is operating below the resonance frequency as an inductor with very large quality factor.

Schmitt trigger U2 serves as an amplifier stage, and supplies at its output a digital circuits necessary for a rectangular voltage waveform, as indicated in the sketch in the blue color.

Pulling capacitor

Since the quartz operates as part of an inductive nature, the remaining circuit has to have the capacitive character of the oscillation frequency. Therefore, in series or in parallel to the oscillating crystal draw the resonance frequency of the whole circuit back to the nominal value of a so-called pulling capacitor with a size of a few picofarads.

Load capacity

The capacity of which "sees" the quartz crystal at its terminals by the rest of the circuit is referred to as load capacitance CL. It is stated by the manufacturer of the quartz crystal in the specification sheet. In commercially available crystals in the range of 20 pF, and it is determined in parallel resonance by the series circuit of the capacitors. In addition to the discrete components C1 and C2, the values ​​in the design of a circuit are easily selectable, enter this other capacity in appearance that you must rather take for granted: the input data or Ausgangskapaztäten Ci and Co of the inverter stage ( Schmitt trigger) and the parasitic capacitance CS, which is determined primarily by the structure of the circuit board and through the housing of the oscillating crystal. The stray capacitance is typically in the range of structures 3 pF to 9 pF.

The load capacitance CL is given as:

An increase in the load capacity is slightly reduced, the frequency of the oscillator.