AN95089 Datasheet, PDF (2/19 Pages) Ramtron International Corporation – BLE Crystal Oscillator Selection and Tuning Techniques

PSoC® 4/PRoC™ BLE Crystal Oscillator Selection and Tuning Techniques
2 Crystal Oscillator Basics
2.1 Crystal Oscillator Circuitry
A typical crystal oscillator circuit is shown in Figure 1. The oscillator circuit has one inverting amplifier, one feedback
resistor (R), two capacitors (C1 and C2), and a quartz crystal (XTAL)
Figure 1. Basic Crystal Oscillator Circuit
Inverting Amplifier
XI
C1
R
XTAL
XO
C2
During normal operation, the crystal and the capacitors form a π-network band-pass filter that provides a 180-degree
phase shift and a voltage gain from the output to input at approximately the resonant frequency of the crystal.
The resistor R acts as a feedback resistance, biasing the inverter in its linear region of operation and effectively
causing it to function as a high-gain inverting amplifier.
The combination of the 180-degree phase shift from the π- network and the negative gain from the inverter results in
a positive loop-gain (positive feedback), making the bias point set by the feedback resistor unstable and leading to
oscillation.
2.2 Load Cap Value (C_L)
The load capacitance is the total capacitance seen by the crystal looking into the rest of the circuit (see Figure 2).
Figure 2. Load Capacitance
Inverting Amplifier
R
XI
XO
C_L
XTAL
The correct operation of a crystal oscillator circuit depends on the value of the total load capacitance value CL that is
composed of not only the two capacitors C1 and C2, but also the parasitic capacitances and pin capacitances.
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Document No. 001-95089 Rev. *A
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