HCMOS Crystal Oscillators With the advent of high speed HCMOS circuits, it is possible to build systems wit h clock rates of greater than 30 MHz. The familiar gate oscillator circuits used at low frequencies work well at higher frequencies and either L–C or crystal resona- tors maybe used depending on the stability required. Above 20 MHz, it becom es expe nsive to fabr icat e fund amen tal mode crystals, so overtone modes are used. Basic Oscillator Theory The equivalent circuit of a quartz crystal, and its reactance characteristics with frequency are shown in Figure 1. F R is called the resonant frequency and is where L 1 and C 1 are in series resonance and the crystal looks like a small resistor R1. The frequency F A is the antiresonant frequency and is the point where L 1 –C 1 look inductive and resonate with C O to form the parallel resonant frequency F A , F R and F A are usually less than 0.1% apart. In specifying crystals, the fre- quency F R is the oscillation frequency to the crystal in a se- ries mode circuit, and F R is the parallel resonant frequency. In a parallel mode circuit, the oscillation frequency will be slig htly below F A wher e the inductiv e comp onent of the L 1 – C 1 arm resonates with C O and the external circuit ca- paci tanc e. The exact freque ncy is ofte n correc ted by the crystal manufacture to a specified load capacitance, usually 20 or 32 picofarads. TABLE 1. Typical Crystal Parameters 32 kHz 200 kHz 2 MHz 30 MHz Par ame ter fun dament al fun dament al fundament al ove rtone R 1 200 kΩ 2 k Ω 100 Ω 20 Ω L 1 7000H 27H 529 mH 11 mH C 1 0.003 pF 0. 0 24 pF 0.012 pF 0.0026 pF C 0 1.7 pF 9 pF 4 pF 6 pF Q 100k 18k 54k 100k The Pierce oscillator is one of the more popular circuits, and is the foundation for almost all single gate oscillators in use today . In this circuit, Figure 2 , the signal from the input to the outp ut of the amplif ier is phase shifted 180 degre es. The crystal appears as a large inductor since it is operating in the parallel mode, and in conjunction with C A and C B , forms a pi network that provides an additional 180 degrees of phase shift from output to the input. C A in series with C B plus any additional stray capacitance form the load capacitance for the crystal. In this circuit, C A is usually made about the same value as C B , and the total value of both capacitors in series is the load capacitance of the crystal which is generally cho- sen to be 32 pF, making the value of each capacitor 64 pF. The approximation equations of the load impe dance , Z 1 , presented to the output of the crystal oscillator’s amplifier by the crystal network is: Where X C =−j/ ωC B and R L is the series resistance of the crystal as shown in Table I. Also ω=2πf where f is the fre- quency of oscillation. The ratio of the crystal network’s input voltage to it’s output voltage is given by: AN005347-1 Crystal Equivalent Circuit AN005347-2 Reactance of Crystal Resonator FIGURE 1. AN005347-3 FIGURE 2. Pierce Oscillator Fairchild Semiconducto r Application Note 340 May 1983 H C O S C r y s t a l O s c i l l a t o r s A N - 3 4 0 © 19 98 Fa irchil d Semicond uctor Corpo ration AN00 5347 www.fairchildsemi.com
