The function of quartz crystal resonator is locking and determines the oscillation frequency of an oscillating circuit which is widely used in electronic industrial. Proper circuit design and components configure is crucial to the stability of oscillating circuit with quartz crystal resonator overtime.

 How can we know the oscillating circuit using crystal resonator is proper designed? What shall hardware engineers consider? Interquip Electronics, manufacturer of quartz crystal resonators and oscillators, would like to share some basic tips:

Negative Resistance

Drive Level
On board oscillating frequency
Negative Resistance of the oscillating circuit

Negative resistance (-R) is a parameter of an oscillating circuit that determined by the gain and terminated capacitances which shall be large enough to ensure a reliable oscillation.

According to the basic rule, the negative feedback loop gain of an oscillation circuit shall be greater than 1 to sustain an oscillation. |-R| divided by Rr represent the negative gain of the oscillating circuit approximately. The recommendations from IEC60444-6 is |-R| / Rr shall be greater than or equal to 3 for reliable oscillation and Rr shall be the maximum Rr in all circumstances. Interquip Electronics recommends 5 times or more which has a better design margin.

The Drive Level of the oscillation circuit

Quartz crystal resonator is a piezoelectric device. The piezoelectric effect provides a coupling between the mechanical properties of a piezoelectric crystal and an electrical circuit. Below is the bulk acoustic wave mode of an AT cut quartz crystal resonator,

The mechanical displacement is proportional to the drive level (power) applied. It induces stress and cause irreversible damage over time in some circumstance if the drive level is extreme large. 1uW to 100uW is recommended for AT cut Mhz quartz crystal resonator in general. 

The drive level is equal to,

     DL=Ix2*RL                 

 RL: Load resistance, which is equal to Rr(1+C0/CL)²

Ix: the current go thru the quartz crystal resonator

C0: Shunt capacitance of quartz crystal resonator

CL: equivalent load capacitance

Ix is the current go thru the quartz crystal resonator which could be measured using a high-frequency current probe.

On board oscillation frequency

The on board oscillating frequency is determined by the quartz crystal resonator characteristic and the on board equivalent load capacitance. The on board equivalent load capacitance CPCB is related to Cg/Cd, terminal capacitance of the integrated circuit (CIC)and stray capacitance (Cs) on board as below:  

 CPCB= (Cg + CIC)×(Cd + CIC)/(Cg + Cd + 2 x CIC) + Cs

The quartz crystal resonator frequency is load capacitance (CL) depended. It is tuned by the manufacturer to the desired tolerance per specification. The on board oscillating frequency shall be within the frequency tolerance when the on board equivalent load capacitance is same as the CL specification of quartz crystal resonator. Otherwise, the on board oscillating frequency will deviate from the nominal frequency. If that happen, adjust Cg and Cd when necessary. 

In addition, it shall be caution when measuring the on board oscillating frequency. The probe capacitance could be part of the on board equivalent load capacitance that contribute measurement error. A non-contact measurement using spectrum analyzer or equivalent is recommended.                                                        

In short, engineers shall verify the negative resistance of the oscillating circuit, the drive level and on-board oscillating frequency when the design of the oscillation circuit is determined. Adjust the circuit component to optimize the performance and lower the risk when necessary. Please feel free to contact Interquip Electronics if there is any question. You are always welcome.