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Figure 10-1 Drive Level Measurement Test Setup with Current Probe

The power dissipated in the crystal is called the drive level and will determine the amount of mechanical vibration inside the crystal. If the drive level, and mechanical vibrations, are too high, the crystal can suffer mechanical damage and cause it to fail or experience a shortened lifetime. The crystal data sheet typically specifies a maximum drive level in µW that should not be exceeded. It cannot be directly measured and must be calculated after determining the RMS current (I_RMS) through the crystal which can be obtained by several methods.

Method 1: Current Probe

Measure the RMS current (I_RMS) through the crystal using a current probe. This method provides the least external load on the oscillator circuit that would result in measurement error, but it requires board level modifications to accommodate the current probe.

Carefully remove the crystal from the board and insert a wire between the crystal pin and the PCB long enough to insert a current probe around the wire. Using a scope, measure the RMS current and or peak-to-peak current.

The RMS current can be calculated from the peak or peak-to-peak current by the following formula:

Equation 20.

I_{R M S} = \frac{I_{p k}}{\sqrt{2}} = \frac{I_{p k - p k}}{2 \sqrt{2}}

Calculate the Drive Level by the following formula:

Equation 21.

D_{L} = {I_{R M S}}^{2} \times R_{L o a d}

Figure 10-2 Drive Level Measurement Test Setup with Differential FET Probe

Method 2: Differential Active FET Probe

An alternative method is to calculate the I_RMS current by measuring the voltage swing across the crystal with a differential active FET probe that has less than 1pF of capacitance. The probe capacitance will need to be factored into the calculation because it will add error to the measurement.

Measure the RMS voltage or peak-to-peak voltage across the crystal.

The RMS voltage can be calculated from the peak-to-peak voltage by the following formula:

Equation 22.

V_{R M S} = \frac{V_{p p}}{2 \sqrt{2}}

The RMS current can be calculated from the RMS voltage by the following formula:

Equation 23.

I_{R M S} = \frac{V_{R M S}}{R} = \frac{V_{R M S}}{X_{c}} = \frac{V_{R M S}}{\frac{1}{2 π F_{c}}} = 2 π F \times V_{R M S} \times C_{t o t a l}

Equation 24.

I_{R M S} = 2 π F \times V_{R M S} \times C_{t o t a l}

Equation 25.

W h e r e : C_{t o t a l} = C_{L o a d} + C_{0} + C_{p r o b e}

The Drive Level (D_L) can then be calculated by the following formula:

Equation 26.

D_{L} = \frac{{(π \times F \times C_{t o t a l})}^{2} \times {V_{p p}}^{2} \times R_{L o a d}}{2}

If the Drive Level exceeds the maximum level specified by the crystal manufacturer, the total load capacitance may need to be adjusted to reduce R_Load, or an external series dampening resistor (Rd) needs to be added between the amplifier output and the crystal + CL1 net.

Figure 10-3 Drive Level Measurement Test Setup with Single-Ended FET Probe

Method 3: Single-ended Active FET Probe

Another alternative method is to calculate the I_RMS current by measuring the voltage swing at the input to the amplifier with an active FET probe that has less than 1pF of capacitance. The probe capacitance will need to be factored into the calculation because it will add error to the measurement. Measure the RMS voltage or peak-to-peak voltage across the CL2 capacitor on the amplifier input side of the crystal. The current flowing through this capacitor is approximately equal to the current flowing through the crystal with only a negligible amount flowing into the amplifier which can usually be neglected.

The RMS voltage can be calculated from the peak-to-peak voltage by the following formula:

Equation 27.

V_{R M S} = \frac{V_{p p}}{2 \sqrt{2}}

The RMS current can be calculated from the RMS voltage by the following formula:

Equation 28.

I_{R M S} = \frac{V_{R M S}}{R} = \frac{V_{R M S}}{X_{c}} = \frac{V_{R M S}}{\frac{1}{2 π F_{c}}} = 2 π F \times V_{R M S} \times C_{t o t a l}

Equation 29.

I_{R M S} = 2 π F \times V_{R M S} \times C_{t o t a l}

W h e r e : C_{t o t a l} = C_{L 2} + (\frac{C_{P C B_S t r a y}}{2}) + C_{p r o b e}

The Drive Level can then be calculated by the following formula:

Equation 30.

D_{L} = \frac{{(π \times F \times C_{t o t a l})}^{2} \times {V_{p p}}^{2} \times R_{L o a d}}{2}

If the drive level exceeds the maximum level specified by the crystal manufacturer, the total load capacitance may need to be adjusted to reduce R_Load, or an external series dampening resistor (Rd) needs to be added between the amplifier output and the crystal + CL1 net.

It is recommended to keep the load capacitance within the crystal manufacturer’s recommendation. However with lower load capacitors, the OSC2 swing can be too close to ground. This could trigger the CLKIN detection circuit that disables the oscillator and stops the oscillations. To avoid the CLKIN detection circuit disabling the oscillator, the load capacitance needs to increase resulting in lower drive levels as well, or a dampening resistor (Rd) needs to be added between the amplifier output and the crystal + CL1 net. This allows lower drive levels to be achieved without increasing the load capacitance beyond the recommended value.

10 Calculate the Drive Level