Quartz Crystal Drive Level Calculation

Quartz Crystal Drive Level

https://www2.renesas.cn/us/en/document/apn/830-quartz-crystal-drive-level

Introduction

Crystal drive level is the amount of power dissipated in a crystal which is usually specified in microwatts or milliwatts. It can be calculated by measuring the excitation current flowing through the crystal. The maximum drive level is the most power the crystal can dissipate while still maintaining operation with all electrical parameters guaranteed. Drive level should be maintained at the minimum levels necessary to initiate proper start-up and assure steady state oscillation. Some of the larger crystal packages like HC49 tend to have higher drive level limits while the smaller crystal packages like 5x7mm and 2.5 x 3.2.mm have lower drive level. To ensure that premature aging or damage to the crystal does not occur, drive level should be considered.

Though drive level is specified in most quartz crystal datasheets, at times it is overlooked. Drive level is an important factor when designing or evaluating an oscillator circuit.

Measuring Drive Level

There are multiple ways of measuring drive level. Using a current probe is most reliable which is similar to current flowing through a resistor. Using a FET probe is also common but has disadvantages on an inverter type oscillator, like a Pierce. On an inverter type oscillator, there is phase difference between the signal at the crystal input and output which make it difficult to accurately measure the voltage across the crystal. The equations and examples below will show the requirements for measuring drive level using a current probe.

The drive level can be calculated by using the RMS drive current (Id) and the load resonance resistance (RL). Refer to equation 1.

Example Calculation

For this example, a Tektronix CT-6 probe was used to measure the drive current. When placing the current probe, special attention should be given to the orientation. For inverter type oscillators, the drive current should be measured on the output side of the crystal in order to contain the influence of the oscillator. Refer to Figure 1.

Oscilloscopes measure voltage versus time, though the display value is in mV, the current probe converts the electric field to AC current. The voltage can be ignored; in reality it is current. Oscilloscopes can display AC current in peak, peak to peak, or RMS. For most cases, we can assume the waveform is a pure sine-wave, hence the equation 6 applies. Refer to equation 6 and Figure 2. 

Using a Saunders & Associates 250B Network Analyzer, the relevant crystal parameters can be characterized. For this example, a 25MHz, HC49 type crystal was used. Refer to Figure 3. 

Using equation 3 in combination with IRMS from figure 2 and RL from values in Figure 3 yields the drive level below. 

The drive level contains variables from both the crystal and oscillator; hence both the crystal type and frequency will affect the drive level. When characterizing an oscillator, it should be verified across minimum and maximum frequency, CL, and RL. For the example above, the drive level is 57.14µW for a 25MHz, HC49 crystal on this specific oscillator. 

This example yielded 57.14µW of drive level. In order to avoid poor aging or crystal damage, this value should be compared to the maximum level recommended by the crystal manufacturer.

AEC-Q200 Crystals

AEC-Q200 - ECS Inc. International

https://ecsxtal.com/store/pdf/ECX-1637Q.pdf