Basic Knowledge of How to Measure the Drive Level | Basic Knowledge of Crystal Units

Drive level

Drive level indicates power consumption by crystal unit while oscillation circuit works.
It is important for crystal to keep within drive level specification.Excessive drive level might cause unexpected change of frequency and equivalent series resistance.

Measurement

Calculation

Drive level could be calculated as follows.

Drive level = I² ･R1

I: Drive current [Effective value]
R1: Equivalent series resistance of crystal unit

Drive current is a current through crystal unit while the oscillation circuit works. Drive current could be measured on actual oscillation circuit. R1 could be measured by network analyzer and so on.

Measurement example

Measurement equipment and device
- PCB
- Crystal unit (with equivalent circuit constants data)
- Oscilloscope, Current probe
Solder a lead wire to crystal and set a current probe as figure shown in below.

Connecting current probe. Desolder one side of crystal pad and connect to PCB by lead wire through current probe.

Supply power voltage to PCB and observe drive current waveform on oscilloscope.
Caluculate effective value of drive current.
If waveform is sine wave or similar, effective value is caluculated by Ip-p/2√2 With digital oscilloscope, effective value could be read by function of oscilloscope.
Drive level is caluculated by above formula.

Oscillation circuit: Normal condition. Measuring current only through crystal. So, current probe should be put at point A.

Judgement

Measured drive level should not exceed drive level specified in crystal specification.

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