This Design Idea shows how to build a reliable, low-cost, and simple inductance tester. The basis for the tester is a Pierce buffered CMOS oscillator (Figure 1). Instead of using the usual quartz crystal, you connect the inductor under test. This oscillator uses a single CMOS inverter biased through resistor R1 in its linear region to form a high-gain inverting amplifier. Because of its high gain, the inverter dissipates lower power than an unbuffered gate; even a small signal drives the output high and low.
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| Figure 1. | Replacing a Pierce oscillator’s crystal with an unknown inductance allows you to measure its value by observing the resulting oscillation’s frequency. |
The LCπ network forms a parallel resonator that ideally resonates at the frequency
which corresponds to a period,
where CS = C1 || C2 = 50 nF. So, you can calculate the inductance, LX, by measuring the resonant frequency, fO, or the period, TO. At the resonant frequency, the LCπ network provides a 180° phase shift from input to output. To oscillate, the phase shift at frequency fO around the oscillator loop must be 360°, and the gain of the oscillator loop must be greater than one. Inverter IC1A provides an additional 180° phase shift from input to output and a high gain to compensate for the attenuation of the network.
Resistor R1 is not critical, and its value can be 1 to 10 MΩ. Resistor R2 isolates the output of gate IC1A from the LCπ network so that you can obtain a nearly clean square wave from the output of the gate itself. In addition, R2 improves frequency stability because it increases the slope of phase shift around the resonant frequency. For best performance, use film capacitors with low self-inductance, such as the MKP1837 polypropylene-film-capacitors series with 1% tolerance from Vishay. You can also use other film capacitors with standard tolerance provided that you select the value with a precision capacitance tester for best accuracy. The low supply current of the circuit allows you to use a battery as a power source.
