Shyam Sunder Tiwari
Electronic Design
Designers seeking a quick and easy frequency source often use a relaxation oscillator (Fig. 1) based on digital gates such as the 74HC14 inverter with hysteresis. The oscillator is also useful in sensor signal digitization when using resistive or capacitive sensors instead of fixed components in the circuit. The oscillator frequency will track changes in the sensor’s resistance or capacitance, providing a basis for measuring the corresponding physical parameters.
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| Figure 1. | This type of relaxation oscillator design is both useful and popular, but raises the question of how stable its output frequency is over time and temperature. |
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This utility of the gate-based oscillator raises the question of how reliably such an oscillator maintains frequency stability over time and temperature variations. I measured the frequency stability of such an oscillator built using metal film resistors and an NPO capacitor to minimize the possibility of component drift. Capacitor C1 had a value of 0.01 µF and R1 was a parallel combination of 1100 Ω and 1 MΩ, giving a frequency between 11 kHz and 12 kHz.
I tracked temperature using an Intersema barometric pressure and temperature recorder. Atmospheric pressure remained stable at 999.95 mbar and relative humidity at 75% during the test. The results show that the oscillator’s stability over temperature (Fig. 2) was no better than 1%/°C and that frequency increased with temperature.
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| Figure 2. | Tests of the design reveal a temperature-dependent drift on the order of 1%/°C, as frequency increases with temperature. |
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No time-dependent drift is evident. The changes seen over time appear to be the result of small fluctuations in temperature. The most likely cause of the oscillator’s limited stability is a change in the internal threshold voltage of the hysteresis circuit.
Thus, this type of oscillator doesn’t provide a frequency source with stability better than 1%/°C. Time-dependent drift wasn’t seen and fell within the fluctuations caused by temperature. I associate this drift with the internal threshold-voltage change in the hysteresis circuit.
