The E-meter

Posted on Feb 6, 2014

To measure the frequency response of the e-meter, a Wein Bridge oscillator, oscilloscope, and single-ended power supply were used. (Ironically, a bridge oscillator is being used to measure a bridge detector. ) The oscillator uses a TL074 quad op-amp, although only two of the op-amps are actually used. Op-amps typically require two power supplies, o

The E-meter
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ne positive and one negative. To avoid this expensive requirement, a "virtual ground" was created for the oscillator at precisely the midpoint between true ground and the supply voltage. The first op-amp (U1A) supplies the virtual ground while the second op-amp (U1B) implements the oscillator. U1A is configured as a "voltage follower", that is to say its output tracks the voltage on U1A+. The 10-turn pot at U1A+ centers virtual ground at one-half of the input voltage. The U1B- 10-turn pot adjusts the gain of the oscillator. When enough gain exists, a sine wave appears at Output. The two diodes stabilize the oscillator. Oscillation occurs when the reactance of one RC arm is equal to the reactance of the other RC arm, and occurs at a frequency of 1/(2*pi*RC). Due to capacitor and resistor tolerances, frequency will deviate somewhat from the equation. Therefore, frequency is measured directly from the oscilloscope. The theory behind the Wein Bridge oscillator is beyond the scope of this discussion. For more information, the reader is directed to a college textbook on oscillator theory. A Dual Trace Storage Oscilloscope was used for this experiment. By storing the waveform, 10 or more cycles can be displayed and a more precise frequency measurement can be obtained. Before each E-meter measurement is taken, the oscillator voltage is precisely controlled to yield an output of 10. 0 Volts peak-to-peak (Vpp), +/-. 05 volts. The...

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