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• PROGRESS IN AN engineering field depends to a considerable degree upon the development of measuring instruments. As the field is extended through laboratory research, instruments are' 'tailor-made" for each project, but later commercial development usually brings with it simple, general-purpose direct-reading instruments. Initially of only moderate accuracy, they eventually are developed to an accuracy comparable with that achieved in older fields.

In the electrical-communication industry, the voltmeter affords an excellent example of this type of development. The frequency range of accurate measurements with voltmeters has been progressively extended from the audio-frequency limits of 20 years ago to the present figure of several hundred megacycles1 approaching the limit to which currently available diodes can be pushed.

'C. A. Woodward, Jr., "A New Vacuum-Tube Voltmeter," General Radio Experimenter, September, 1946, pp. 1-10.

Figure 1. Ponel view of the Type 1802-A Crystal Galvanometer.

'C. A. Woodward, Jr., "A New Vacuum-Tube Voltmeter," General Radio Experimenter, September, 1946, pp. 1-10.

Figure 1. Ponel view of the Type 1802-A Crystal Galvanometer.

For still higher frequencies, there is an obvious need for a direct-reading voltmeter. Such an instrument must necessarily be of the pioneering type, in which accuracy is sacrificed for convenience and extended frequency range. The new Type 1802-A Crystal Galvanometer is exactly this type of instrument. Its range of direct measurement is 0.1 to 1 volt with an accuracy of ±5%, and two multipliers are furnished to extend the range to 10 volts and 100 volts. It can be used as a direct-reading instrument at frequencies up to 1000 megacycles and for the measurement of voltage ratios, the frequency limit is well above 1000 megacycles.

Functionally, this new voltmeter is a peak-reading instrument, consisting of a rectifier and a d-c amplifier. The extended frequency range is obtained through the use of a crystal rectifier in place of the thermionic diode used in vacuum-tube voltmeters. Crystal rectifiers, however, cannot yet be produced to a degree of uniformity comparable with that of the thermionic diode, and this fact explains the difference in accuracy between the crystal instrument and the vacuum-tube voltmeter. The scale is calibrated directly in volts, but the instrument has been named a crystal galvanometer to indicate that its accuracy is not as good as the 2°/0 we have come to expect from the vacuum-tube voltmeter.

The crystal rectifier used in this voltmeter is one of the new units developed during the war2,3, wrhich were widely used as mixers and as uncalibrated voltage indicators. In a previous article3 the characteristics of these crystals as voltmeter rectifiers were dismissed, and it was pointed out that their excellent high-frequency characteristics considerably outweighed their lack of uniformity.

These crystals are commonly used as simple rectifiers in series with a meter. The use of a peak-reading circuit, however, has two important advantages: the input resistance is higher; and the variation in response between different crystals is less. The resistance of the lN21B-type crystal is of the order of a few hundred ohms in the "forward" direction and from 15,000 to 100,000 ohms in the "reverse" direction. In the simple crystal-meter circuit the input resistance is approximately twice the forward resistance, while in the peak-reading circuit it is approximately one-third the back resistance, or from 5,000 to 30,000 ohms. Variations in crystal forward resistance affect the calibration directly in the series circuit, but in the peak-reading circuit, only the small difference between the peak applied voltage and the developed d-c voltage depends upon the crystal characteristics.

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