New Highsensitivity Electrometer

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A Low-Cost Microwave Signal

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New Dial Brings New Convenience to Octave-Band Noise Measurements S

. The new Type 1230-A D-C Amplifier and Electrometer is basically a high-resistance millivoltmeter. Voltage, current, and resistance is indicated on a panel meter and can also be indicated on a recorder.

Because of its high sensitivity and excellent stability, this instrument has a wide range of applications in science, engineering, and industry. Typical examples include the measurement of:

Ionization currents, photo currents, grid currents in electron tubes, and time-current curves of capacitors during charge and discharge.

Piezo-electric potentials, bioelectric potentials, contact potentials, electrostatic field potentials, and pH indications.

Silicon-diode back resistance, inter-conductor resistance of cables, insula tion resistance of electrical equipment and voltage coefficient of resistance.

The amplifier in this instrument is strictly direct coupled. It uses neither the relatively low input-resistance chopper system nor the high-cost vibrating capacitor system. Its stability is due to excellent supply regulation, shock mounting, liberal use of wire-wound resistors at the important places, and adequate aging of both tubes and corn-

Figure 1. Panel view of the Type 1 230-A D-C Amplifier and Electrometer.

Western Electric D96475

volts ohms

INPUT

INPUT

Coupled Cathode Follower

OUTPUT

Figure 2. Elementary schematic of the Electrometer. The circuit is, fundamentally, a cathode follower in which the "tube" is a 3-stage, direct-coupled amplifier. The magnitude of the cathode resistor, Rb, determines the voltage sensitivity.

OUTPUT

Figure 2. Elementary schematic of the Electrometer. The circuit is, fundamentally, a cathode follower in which the "tube" is a 3-stage, direct-coupled amplifier. The magnitude of the cathode resistor, Rb, determines the voltage sensitivity.

ponents. As a consequence, drift after warm-up is normally less than 2 millivolts per hour.

Grid current at the input of the 3-tube direct-coupled amplifier is negligible, because the tube in the first stage is an electrometer type. The input resistance is determined by the setting of a switch that provides resistance standards in decimal steps from 10 kilohms to a hundred thousand megohms (1011 ohms).

The ability to measure from 30 millivolts full-scale to 10 volts full-scale, coupled with the wide range of resistance standards, permits current measurements from one milliampere full scale to 0.3 micro-microampere (3 X 10~13 amp.) full scale at an effective "ammeter" resistance appreciably less than the value of the resistance standard .

An internal stabilized-voltage source permits direct-reading resistance measurements from 300 kilohms to ten mega-megohms at full scale (5 X 1014 ohms at the smallest meter division). Through use of the most sensitive voltage range and readily available external batteries, the resistance range can be extended by a factor of two hundred or more.

Circuit

Fundamentally, the circuit is a simple cathode follower where the "tube" is a three-stage amplifier as shown in the elementary schematic of Figure 2. Figure 3 shows the elementary circuit for each type of measuremeift. The effective transconductance is the product of the trans-conductance of the third stage and the voltage gain of the first two stages. The result is a transconductance in the millions of micromhos. Consequently, the input voltage is duplicated within a few microvolts across the cathode resistor, and excellent linearity is obtained even at the 30-millivolt scale. Voltage ranges are selected by changing the value of the cathode resistor.

The first two stages of the amplifier use sub-miniature tubes with ten-milliampere filaments. The filaments are in a resistor chain fed from a doubly stabilized voltage-regulating system. The plate and screen voltages of the first stage, as well as the screen voltage of the second stage, are obtained from this same highly stabilized supply. As a consequence of the great care used for stabilization, line voltage fluctuations have a negligible effect on performance. Balanced amplifier systems were tried but more reliable results were obtained by using the fully stabilized supply rather than the balancing method, which depends on perfect matching for adequate results.

Figure 3. Elementary schematics, showing, left to right, the circuits for measuring current, resistance, and voltage. The batteries are symbolic only; the instrument is entirely a-c operated.

. INPUT

high Input Resistance

The input resistance of the amplifier is about 1014 ohms when the input-resistance switch is at the open position. This extremely high resistance level is due not only to the use of an electrometer tube but also to unusual construction features. Every effort was made to obtain reliable operation under high humidity conditions. The glass envelope around the grid lead is treated with silicone. The resistance-standard selector switch uses switch contacts that are mounted on individual teflon bushings set in a metal base that connects to a guard point.

Internal Standards Calibration

To permit checking the high-resistance internal standards in terms of the low-resistance wire-wound standards, a check position is provided 011 the function switch. This has meant further elaboration of a switch already unusual in construction to meet the requirement of excellent performance at a 1014-ohm level under adverse humidity conditions. The effort is well repaid in the ease with which the resistance of even the 10n-ohm standard can be checked. A photograph of the switch is shown in Figure 4.

No Switching Transients

A switch is provided for readily disconnecting the unknown from the input without otherwise disturbing either

Figure 4. View of the function switch.

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Telaio Meccanico Cartwright
Figure 5. View of the Type 1230-P1 Component Shield plugged into the rear of the Electrometer.

the unknown circuit or the electrometer input circuit; to accomplish the switching without causing an electrostatic surge (due to friction of metal on dielectric) and without causing a change in capacitance with resultant voltage surge due to redistribution of charge, the contactor is raised by a teflon button with a metal rim in permanent contact with one of the blades.

Shielaing

Complete shielding of the shock-mounted electrometer stage is important, to eliminate grid currents due to ambient light, to prevent dust from entering and affecting the high resistance, and especially to isolate the input from random electrostatic potentials that are not usually noticed, but that become obvious at resistance levels in excess of l()tt ohms.

The input connection is through a teflon-insulated coaxial terminal, and available accessories permit extension of the complete shielding to the unit under test. In particular, the Type 1230-P1 Component Shield provides a fully-shielded compartment within which components under measurement can be quickly and easily connected. The ground and guard terminals are duplicated 011 the panel of the Component Shield for greatest adaptability. Figure 5 shows the Component Shield plugged into the rear of the Electrometer.

Guard Terminals

While most measurements can be made by connecting the unknown (voltage, current or resistance) from the high input terminal to ground, there are some applications, especially in three-terminal resistance measurements, where guard points are necessary. Accordingly, the Type 1230-A Amplifier is provided with three guard terminals which can be grounded or not as desired. This arrangement is shown in Figure 6.

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