2.5 Volt Voltage Meters

2.5.1 Pulsed Voltage Measurements

Pulse voltages are measured using pulsed voltmeters constructed according to the scheme shown in Fig. 2.8, a. In this scheme, it is possible to measure the amplitude of only positive pulses; for negative ones, the reverse connection of the diode is necessary. Special pulse voltmeters are graduated in amplitude (peak) values.

In the study of radio pulses, the processes in the voltmeter circuit proceed in the same way as when measuring video pulses. However, the capacitor charge occurs only at positive half-periods of the carrier frequency, i.e. with a positive envelope. The measurement error in this case may increase.

In the case of measuring pulse voltages, it must be borne in mind that the frequency spectrum occupied by the pulses is wide, especially the spectrum of radio pulses of short duration. Spectrum components can be located in the high-frequency region where additional errors appear.

2.5.2 Measurement of noise voltage

Most accurately the rms value of the noise voltage can be measured with a quadratic detector voltmeter. Graduation of such a voltmeter does not depend on the form of voltage, and therefore, is suitable in this case.

When measuring noise voltages, it is necessary to take into account a number of specific requirements:

1. Noise voltage can have large emissions

exceeding 3-4 times its average square value.

Therefore, the length of the quadratic part of the characteristics of the detector should be large, while there should be no limitation of the noise voltage in the amplifiers connected to the detector.

The amplitude characteristic of the input amplifier should be linear to a level that the noise voltage is less likely to exceed;

2. Spectral density of noise voltage usually takes wide bandwidth. Amplifiers connected to a non-linear device should not introduce linear distortion;

3. When measuring, the readings of the voltmeter are determined by the implementation of the process under study for a finite accumulation time the mean square value of the individual noise voltage realizations is measured.

Let the noise under study be a stationary random process and its expectation and variance do not depend on time. The voltmeter readings will be different for different implementations, i.e. there is a measurement error due to the finite accumulation time. The scatter of readings from one implementation to another is the smaller, the longer the accumulation time. In this case, the measurement error is also reduced.

Test questions:

1. How to measure the pulse voltage?

2. The principle of measuring noise voltages.

3. Explain the operation of the pulse voltage voltmeter.