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Q-6.
The frequency response of test equipment refers to what aspect of ac voltage measurements?
Multimeter Method
As previously stated, an analog multimeter’s usefulness is limited by its low input impedance and
poor accuracy (typically 2%). However, rugged construction and ease of operation make analog
multimeters extremely useful whenever poor accuracy and low input impedance can be tolerated. When
performing ac voltage measurements with a multimeter, be certain that the frequency of the signal being
measured falls within the upper and lower frequency limitations of the meter.
Oscilloscope Method
A major advantage of using an oscilloscope for ac voltage measurements is that the waveform can be
observed; consequently, errors in measuring complex peak voltages are minimized. An oscilloscope may
be used as a high-impedance ac voltmeter. In standard oscilloscopes, the vertical amplifier input
impedance is generally greater than 1 megohm, making it possible to measure voltages in high-impedance
circuits. If the signal is applied directly to the plates, rather than at the vertical amplifier input, the input
impedance is increased considerably.
Voltage measurements are most easily made when the deflection of the trace extends across the
major portion of the oscilloscope screen; whenever possible, the trace should cover at least 60% of the
vertical viewing area of the screen. If the amplitude of the measured voltage is very low, the trace
dimensions may be small. If a voltage to be measured is large and cannot be attenuated to a usable value
by attenuation circuits within the oscilloscope, an external resistive or capacitive voltage divider can be
used. Such voltage dividers are often furnished with oscilloscope test sets and are called HIGH
VOLTAGE PROBES. When the voltage of pulses or other complete waveforms is being measured, the
high voltage probe selected must be so designed as not to distort the measured signal. Most probes have
adjustable (compensating) capacitors that are used to adjust the symmetry of the displayed waveform.
You adjust the probe by monitoring either the calibrator output of the oscilloscope or a known good
signal and adjusting the probe for a symmetrical display. Oscilloscopes are calibrated to display peak-to-
peak values. To determine the rms voltage of a sinusoidal signal, divide the number of graticule units
from the positive to the negative peaks by two and multiply this value by 0.707. When using the
oscilloscope for ac voltage measurements, ensure the upper frequency range of the oscilloscope is not
exceeded; otherwise, inaccurate values will be displayed. Most commonly used oscilloscopes have a
frequency response from dc up to 100 megahertz.
Q-7.
Ideally, an oscilloscope presentation should cover what vertical portion of the screen?
Digital Multimeter Method
As previously mentioned, digital multimeters present a high input impedance to the circuit under test
and are fairly accurate. Many earlier models had very limited frequency responses. Even today the upper
frequency limitations of digital multimeters vary from 20 kilohertz to over 300 kilohertz, depending on
the model. Their upper frequency limitations can, however, be significantly extended by using optional rf
probes. When you perform ac voltage measurements with a digital multimeter, remember that they are
true rms indicating devices.
Differential Voltmeter Method
Most differential voltmeters can be used to measure both ac and dc voltages. The differential
voltmeter method of measuring ac voltage is the most accurate of the common measurement techniques.
Typical accuracies are 0.05% when operated in the ac mode.
