Chapter 13
AC Measurements
• Objectives
– After completing this chapter, the student
should be able to:
• Identify the types of meters available for AC
measurements.
• Identify the types of meter movements used to make
AC measurements.
• Explain the function of an oscilloscope.
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• Identify the basic parts of an oscilloscope and
explain their functions.
• Demonstrate the proper setup of an oscilloscope.
• Describe how to use an oscilloscope to make a
measurement.
• Explain how a counter works.
• Identify the basic parts of a counter.
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• AC Meters
– Moving-coil meter movement.
• referred to as d’Arsonval meter movement.
• designed to measure DC current.
• AC current must be converted to DC current to be
measured.
• The process is called rectification.
• The rectifiers convert the sine wave into a pulsating
DC current.
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– Iron-vane meter movement
• Does not require the conversion of AC to DC.
• Consists of two iron vanes within a coil.
– One stationary vane
– One movable vane
• Used primarily for 60 hertz applications.
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• Clamp-on meter
– Uses a split-core transformer.
– Used for measuring high values of AC current.
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• Oscilloscopes
– Most versatile piece of test equipment available
for working on electronic equipment and
circuits.
– Provides a visual display of what is occurring
in the circuit.
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• Oscilloscopes provide:
– The frequency of a signal.
– The duration of a signal.
– The phase relationship between signal
waveforms.
– The shape of a signal’s waveform.
– The amplitude of a signal.
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• The basic parts of an
oscilloscope are:
– A cathode ray tube
(CRT).
– A sweep generator.
– Horizontal and vertical
deflection amplifiers.
– Power supplies.
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• Cathode-ray tube
(CRT)
– A phosphor screen.
– Deflection plates.
– An electron gun.
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• Faceplate
– Marked in centimeters along the vertical and
horizontal axis.
– Can be calibrated with a known voltage before
testing an unknown signal.
– Called a graticule, and is separate from the
oscilloscope.
– Mounted in front of the CRT.
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• Power switch
–
–
–
–
Usually on the front panel.
May be a toggle, push-button or rotary switch.
Mounted separately or with another switch.
Used to apply line voltage to operate the
oscilloscope.
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• Intensity switch
–
–
–
–
Also called brightness.
Controls the electron beam within the CRT.
It is a rotary control.
Too much intensity for too long can burn a hole
or etch a line in the phosphor screen.
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• Focus and astigmatism controls
– Connected to the electron gun.
– Used to adjust the electron beam size and
shape.
– Rotary controls.
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• Horizontal and vertical position controls
– Rotary controls.
– Allows the electron beam to be positioned
anywhere on the face of the CRT.
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• Horizontal block
– Consists of:
• A vertical input jack.
• An AC/DC switch.
• A volts/cm rotary switch.
– The oscilloscope probe is connected to the
input jack.
– The probe is then connected to the circuit to be
tested.
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• Horizontal block
– Also called the time base.
– Consists of:
• A time/cm rotary switch.
• A trigger-control switch.
• A triggering level control.
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• Level control
– Sets the amplitude that the triggering signal
must exceed before the sweep generator starts.
• Initial oscilloscope control settings:
–
–
–
–
Intensity: set to the center of range.
Focus: set to the center of range.
Astigmatism: set to the center of range.
Position: set to the center of range.
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–
–
–
–
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Triggering: INT +
Level: AUTO
Tine/cm: 1 msec
Volts/cm: 0.02
Power: ON
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• Frequency counters
– Measures frequency by comparing a known
frequency against an input frequency.
– Consist of:
•
•
•
•
•
•
A time base.
An input-signal conditioner.
A gate-control circuit.
A main gate.
A decade counter.
A display.
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• The electronic counter is used in/on:
–
–
–
–
electronics repair shops.
engineering departments.
ham radio shacks.
industrial production lines.
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• Wide use of the electronics counter can be
attributed to the integrated circuit, which
has:
–
–
–
–
–
reduced the size and price.
increased its accuracy.
increased its reliability.
increased its stability.
increased its frequency range.
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• Bode Plots
–
–
–
–
Named for H.W. Bode.
Used for studying amplifier feedback.
Required semi-log graph paper.
Required two graphs.
• gain in decibels.
• phase shift in degrees.
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• Bode plotters today
– Computer simulations make them easier to use.
– Used to measure voltage gain or phase shift of a
signal.
– Produces a graph of circuit’s frequency
response.
– Useful in analyzing filter circuits.
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• In Summary
– Measuring AC current with:
• A moving coil meter.
• Iron-vane meter.
• A clamp-on meter.
– An oscilloscope provides:
• Frequency of the signal.
• Duration of the signal.
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• Phase relationships between signal waveforms.
• Shape of the signal’s waveform.
• Amplitude of the signal.
– The basic parts of an oscilloscope are:
•
•
•
•
•
Cathode-ray tube (CRT).
Sweep generator.
Horizontal deflection amplifier.
Vertical deflection amplifier.
Power supply.
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– Frequency counters
– Basic parts of a frequency counter are:
•
•
•
•
•
•
Time base
Input signal conditioner
Gate-control circuit
Main gate
Decade counter
Display
– Bode plotters
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