AC Test Equip
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AC TEST EQUIPMENT
There are two main forms of AC test equipment, signal-measuring and signal-generating.
Multimeters generally allow AC measurements to be measured by means of a rectifier circuit. A rectifier converts AC to DC.
Voltage and Current Measurements
There can be different scales to read on analog meters for AC voltage and current measurements. The AC scale is calibrated to read in RMS.
Current Probes and Clamps
A current probe allows current to be measured without breaking the circuit and inserting the meter. A hall-effect device is used to measure the current based on the magnetic field surrounding the conductor.
True RMS Voltmeter
The standard multimeter using a rectifier to measure RMS AC is only effective for sine waves with no distortion. Some True RMS voltmeters use a thermal device to measure the voltage. Some digital meters obtain the true RMS value by sampling the signal over time.
Radio-Frequency (RF) Probes
Most multimeters are not able to measure AC signals with frequencies higher than 2kHz accurately. RF probes contain a capacitor and a rectifier to allow higher frequencies to be measured.
Most voltmeters measure 1000V maximum. High-voltage probes can be used with a multiplier resistor to allow higher voltages to be measured.
Oscilloscopes allow a waveform to be displayed on a CRT or LCD screen.
The voltage of the input signal provides the vertical control of the beam. Higher volt raises the beam. Lower voltage lowers it. A sweep oscillator is used to move the beam from left to right gradually over time. When the sweep ends the beam jumps back to the left for the next sweep.
The import signal is directly amplified and used to control the electron beam producing the image on the screen.
Oscilloscope Constraints: Bandwidth (BW)
The bandwidth rating of an oscilloscope should be at least three times higher than the fundamental frequency of the fastest signal to be measured for accurate readings.
Digital oscilloscopes sample the input signal and convert it to a digital signal. The digital information is used to reconstruct the waveform on the display. Because the waveforms are stored in memory, digital oscilloscopes are commonly called DSOs or digital storage oscilloscopes.
The Digital Phosphor Oscilloscope (DPO)
The digital phosphor oscilloscope emulates the intensity variations a waveform may show on an analog oscilloscope.
ANALOG VERSUS DIGITAL: CHOOSING THE SCOPE TO USE
Everything in an analog scope operates at the same speed as the input signal.
Analog scopes are very responsive.
A digital scope may not be as responsive as an analog scope because of sampling.
A digital scope may provide automatic measurements.
A digital scope may handle multiple inputs better because it is not necessary to chop or alternate the signal processing.
Digital scopes can store waveforms for reference.
Digital scopes can be connected to printers and computers.
To minimize circuit loading a 10X (times ten) attenuator probe is commonly used.
A 1X probe is useful for measuring signals of less than 10 mV.
Most probes have a compensation adjustment. A terminal with a square wave output is provided by the oscilloscope to make the adjustment so that the display shows a proper square wave.
COMMON TEST EQUIPMENT
A spectrum analyzer graphically presents a plot of signal amplitude as a function of frequency for a selected portion of the spectrum.
A frequency counter can provide a digital display of the frequency of a signal.
Signal generators an alternating current at a specific frequency and amplitude.
The function generator is a common signal generator that provides sine, square, and triangular waveforms.
SPECIALTY TEST EQUIPMENT
NTSC Color Bar Pattern Generator
The NTSC Color Bar Pattern Generator provides a test pattern for repairing and adjusting TVs.
The wow-flutter meter measures sound distortion in audio-video equipment.
Automatic Distortion Meter
The automatic distortion meter measures distortion in the audio range.
AM/FM Synthesized Signal Generator
The AM/FM synthesized signal generator displays RF signals with a built-in frequency counter.