In the most simplistic sense, the oscilloscope is essentially a visual voltmeter. However, to the novice, this meter, with all its controls and visual screen, induces both fascination and intimidation. The oscilloscope can be one of the most valuable types of equipment in troubleshooting.
The basic advantage of the oscilloscope is that it provides a visible display of the waveform being measured. Most oscilloscopes use electrostatic deflection. The beam sent from the electron gun is deflected vertically or horizontally by pairs of vertical and horizontal plates.
Although the oscilloscope is largely used to measure peak-to-peak voltage, other measurements that can be taken are the frequency, time periods, wave slopes, phase angles, and frequency response. The picture below illustrates a typical oscilloscope.
Functions In The Oscilloscope
The basic operating controls and their functions in an oscilloscope are as follows:
- Intensity—controls the brightness of the electron beam
- Focus—adjusts the sharpness of the beam
- Vertical center control—controls the vertical positioning of the electron beam
- Horizontal center control—controls the horizontal positioning of the electron beam
- Vertical gain—adjusts the height of the waveform
- Horizontal gain—adjusts the width of the waveform
- Sweep control—adjusts the frequency of the horizontal sweep oscillator
- Sync selector—permits use of an internal or external synchronization
- Z axis—varies the intensity of modulation of the trace
- Calibration scale—permits a scale for measurement of voltage waveforms
Basic Setup For Oscilloscope
The basic setup for an oscilloscope consists of the following:
- Turn intensity, focus gain, and sync amplitude to minimum.
- Turn vertical and horizontal controls to midrange.
- Turn on scope and adjust the intensity control to minimum brightness.
- Allow 1 to 2 minutes (min) for the scope to warm up and then adjust the focus control for a sharp trace.
- Center the trace signal by adjusting the vertical and horizontal controls.
- Connect a 6.3-V (volt) alternating-current (ac) source to the vertical input for calibration.
- Because 6.3 V root mean square (rms) equals 9 V peak voltage or 18 V peak to peak, it should be adjusted to display 1.8 divisions on the cathode-ray tube (CRT) screen.
A calibrated display of an oscilloscope
- Adjust the sync control until a stationary pattern displaying three sine waves appears.
- Now the oscilloscope is “set up” and calibrated.
Calibrate An Oscilloscope
Each division will now equal 10 V peak. The vertical attenuator can be used to multiply the divisions by 0.1, 1, 10, etc., as desired for proper measurements.
When you calibrate an oscilloscope with an internal calibrator, the scope can be calibrated by adjusting a fixed pattern of 1 V peak to peak.
More sophisticated oscilloscopes have features such as built-in calibrators for calibration checks, separate and independent comprehensive triggering facilities, and beam finders.
When you are troubleshooting with an oscilloscope, three basic accessory probes are commonly used:
- Low-capacitance probe
- Demodulation probe or radio-frequency (rf) probe
- Voltage-divider probe
The low-capacitance probe is generally used to measure high-frequency or high-impedance circuits. The “loading effect” is reduced by using this probe, which increases the accuracy of measurement.
The demodulation probe or rf probe is often used to measure rf signals where the signal must be detected before being displayed on the scope.
The voltage-divider probe is used when the voltage being measured is desired to be “stepped down” (reduced). The usual voltage division ratio is 10:1 or 100:1.
When you select an oscilloscope, it is important to consider the bandwidth or rise time, triggering, and other specialized requirements. Bandwidths can vary from 10 megahertz (MHz) to more than 100 MHz. For example, personal computers used to utilize 5-MHz clocks but are now often seen at 20 MHz. There can be dramatic differences in measurements of the actual waveforms between two different oscilloscopes, especially with digital pulses. An oscilloscope with 10 MHz may be sufficient for automotive requirements but insufficient for video equipment and industrial programming circuitry.
The selection of an analog versus a digital oscilloscope is also important depending on the application. Analog oscilloscopes generally are less costly and are best for measuring analog and high-frequency signals, whereas digital oscilloscopes are used for special digital and storage applications. Also, recent technology now offers analog-digital combinations incorporating digital recording and control with analog familiarity.
Other specialized applications require waveform recording capabilities. The electromyography (EMG) unit, used in biomedical diagnostic testing, is a typical example. The EMG unit uses a built-in oscilloscope to measure the electrical impulses and the nerve conduction velocity that stimulate muscles and allow sensation. In simplistic terms, electrodes record both the electrical activity that travels from one point of the body to another and the muscle or nerve activity at a fixed point. It is important for the practitioner to be able to view more than one signal, freeze a signal, produce an instant hardcopy from a printer, or file a waveform for multiple comparisons. Special triggering requirements, such as delayed sweeps or expansion of full rise time, of a pulse are also available for troubleshooters.
In addition to bench oscilloscopes, there are several types of portable oscilloscopes. These instruments provide sufficient power and high-performance capabilities needed to troubleshoot devices on the job site. For example, some oscilloscopes combine high safety ratings and performance equivalent to many bench oscilloscopes with the extra protection of durable portability. These instruments are useful for plant maintenance engineers and technicians who need a durable test instrument, especially in harsh and hazardous conditions. Some of the features of portable oscilloscopes include quick sampling rates up to 5 GS/s and 200 pixel shader (ps) resolution, memory capability in excess of 10,000 samples per channel, and waveform with zoom capabilities. They are exceptionally good instruments for three-phase testing in diagnosing industrial systems, power inverters, and converters. Other features include circuit voltage and amperage overloading, signal timing measurements, signal fluctuation, harmonic testing and transience loads, and three-phase power input. One of the more useful functions of portable oscilloscopes includes the paperless recording. This function can be convenient for collecting data by allowing the plotting of values over an extended period of time.
In addition, voltages, amperages, temperature, and frequency measurements can be recorded and stored in the unit. The more advanced bench oscilloscopes provide an extensive range of capabilities. For example, the phosphor oscilloscopes contain advanced triggering and protocol decode and search capabilities with bandwidths from 500 MHz to 3.5 gigahertz (GHz) and a fast digital phosphor display. Other features include rapid capture of signal anomalies, serial triggering and analysis options, and software analysis in oscilloscope packages.