Oscilloscope

Oscilloscope

An interactive graphic representing the controls of a typical oscilloscope.

a device for viewing oscillations, as of electrical voltage or current, by a display on the screen of a cathode ray tube.An oscilloscope is easily the most useful instrument available for testing circuits because it allows you to see the signals at different points in the circuit. The best way of investigating an electronic system is to monitor signals at the input and output of each system block, checking that each block is operating as expected and is correctly linked to the next. With a little practice, you will be able to find and correct faults quickly and accurately. The diagram shows a Hameg HM 203-6 oscilloscope, a popular instrument in UK schools. Your oscilloscope may look different but will have similar controls.

An oscilloscope, previously called an oscillograph,and informally known as a scope, CRO (for cathode-ray oscilloscope), or DSO (for the more modern digital storage oscilloscope), is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional plot of one or more signals as a function of time. Other signals (such as sound or vibration) can be converted to voltages and displayed.

Oscilloscopes are used to observe the change of an electrical signal over time, such that voltage and time describe a shape which is continuously graphed against a calibrated scale. The observed waveform can be analyzed for such properties as amplitude, frequency, rise time, time interval, distortion and others. Modern digital instruments may calculate and display these properties directly. Originally, calculation of these values required manually measuring the waveform against the scales built into the screen of the instrument.

The oscilloscope can be adjusted so that repetitive signals can be observed as a continuous shape on the screen. A storage oscilloscope allows single events to be captured by the instrument and displayed for a relatively long time, allowing observation of events too fast to be directly perceptible.

Oscilloscopes are used in the sciences, medicine, engineering, and telecommunications industry. General-purpose instruments are used for maintenance of electronic equipment and laboratory work. Special-purpose oscilloscopes may be used for such purposes as analyzing an automotive ignition system or to display the waveform of the heartbeat as an electrocardiogram.

Before the advent of digital electronics, oscilloscopes used cathode ray tubes (CRTs) as their display element (hence were commonly referred to as CROs) and linear amplifiers for signal processing. Storage oscilloscopes used special storage CRTs to maintain a steady display of a single brief signal. CROs were later largely superseded by digital storage oscilloscopes (DSOs) with thin panel displays, fast analog-to-digital converters and digital signal processors. DSOs without integrated displays (sometimes known as digitisers) are available at lower cost and use a general-purpose digital computer to process and display waveforms.

What Can Scopes Measure:

In addition to those fundamental features, many scopes have measurement tools, which help to quickly quantify frequency, amplitude, and other waveform characteristics. In general a scope can measure both time-based and voltage-based characteristics:

  • Timing characteristics:
    • Frequency and period – Frequency is defined as the number of times per second a waveform repeats. And the period is the reciprocal of that (number of seconds each repeating waveform takes). The maximum frequency a scope can measure varies, but it’s often in the 100’s of MHz (1E6 Hz) range.
    • Duty cycle – The percentage of a period that a wave is either positive or negative (there are both positive and negative duty cycles). The duty cycle is a ratio that tells you how long a signal is “on” versus how long it’s “off” each period.
    • Rise and fall time – Signals can’t instantaneously go from 0V to 5V, they have to smoothly rise. The duration of a wave going from a low point to a high point is called the rise time, and fall time measures the opposite. These characteristics are important when considering how fast a circuit can respond to signals.
  • Voltage characteristics:
    • Amplitude – Amplitude is a measure of the magnitude of a signal. There are a variety of amplitude measurements including peak-to-peak amplitude, which measures the absoluted difference between a high and low voltage point of a signal. Peak amplitude, on the other hand, only measures how high or low a signal is past 0V.
    • Maximum and minimum voltages – The scope can tell you exactly how high and low the voltage of your signal gets.
    • Mean and average voltages – Oscilloscopes can calculate the average or mean of your signal, and it can also tell you the average of your signal’s minimum and maximum voltage.

When to Use an O-Scope:

The o-scope is useful in a variety of troubleshooting and research situations, including:

  • Determining the frequency and amplitude of a signal, which can be critical in debugging a circuit’s input, output, or internal systems. From this, you can tell if a component in your circuit has malfunctioned.
  • Identifying how much noise is in your circuit.
  • Identifying the shape of a wave – sine, square, triangle, sawtooth, complex, etc.
  • Quantifying phase differences between two different signals.
  • Bandwidth – Oscilloscopes are most commonly used to measure waveforms which have a defined frequency. No scope is perfect though: they all have limits as to how fast they can see a signal change. The bandwidth of a scope specifies the range of frequencies it can reliably measure.
  • Digital vs. Analog – As with most everything electronic, o-scopes can either be analog or digital. Analog scopes use an electron beam to directly map the input voltage to a display. Digital scopes incorporate microcontrollers, which sample the input signal with an analog-to-digital converter and map that reading to the display. Generally analog scopes are older, have a lower bandwidth, and less features, but they may have a faster response (and look much cooler).
  • Channel Amount – Many scopes can read more than one signal at a time, displaying them all on the screen simultaneously. Each signal read by a scope is fed into a separate channel. Two to four channel scopes are very common.
  • Sampling Rate – This characteristic is unique to digital scopes, it defines how many times per second a signal is read. For scopes that have more than one channel, this value may decrease if multiple channels are in use.
  • Rise Time – The specified rise time of a scope defines the fastest rising pulse it can measure. The rise time of a scope is very closely related to the bandwidth. It can be calculated as Rise Time = 0.35 / Bandwidth.
  • Maximum Input Voltage – Every piece of electronics has its limits when it comes to high voltage. Scopes should all be rated with a maximum input voltage. If your signal exceeds that voltage, there’s a good chance the scope will be damaged.
  • Resolution – The resolution of a scope represents how precisely it can measure the input voltage. This value can change as the vertical scale is adjusted.
  • Vertical Sensitivity – This value represents the minimum and maximum values of your vertical, voltage scale. This value is listed in volts per div.
  • Time Base – Time base usually indicates the range of sensitivities on the horizontal, time axis. This value is listed in seconds per div.
  • Input Impedance – When signal frequencies get very high, even a small impedance (resistance, capacitance, or inductance) added to a circuit can affect the signal. Every oscilloscope will add a certain impedance to a circuit it’s reading, called the input impedance. Input impedances are generally represented as a large resistive impedance (>1 MΩ) in parallel (||) with small capacitance (in the pF range). The impact of input impedance is more apparent when measuring very high frequency signals, and the probe you use may have to help compensate for it.

Faced with an instrument like this, students typically respond either by twiddling every knob and pressing every button in sight, or by adopting a glazed expression. Neither approach is specially helpful. Following the systematic description below will give you a clear idea of what an oscilloscope is and what it can do.The function of an oscilloscope is extremely simple: it draws a V/t graph, a graph of voltage against time, voltage on the vertical or Y-axis, and time on the horizontal or X-axis.As you can see, the screen of this oscilloscope has 8 squares or divisions on the vertical axis, and 10 squares or divsions on the horizontal axis. Usually, these squares are 1 cm in each direction:

Many of the controls of the oscilloscope allow you to change the vertical or horizontal scales of the V/t graph, so that you can display a clear picture of the signal you want to investigate. 'Dual trace' oscilloscopes display two V/t graphs at the same time, so that simultaneous signals from different parts of an electronic system can be compared.

Oscilloscope

Setting up the Oscilloscope:

1. Someone else may have been twiddling knobs and pressing buttons before you. Before you switch the oscilloscope on, check that all the controls are in their 'normal' positions. For the Hameg HM 203-6, this means that:

  • all push button switches are in the OUT position
  • all slide switches are in the UP position
  • all rotating controls are CENTRED
  • the central TIME/DIV and VOLTS/DIV and the HOLD OFF controls are in the calibrated, or CAL position

Check through all the controls and put them in these positions:

Oscilloscope

2. Set both VOLTS/DIV controls to 1 V/DIV and the TIME/DIV control to 0.2 s/DIV, its slowest setting:

Oscilloscope

                        Volts/Division                                                                    Time/Division

3. Switch ON, red button, top centre: The green LED illuminates and, after a few moments, you should see a small bright spot, or trace, moving fairly slowly across the screen.

Oscilloscope

4. Find the Y-POS 1 control: The Y-POS 1 allows you to move the spot up and down the screen. For the present, adjust the trace so that it runs horizontally across the centre of the screen.

Oscilloscope

5. Now investigate the INTENSITY and FOCUS controls: When these are correctly set, the spot will be reasonably bright but not glaring, and as sharply focused as possible. (The TR control is screwdriver adjusted. It is only needed if the spot moves at an angle rather than horizontally across the screen with no signal connected.)

Oscilloscope

6. The TIME/DIV control determines the horizontal scale of the graph which appears on the oscilloscope screen.

With 10 squares across the screen and the spot moving at 0.2 s/DIV, how long does it take for the spot to cross the screen? The answer is 0.2 x 10 = 2 s. Count seconds. Does the spot take 2 seconds to cross the screen?

Now rotate the TIME/DIV control clockwise:

With the spot moving at 0.1 s/DIV, it will take 1 second to cross the screen.

Continue to rotate TIME/DIV clockwise. With each new setting, the spot moves faster. At around 10 ms/DIV, the spot is no longer separately visible. Instead, there is a bright line across the screen. This happens because the screen remains bright for a short time after the spot has passed, an effect which is known as the persistence of the screen. It is useful to think of the spot as still there, just moving too fast to be seen.

Keep rotating TIME/DIV. At faster settings, the line becomes fainter because the spot is moving very quickly indeed. At a setting of 10 µs/DIV how long does it take for the spot to cross the screen?

Oscilloscope

7. The VOLTS/DIV controls determine the vertical scale of the graph drawn on the oscilloscope screen.

Check that VOLTS/DIV 1 is set at 1 V/DIV and that the adjacent controls are set correctly:

Oscilloscope

The Hameg HM 203-6 has a built in source of signals which allow you to check that the oscilloscope is working properly. A connection to the input of channel 1, CH 1, of the oscilloscope can be made using a special connector called a BNC plug, as shown below:

Oscilloscope

The diagram shows a lead with a BNC plug at one end and crocodile clips at the other. When the crocodile clip from the red wire is clipped to the lower metal terminal, a 2 V square wave is connected to the input of CH 1.

Adjust VOLTS/DIV and TIME/DIV until you obtain a clear picture of the 2 V signal, which should look like this:

Oscilloscope

Check on the effect of Y-POS 1 and X-POS:

Oscilloscope                                     Oscilloscope

Oscilloscope                    Oscilloscope

What do these controls do?

Y-POS 1 moves the whole trace vertically up and down on the screen, while X-POS moves the whole trace from side to side on the screen. These control are useful because the trace can be moved so that more of the picture appears on the screen, or to make measurements easier using the grid which covers the screen.

You have now learned about and used the most important controls on the oscilloscope.

You know that the function of an oscilloscope is to draw a V/t graph. You know how to put all the controls into their 'normal' positions, so that a trace should appear when the oscilloscope is switched on. You know how the change the horizontal scale of the V/t graph, how to change the vertical scale, and how to connect and display a signal.

What is needed now is practice so that all of these controls become familiar.

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