The Controls

This section briefly describes the basic controls found on analog and digital oscilloscopes. Remember that some controls differ between analog and digital oscilloscopes; your oscilloscope probably has controls not discussed here.

Display Controls

Display systems vary between analog and digital oscilloscopes. Common controls include:

Vertical Controls

Use the vertical controls to position and scale the waveform vertically. Your oscilloscope also has controls for setting the input coupling and other signal conditioning, described in this section. Figure 1 shows a typical front panel and on-screen menus for the vertical controls.



Figure 1: Vertical Controls

Position and Volts per Division

The vertical position control lets you move the waveform up or down to exactly where you want it on the screen.

The volts per division (usually written volts/div) setting varies the size of the waveform on the screen. A good general purpose oscilloscope can accurately display signal levels from about 4 millivolts to 40 volts.

The volts/div setting is a scale factor. For example, if the volts/div setting is 5 volts, then each of the eight vertical divisions represents 5 volts and the entire screen can show 40 volts from bottom to top (assuming a graticule with eight major divisions). If the setting is 0.5 volts/div, the screen can display 4 volts from bottom to top, and so on. The maximum voltage you can display on the screen is the volts/div setting times the number of vertical divisions. (Recall that the probe you use, 1X or 10X, also influences the scale factor. You must divide the volts/div scale by the attenuation factor of the probe if the oscilloscope does not do it for you.)

Often the volts/div scale has either a variable gain or a fine gain control for scaling a displayed signal to a certain number of divisions. Use this control to take rise time measurements.

Input Coupling

Coupling means the method used to connect an electrical signal from one circuit to another. In this case, the input coupling is the connection from your test circuit to the oscilloscope. The coupling can be set to DC, AC, or ground. DC coupling shows all of an input signal. AC coupling blocks the DC component of a signal so that you see the waveform centered at zero volts. Figure 2 illustrates this difference. The AC coupling setting is handy when the entire signal (alternating plus constant components) is too large for the volts/div setting.



Figure 2: AC and DC Input Coupling

The ground setting disconnects the input signal from the vertical system, which lets you see where zero volts is on the screen. With grounded input coupling and auto trigger mode, you see a horizontal line on the screen that represents zero volts. Switching from DC to ground and back again is a handy way of measuring signal voltage levels with respect to ground.

Bandwidth Limit

Most oscilloscopes have a circuit that limits the bandwidth of the oscilloscope. By limiting the bandwidth, you reduce the noise that sometimes appears on the displayed waveform, providing you with a more defined signal display.

Channel Invert

Most oscilloscopes have an invert function that allows you to display a signal "upside-down." That is, with low voltage at the top of the screen and high voltage at the bottom.

Alternate and Chop Display

On analog scopes, multiple channels are displayed using either an alternate or chop mode. (Digital oscilloscopes do not normally use chop or alternate mode.)

Alternate mode draws each channel alternately - the oscilloscope completes one sweep on channel 1, then one sweep on channel 2, a second sweep on channel 1, and so on. Use this mode with medium- to high-speed signals, when the sec/div scale is set to 0.5 ms or faster.

Chop mode causes the oscilloscope to draw small parts of each signal by switching back and forth between them. The switching rate is too fast for you to notice, so the waveform looks whole. You typically use this mode with slow signals requiring sweep speeds of 1 ms per division or less. Figure 3 shows the difference between the two modes. It is often useful to view the signal both ways, to make sure you have the best view.



Figure 3: Multi-Channel Display Modes

Math Operations

Your oscilloscope may also have operations to allow you to add waveforms together, creating a new waveform display. Analog oscilloscopes combine the signals while digital oscilloscopes mathematically create new waveforms. Subtracting waveforms is another math operation. Subtraction with analog oscilloscopes is possible by using the channel invert function on one signal and then use the add operation. Digital oscilloscopes typically have a subtraction operation available. Figure 4 illustrates a third waveform created by adding two different signals together.



Figure 4: Adding Channels

Horizontal Controls

Use the horizontal controls to position and scale the waveform horizontally. Figure 5 shows a typical front panel and on-screen menus for the horizontal controls.



Figure 5: Horizontal Controls

Position and Seconds per Division

The horizontal position control moves the waveform from left and right to exactly where you want it on the screen.

The seconds per division (usually written as sec/div) setting lets you select the rate at which the waveform is drawn across the screen (also known as the time base setting or sweep speed). This setting is a scale factor. For example, if the setting is 1 ms, each horizontal division represents 1 ms and the total screen width represents 10 ms (ten divisions). Changing the sec/div setting lets you look at longer or shorter time intervals of the input signal.

As with the vertical volts/div scale, the horizontal sec/div scale may have variable timing, allowing you to set the horizontal time scale in between the discrete settings.

Time Base Selections

Your oscilloscope has a time base usually referred to as the main time base and it is probably the most useful. Many oscilloscopes have what is called a delayed time base - a time base sweep that starts after a pre-determined time from the start of the main time base sweep. Using a delayed time base sweep allows you to see events more clearly or even see events not visible with just the main time base sweep.

The delayed time base requires the setting of a delay time and possibly the use of delayed trigger modes and other settings not described in this book. Refer to the manual supplied with your oscilloscope for information on how to use these features.

Trigger Position

The trigger position control may be located in the horizontal control section of your oscilloscope. It actually represents "the horizontal position of the trigger in the waveform record." Horizontal trigger position control is only available on digital oscilloscopes.

Varying the horizontal trigger position allows you to capture what a signal did before a trigger event (called pretrigger viewing).

Digital oscilloscopes can provide pretrigger viewing because they constantly process the input signal whether a trigger has been received or not. A steady stream of data flows through the oscilloscope; the trigger merely tells the oscilloscope to save the present data in memory. In contrast, analog oscilloscopes only display the signal after receiving the trigger.

Pretrigger viewing is a valuable troubleshooting aid. For example, if a problem occurs intermittently, you can trigger on the problem, record the events that led up to it and, possibly, find the cause.

Magnification

Your oscilloscope may have special horizontal magnification settings that let you display a magnified section of the waveform on-screen.

XY Mode

Most oscilloscopes have the capability of displaying a second channel signal along the X-axis (instead of time). This is called XY mode; you will find a longer discussion later in this book.

Trigger Controls

The trigger controls let you stabilize repeating waveforms and capture single-shot waveforms. Figure 6 shows a typical front panel and on-screen menus for the trigger controls.



Figure 6: Trigger Controls

The trigger makes repeating waveforms appear static on the oscilloscope display. Imagine the jumble on the screen that would result if each sweep started at a different place on the signal (see Figure 7).



Figure 7: Untriggered Display

Trigger Level and Slope

Your oscilloscope may have several different types of triggers, such as edge, video, pulse, or logic. Edge triggering is the basic and most common type and is the only type discussed in this book. Consult your oscilloscope instruction manual for details on other trigger types.

For edge triggering, the trigger level and slope controls provide the basic trigger point definition.

The trigger circuit acts as a comparator. You select the slope and voltage level of one side of the comparator. When the trigger signal matches your settings, the oscilloscope generates a trigger.

Figure 8 shows you how the trigger slope and level settings determine how a waveform is displayed.



Figure 8: Positive and Negative Slope Triggering

Trigger Sources

The oscilloscope does not necessarily have to trigger on the signal being measured. Several sources can trigger the sweep:

Most of the time you can leave the oscilloscope set to trigger on the channel displayed.

Note that the oscilloscope can use an alternate trigger source whether displayed or not. So you have to be careful not to unwittingly trigger on, for example, channel 1 while displaying channel 2.

Trigger Modes

The trigger mode determines whether or not the oscilloscope draws a waveform if it does not detect a trigger. Common trigger modes include normal and auto.

In normal mode the oscilloscope only sweeps if the input signal reaches the set trigger point; otherwise (on an analog oscilloscope) the screen is blank or (on a digital oscilloscope) frozen on the last acquired waveform. Normal mode can be disorienting since you may not see the signal at first if the level control is not adjusted correctly.

Auto mode causes the oscilloscope to sweep, even without a trigger. If no signal is present, a timer in the oscilloscope triggers the sweep. This ensures that the display will not disappear if the signal drops to small voltages. It is also the best mode to use if you are looking at many signals and do not want to bother setting the trigger each time.

In practice, you will probably use both modes: normal mode because it is more versatile and auto mode because it requires less adjustment.

Some oscilloscopes also include special modes for single sweeps, triggering on video signals, or automatically setting the trigger level.

Trigger Coupling

Just as you can select either AC or DC coupling for the vertical system, you can choose the kind of coupling for the trigger signal.

Besides AC and DC coupling, your oscilloscope may also have high frequency rejection, low frequency rejection, and noise rejection trigger coupling. These special settings are useful for eliminating noise from the trigger signal to prevent false triggering.

Trigger Holdoff

Sometimes getting an oscilloscope to trigger on the correct part of a signal requires great skill. Many oscilloscopes have special features to make this task easier.

Trigger holdoff is an adjustable period of time during which the oscilloscope cannot trigger. This feature is useful when you are triggering on complex waveform shapes, so that the oscilloscope only triggers on the first eligible trigger point. Figure 9 shows how using trigger holdoff helps create a usable display.



Figure 9: Trigger Holdoff

Acquisition Controls for Digital Oscilloscopes

Digital oscilloscopes have settings that let you control how the acquisition system processes a signal. Look over the acquisition options on your digital oscilloscope while you read this description. Figure 10 shows you an example of an acquisition menu.



Figure 10: Example of an Acquisition Menu

Acquisition Modes

Acquisition modes control how waveform points are produced from sample points. Recall from the first section that sample points are the digital values that come directly out of the Analog-to-Digital-Converter (ADC). The time between sample points is called the sample interval. Waveform points are the digital values that are stored in memory and displayed to form the waveform. The time value difference between waveform points is called the waveform interval. The sample interval and the waveform interval may be but need not be the same. This fact leads to the existence of several different acquisition modes in which one waveform point is made up from several sequentially acquired sample points. Additionally, waveform points can be created from a composite of sample points taken from multiple acquisitions, which leads to another set of acquisition modes. A description of the most commonly used acquisition modes follows.

Stopping and Starting the Acquisition System

One of the greatest advantages of digital oscilloscopes is their ability to store waveforms for later viewing. To this end, there are usually one or more buttons on the front panel that allow you to stop and start the acquisition system so you can analyze waveforms at your leisure. Additionally, you may want the oscilloscope to automatically stop acquiring after one acquisition is complete or after one set of records has been turned into an envelope or average waveform. This feature is commonly called single sweep or single sequence and its controls are usually found either with the other acquisition controls or with the trigger controls.

Sampling Methods

In digital oscilloscopes that can use either real-time sampling or equivalent-time sampling as described earlier, the acquisition controls will allow you to choose which one to use for acquiring signals. Note that this choice makes no difference for slow time base settings and only has an effect when the ADC cannot sample fast enough to fill the record with waveform points in one pass.

Other Controls

So far we have described the basic controls that a beginner needs to know about. Your oscilloscope may have other controls for various functions. Some of these may include:

Look over the other options available to you and read your oscilloscope's manual to find out more about these other controls.

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