|Introduction||Display Controls||Input Amplifiers|
|Trigger Control||Timebase Setting||Dual Trace Operation|
XYZs of Analog and Digital Oscilloscopes, from Tektronix.
Although we use a simple analog scope in the PHYS345 lab, it is worth looking over all sections of this online tutorial to familiarize yourself with digital and sampling oscilloscopes.
BK Precision 2020 Dual Trace Oscilloscope (or equivalent).
Pasco 8000 Power Supply (or equivalent) with 5.0 V dc and 6.3 V ac.
2 BNC male-to-double banana jack adapters.
At least 3 banana plug patch cords (including black and red).
Figure 1. BK Precision 2120 Oscilloscope.
The single most important diagnostic tool used by experimental physicists is the oscilloscope. Certainly all scientists and engineers should be familiar with this common instrument, shown in the Fig. 1. An oscilloscope (scope for short) can be used to "see" an electrical signal by displaying a replica of a voltage signal as a function of time. The display is generated by a sweeping electron beam striking a fluorescent screen, the same principle behind common television displays. The purpose of this lab exercise is to introduce the fundamentals of oscilloscope operation and its practical use.
The oscilloscope is used to obtain "voltage versus time" pictures of electrical signals. The display consists of a tube with an electron gun, x and y-deflection plates, and a phosphor screen which glows in response to an internal electron beam. In normal operation the beam is swept continuously from left to right at a uniform speed. (The beam is shut off during its rapid return to the left side.) A triggered ramp generator generates this sweep motion by applying a "sawtooth" voltage to the x-deflection plates, as indicated in Fig. 2. The sweep rate and method of triggering can be varied using controls on the front panel. An external voltage connected to one of the oscilloscope inputs can be internally amplified and applied to the y-deflection plates. The combination of the x and y-motions then causes the beam to trace out a plot of the input voltage as a function of time.
Admittedly, the dozen or so knobs and switches on the front panel of a typical oscilloscope can be a bit daunting to a beginning user. The relatively inexpensive scopes found in most introductory physics labs provide essentially the same features though, so that familiarity gained in this exercise should provide the necessary background for the successful operation of any oscilloscope. The four main scope control areas are 1) display controls, 2) input amplifiers, 3) trigger selection, and 4) timebase setting; each will be discussed in turn.
Figure 2. Block diagram of oscilloscope operation.
Figure 3. Front panel of BK Precision 2120 Oscilloscope.
To begin, plug the cord into the wall socket, adjust the front support for easy viewing, and turn on the scope by depressing the red power button at the lower right of the front panel. Configure the knob and switch settings as shown in Fig. 3 and listed in Table 1. Most of these settings will be discussed at length.
After the scope warms up, with these initial settings, a horizontal line (or possibly two) should appear on the screen. If not, try the positioning knobs explained below or ask the lab instructor for help. Use the INTENSITY knob to adjust the brightness of the so-called trace, but no more intense than necessary as this has a detrimental effect on the phosphor of the display. Use the FOCUS knob to adjust the sharpness of the trace.
Positioning of the trace is accomplished with the knobs labeled with vertical or horizontal arrows. In the upper right hand corner, moves the trace horizontally. The position knobs in the middle of the front panel move their respective traces vertically. Can the trace be sent completely off the screen in any direction by adjusting the positioning knobs?
|Table 3.1. Initial Oscilloscope Settings|
|vertical mode:||all buttons out|
|NORM CH1 MONO ALT|
|CH1/2 inputs:||sensitivity VOLTS/DIV||0.2 V/div|
|input coupling AC/GND/DC||DC|
|variable sensitivity||full clockwise (CAL)|
|LEVEL||midrange, pushed in (AUTO)|
|variable sweep||full clockwise (CAL)|
|PULL x10||pushed-in (normal)|
The input stage of the oscilloscope is used to couple the signal of interest to the input amplifiers, needed to increase the signal amplitude to a level appropriate to drive the electron beam deflection circuitry. The type of coupling can be selected (dc/ac) as well as the overall gain of the amplifier (VOTLS/DIV) to control the overall vertical "height" of the trace, as discussed further below.
A calibration output, available at the metal tab labeled CAL 0.2 Vpp near the power switch, will be used as the input signal for the course of this exercise. It provides a known, regular waveform by which the oscilloscope may be calibrated to ensure that subsequent readings are accurate. The calibration signal, shown in Fig. 4 is a 0.2 V amplitude square wave, so called because of the appearance that results from the "high" voltage level (0.2 V) lasting for the same duration as the low level (0.0 V). The calibration signal has a frequency of 1000 Hz (period 1.0 ms).
Figure 4. Calibration signal.
Connect the CAL signal to the scope input by attaching a hook-tip scope probe to the channel 1 (CH1) input. Push in the adapter plug and twist until it locks in place (thus the "bayonet" action of the "B"NC) and connect the hook tip of the probe to the BNC signal. In general, both leads of the scope probe would need to be connected to measure a potential difference. However, since the signal is derived from the oscilloscope itself, the ground reference is already present and no additional connection is required; generally an additional connection would be made between the black banana clip of the probe and the ground of the circuit being measured. While using the probe in this exercise, please ensure that x1 sensitivity is selected on the probe handle.
To facilitate accurate readings, the ground level for the trace should be checked periodically, Move the slide switch for CH1 marked AC/GND/DC to the GND position. Adjust the vertical position of the trace to an easily remembered position on the display, generally the axis of the reticle.
Now move the slide switch to the DC setting. If the sensitivity is set to 0.2 V/div and the timebase is at 0.5 ms/div, a trace should appear similar to that shown in Fig. 4. If not, see your lab instructor for assistance. Note: the trigger LEVEL may need to be adjusted slightly to obtain a stable trace, discussed at length later in the section on triggering.
Study the effects of changing the sensitivity (VOLTS/DIV). Draw the waveform seen on the display and record the voltage from top to bottom of the square wave. To take a voltage reading: 1) measure the height of the square wave, using the calibrated grid where each square is 1 div x 1 div, and 2) multiply the number of divisions by the sensitivity setting (V/div). Take readings at several sensitivities; discuss which provides the greatest accuracy. Ensure that the outer sensitivity knob, which provides a variable gain if needed, is in the CAL position (fully clockwise and in the detent position) before making the readings.
The difference between dc and ac coupling can be understood by examining Fig. 2. As you will see later in the course, the capacitor shown in the ac stage prevents the average dc voltage from reaching the CH1 amplifier. Determine the average dc level associated with the CAL signal by switching between ac and dc coupling and observing the change in the vertical level of the trace.
The real utility of ac coupling becomes apparent when trying to measure small variations "riding" on a sizable dc level. As an example, consider the relatively small "ripple" (~5 mV) on the output of a typical dc power supply (say 5 V). With dc coupling, increasing the input sensitivity in an attempt to see the small variation will send the trace off the top of the screen. With ac coupling however, the 5 V level will be suppressed and the sensitivity can be greatly increased while retaining the trace on the display.
Use the 5 V dc power supply to confirm this behavior. Use a BNC-banana adapter; this adapter is color-coded, red for the signal input and black for the ground reference (presumably 0.0 V). With dc coupling, first confirm that the output voltage of the power supply is correct. If you then try to examine the ripple more closely by increasing the sensitivity, the trace is soon driven off the edge of the display. With ac coupling, the dc level is suppressed and the ripple can be observed in detail; measure its amplitude and compare to the dc level. (You may wish to slow the timebase to 5 ms/div.)
The triggering circuitry is the means by which the oscilloscope is able to provide repeated "snapshots" of a repetitive signal. By controlling the threshold voltage of triggering and slope polarity, the user is provided with great flexibility in the appearance of the display and the utility of the scope for capturing elusive signals. Some further details of scope operation are required to understand the concept of triggering.
The horizontal sweep of the electron beam is controlled by the timebase circuitry to be discussed in the next section. The beam sweep is driven by the application of a ramp voltage to the horizontal deflection plates, which serves to move the beam across the face a distance proportional to the elapsed time from a trigger event. The occurence of the trigger event marks the start of the linear ramp. By slowing the timebase to about 0.1 sec/div, the actual deflection of the beam can be slowed to the point where the resulting luminous "dot" can be observed crossing the display from left to right.
Figure 5. Effects of slope and threshold on triggering.
To examine the effects of the trigger settings on the triggering of the display, use the 6.3 V ac (60 Hz) output available on the power supply as the signal for CH1 and increase the sensitivity to 5 V/div. In addition, select 2 ms/div as the sweep speed. With the trigger SOURCE set for CH1, the LEVEL set to the midrange, and SLOPE set as +, a stable trace showing the sinsusoidal wave should appear. Adjust the horizontal position of the trace until its starting position is seen on the display. Adjust the LEVEL control to the right (positive threshold voltage) and to the left (negative threshold voltage), noting the change in the starting point of the trace. As the threshold for the trigger in increased, the starting point comes later at a higher voltage. Refer to Fig. 5 for a pictorial representation of the various triggering combinations. Setting the trigger level too low or too high (outside the bounds of the signal amplitude) results in loss of triggering and an unstable "running" waveform.
Switch the trigger SOURCE control to CH1 and observe the change. The +/- of the SLOPE selector chooses whether the trigger event will occur on moving through the threshold from high voltage to low (negative slope, thus -) or from low to high (positive slope). Refer again to Fig. 5.
Time may permit examination of the external trigger selection ext. It is on this setting that an additional, better-conditioned signal can be used to trigger the scope while observing a relatively weak signal, one too weak to use the internal triggering discussed above. For example, you may observe again the ripple on the dc power supply or the stray signal picked up by a loose lead, using the 60 Hz, 6.3 V ac signal to trigger the scope externally. External triggering is accomplished by connecting the signal to be used fro triggering to the external trigger input of the scope and selecting EXT on the trigger SOURCE selector.
The sweep speed of the electron beam is controlled with the timebase setting (TIME/DIV). Looking again at the 60 Hz, 6.3 V ac signal, start from 5 ms/div and increase the timebase to larger settings (slower sweeps) -- a longer "snapshot" of the signal is displayed but with less detail. Remember that the 60 Hz signal has not changed; the beam sweeps more slowly, so more transitions are displayed. Moving to much smaller timebase settings (faster sweeps), more detail can be seen at the expense of the "big picture". Notice the decrease in intensity of the trace as the electron beam moves more swiftly over the screen at faster timebase settings.
Returning to a timebase around 5 ms/div, confirm that the period of the 6.3 V ac signal is that of a 60 Hz oscillation. Adjust the trigger level if necessary to achieve a stable display. Measure the horizontal distance between the two closest points where the waveform crosses the baseline (zero volt level) with the same slope. (Note that there are two zero crossings per cycle, each with different slope.) Multiply this number of divisions by the timebase setting (time/div); the resulting time T is known as the period. Repeat for the scope's CAL signal.
How short a time can be read by this oscilloscope? This can be estimated by the smallest timebase setting. In practice, this setting may result in a trace to weak to be seen; thus the so-called writing speed of the oscilloscope may also limit the shortest time that can be accurately determined, as will the bandwidth of the input amplifiers.
Dual Trace Operation
The availability of two independent input channels greatly facilitates comparison of two signals. This is implemented by depressing the button marked MONO/DUAL in the vertical mode section of the front panel. While displaying the 6.3 V ac signal on CH1, using it as a trigger source, observe the signal from a loose lead on CH2. You will probably observe "spikes" and other types of electrical noise. You should attempt to explain why this noise is "locked" to the signal on CH1. For a more dramatic effect, touch your finger to the CH2 input jack. The noise probably has much higher amplitude and is still locked to the calibation signal. Finally, while holding the bare lead in one hand, touch the metal frame of the table and observe the noise signal on CH2. Attempt to explain your observations.
There are two modes of dual-trace operation available on most oscilloscopes. The CHOPPED mode is the more useful one at low frequencies. In this mode, the beam alternately displays the two signals. The switching occurs at a relatively fast rate (2 microsec in the Model 2120 you are using) with blanking in between, so that all that should be seen on the screen are the two separate traces. The other dual-trace display mode is called the alternating mode. In this mode the output of CH1 is displayed for a full sweep, then the output of CH2 is displayed for a full sweep, and so on. When the two inputs are repetitive and synchronized this leads to a stable display. You can observe the difference between the chopped and alternate modes by looking again at the 6.3 V ac signal, first with a slow timebase setting of 5 ms/div and then with a much faster setting, while switching between the ALT and CHOP positions.
XYZs of Oscilloscopes, from Tektronix
Oscilloscopes from B+K Precision