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HB 01-26-09 Oscilloscope (1) Lab 5 1

Oscilloscope Lab 5

Equipment SWS, BK Precision model 2120B oscilloscope,Wavetek FG3C function generator,

2-3 foot coax cable with male BNC connectors,

Reading How To Use Science Workshop, pp. 170-183 (scope display). Review the “Voltage,

Current, and Resistance” writeup for the use of the SWS signal generator.

Remark The oscilloscope is a diﬃcult instrument to understand, but it is also a very impor-

tant instrument. Studying this write-up carefully before the lab will be worthwhile. This

lab cannot be approached casually.

1 Introduction

The standard instrument for examining time dependent voltages in a circuit is the oscillo-

scope, or “scope” for short. Even if you are doing a DC experiment it is a good idea to

look at the circuit with a scope. You might ﬁnd a large AC signal symmetric with respect

to ground that does not show up on DC instruments but is aﬀecting your experiment in

undesirable ways, such as overloading a sensitive ampliﬁer. Oscilloscopes are ubiquitous in

the medical profession. They are seen next to hospital beds showing the electrical output of

a heart. They are used extensively in operating rooms.

A voltage which depends on time can be drawn on graph paper with the voltage on the

vertical axis and the time on the horizontal axis. If the voltage is periodic (repeats itself

after one cycle) an oscilloscope will present a similar curve. By adjusting the scope it is

possible to display part of one cycle or many cycles of the waveform. Properties of a periodic

voltage, such as shape, peak-to-peak amplitude and period, can be measured. Is that sine

wave from your signal generator distorted? Feed it into a scope and ﬁnd out. Scopes have

other uses, but these are the primary subjects of this lab.

The oscilloscope is a sophisticated instrument. Competent use depends on understanding

how this instrument works. It is often necessary to consult the instruction manual for a

particular scope to use the scope eﬀectively. The basic ideas are usually the same but are

implemented in diﬀerent ways.

In this lab you will become familiar with 2 oscilloscopes, the model 2120B made by BK

Precision, and the SWS scope. Both these instruments do very much the same thing except

that the controls on the BK are mostly knobs and switches and those on the SWS are icons

such as buttons. The SWS scope is digital. The input waveform is digitized and stored, at

least for awhile. The BK scope is analogue. The input voltage may be ampliﬁed, but it is

not digitized and stored.

2 An Analogy, or the Case of the Vanishing Ink

A slowly varying periodic voltage is observed on a voltage meter. A curve of the voltage

versus time is desired. On a long strip of paper are put a vertical axis for voltage and a

horizontal axis for time. At time t

a plot of voltage versus time is started when the voltage

has the value V

and a positive slope (voltage increasing). See Fig. 1. At time t

it is noticed

that the ink around the point at time t

has started to fade. There is good reason to believe

that after not too long a time the ink will vanish. The plotting is stopped at time t

. The

Summary of content (16 pages)

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HB 01-26-09 Oscilloscope (1) Oscilloscope Lab 5 1 Lab 5 Equipment SWS, BK Precision model 2120B oscilloscope,Wavetek FG3C function generator, 2-3 foot coax cable with male BNC connectors, Reading How To Use Science Workshop, pp. 170-183 (scope display). Review the “Voltage, Current, and Resistance” writeup for the use of the SWS signal generator. Remark The oscilloscope is a difficult instrument to understand, but it is also a very important instrument.
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HB 01-26-09 Oscilloscope (1) Lab 5 2 time interval plotted is P = t2 − t1 . In Fig. 1 the curve to the right of t2 is dotted. No plotting is done for a time interval N but the actual voltage is following the dotted curve. At time t3 = t2 + N , when the voltage has positive slope and the value VT (the same values at time t1 ) the plotting on the paper is resumed not at horizontal axis point t3 but at horizontal axis point t1 . Again the plotting is stopped at horizontal axis point t2 .
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HB 01-26-09 Oscilloscope (1) Lab 5 3 passes to the outside of the tube and can be observed. If the electron beam is suddenly cut off, the light from the phosphor does not stop immediately but decays in less than a second. The flat part of the tube looked at is called the screen. When you are looking at the screen the electron beam is coming toward you.
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HB 01-26-09 Oscilloscope (1) Lab 5 4 screen. When the ramp voltage across the horizontal deflection plates is VL , the electron beam is at the left of the screen. When the ramp voltage is zero the beam is in the horizontal center of the screen. When the ramp voltage is VR the beam is at the right of the screen. We assume that |VL |=|VR |. How quickly the electron beam is swept across the screen is controlled by the TIME BASE switch.
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HB 01-26-09 Oscilloscope (1) Lab 5 5 voltage and always tracing out the same curve. The result is a stationary trace of the input voltage on the scope screen. In the example shown in Fig. 4, note that somewhat less than one cycle of the input voltage will be displayed. During the time intervals “N” the electron beam is blanked and is not being swept across the phosphor. An additional point is that once a sweep starts it is always completed, even if the trigger conditions are met during the sweep.
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HB 01-26-09 Oscilloscope (1) Lab 5 6 • POWER(1,3)-Button (3) turns the scope on and off. LED (1) indicates when the scope is on. • CRT(4-6) (cathode ray tube)- controls the intensity and focus of the electron beam, and the sharpness of the trace. • GND and CAL(9-10)- This box is not named but provides a calibration voltage and ground. These will not be used in this lab. • VERTICAL(12-22)- This box is really divided into two boxes. The scope has two voltage amplifiers for two possible input voltages.
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HB 01-26-09 Oscilloscope (1) Lab 5 7 12. VERTICAL MODE switch. Set to CH1 (channel 1 or X input). 13. CH1 AC-GND-DC switch. Set this switch at AC. This puts a blocking capacitor in the input to the scope, suitable for AC signals. If in the future you wish to look at DC signals, choose DC. The GND position disconnects the input signal and grounds the scope input. This is very useful if you want to know what vertical position of the trace corresponds to zero volts. 14. CH1 INPUT JACK.
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HB 01-26-09 Oscilloscope (1) Lab 5 8 When this control is in the CAL position, control 23 is calibrated. Set in CAL position (fully clockwise). 27. HORIZONTAL POSITION/PULL control. Adjusts the horizontal position of the trace. Set pointer up. This control knob can be pulled out to expand the trace horizontally. Set the knob in. 29. X-Y button. This button should be out. (button is “in” for x-y operation.) 30. HOLDOFF/PULL CHOP control. Check that this knob is in and full counterclockwise. 31.
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HB 01-26-09 Oscilloscope (1) Lab 5 9 the effect of turning the variable sweep rate knob (26). Increase the sweep speed one click at a time and note how the moving spot becomes a solid line. Observe that there is a range of sweep speeds for which the trace “flickers.” This is due to the persistence of the phosphor or of the human eye, or perhaps both. When you need to make measurements of an input signal (peak voltage, period, etc.
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HB 01-26-09 Oscilloscope (1) Lab 5 10 The controls you have just used are the ones most frequently employed in adjusting the trace. It is instructive to look at a low frequency signal from the Wavetek oscillator. If the frequency is appropriate, the time development of a trace as shown in Fig. 4 can be followed. Set the frequency of the Wavetek oscillator to about 5 Hz and the time base to 50 ms. Observe the development of the trace carefully and note the effect of changing the trigger parameters.
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HB 01-26-09 6.6 Oscilloscope (1) Lab 5 11 Scope Probes In this experiment the function generator is connected to the scope by a cable with connectors on both ends. Very often voltages in a circuit are of interest and a scope probe is used. This is a cable with a connector on one end for plugging into the scope input. The other end of the probe cable has an electrode for touching the point in the circuit whose voltage waveform is desired. There is a clip for making a connection to ground.
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HB 01-26-09 Oscilloscope (1) Lab 5 12 window. Click auto. Drag the scope display icon to the terminals labeled V to open the scope display. Observe the different signals from the signal generator with the scope display and thoroughly familiarize yourself with the controls of the SWS scope. You may have trouble getting a trace with the 3 positive only waveforms from the SWS signal generator. This may be due to the fact that the default trigger level is 0.
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HB 01-26-09 Oscilloscope (1) Lab 5 13 You will find that the two knobs labeled SWEEP TIME and MOD/DEPTH-SWEEP RATE affect the waveform. Can you figure out exactly what these knobs do? Does it make any difference whether these knobs are in or out? When you are finished, put the frequency knob back in. • MODULATION A “carrier (wave)” is a single frequency (sinusoidal) electromagnetic wave. For radio broadcasting in the U.S.
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HB 01-26-09 Oscilloscope (1) Figure 1: Figure 2: Lab 5 14
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HB 01-26-09 Oscilloscope (1) Figure 3: Figure 4: Lab 5 15
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HB 01-26-09 Oscilloscope (1) Figure 5: Lab 5 16