Instruction manual

ManualsBrandsB&K Precision ManualsPower Tools2120B

HB 08-18-2010 1

Oscilloscope

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

2-3 foot coax cable with male BNC connectors, 2 voltage sensors

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 in order to use it 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

and 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 Analogue Oscilloscope Basics

The heart of an analogue scope is a cathode ray tube (CRT). See Fig. 2. A cathode ray is

an historical name for a beam of electrons. The “tube” refers to the glass vacuum envelope.

Electrons are thermally emitted from a hot cathode. They are accelerated (through 2 kV

in the BK scope) by an electrode and then focused into a thin beam by electrostatic lenses.

A vacuum is necessary so that the electrons will not be scattered by air molecules and so

that the cathode will not burn up. The electron beam is then passed through 2 pairs of

Summary of content (16 pages)

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HB 08-18-2010 1 Oscilloscope Equipment SWS, BK Precision model 2120B oscilloscope,Wavetek FG3C function generator, 2-3 foot coax cable with male BNC connectors, 2 voltage sensors 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 08-18-2010 2 deflection plates. A voltage across one pair of deflection plates deflects the electron beam in the vertical direction, while a voltage across the other pair of deflection plates deflects the electron beam in the horizontal direction. After passing through the deflection plates the electron beam strikes a phosphor material that covers the inside surface of a flat portion of the vacuum tube.
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HB 08-18-2010 3 3 Description of How a Scope Displays a Single Periodic Voltage A periodic function in time is one that repeats itself again and again, such as a sine wave or a square wave. A scope can display a voltage that is periodic in time in exactly the same way. The periodic voltage is connected to the vertical input of the scope, and if the scope is adjusted correctly, a graph of the voltage as a function of time appears as a stationary trace on the screen of the scope.
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HB 08-18-2010 4 We consider the case where the trigger pulse is derived from the input voltage, which is the voltage that is to be observed. The operator of the scope, by using a scope control usually marked “trigger level”, chooses a value of the input voltage at which the trigger pulse is to occur. We call this voltage VT . This can be a positive or negative voltage, but for the trigger pulse to occur VT must be between between the maximum and minimum values of the input voltage.
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HB 08-18-2010 5 buttons, connectors, etc. The controls that are used mainly for dual trace or x-y use will not be discussed other than to adjust them for single trace use. There are a lot of controls, and unless you have a physical picture of what the controls do you will not be able to use the scope in a reasonable sort of way. Review the previous section and Fig. 1 as necessary. Fig. 5 is a picture of the front panel of the BK scope. The controls are numbered.
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HB 08-18-2010 6 1. ON indicator. Lights when scope is on. 3. POWER pushbutton. Turns scope on and off. Leave scope off (button out). 4. INTENSITY control knob. Adjusts the current in the electron beam and therefore the brightness of the trace. Set pointer up. 5. Trace rotation. Do not adjust. 6. FOCUS control knob. Adjusts trace focus. Set pointer up. 9. Ground. Not used in this lab. 10. Calibrate. Provides a 2 V p-p 1 kH calibration square wave. Not used in this lab. 12. VERTICAL MODE switch.
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HB 08-18-2010 7 20. CH2 VARIABLE/PULL control. Adjusts the vertically sensitivity of channel 2 continuously. 21. CH2 INPUT JACK. Not needed in this lab. 22. CH2 AC-GND-DC switch. Not needed in this lab, but set at GND. 23. Horizontal TIME/DIV control. Adjusts the rate, in discreet steps, at which the electron beam is swept across the screen. For the numbers to be valid, control 26 (VAR SWEEP) must be in the CAL position.
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HB 08-18-2010 8 helpful to refer to previous sections during these exercises. Reminder: the VOLTS/DIV and TIME/DIV switches are not calibrated unless the knobs in the center of these switches are fully clockwise in the calibrated positions. 5.1 Preliminaries Remove any input cables to the scope. Turn on the scope (control 3) and observe LED (1). In a few seconds you should see a trace which is a horizontal line (it takes the cathode a few seconds to warm up).
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HB 08-18-2010 9 1. the VOLTS/DIV controls are changed (15 and 16)? 2. the TIME/DIV controls are changed (23 and 26)? 3. the trigger level (33) voltage is changed (33)? Note that if the trigger voltage is set too high or too low the trace does not “freeze” but “runs” to the right or the left if the trigger coupling switch is on AUTO. 4. With the trigger COUPLING switch in the NORMal mode, can you make the trace disappear by rotating the TRIG LEVEL knob (33) both ways? Explain. 5.
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HB 08-18-2010 10 up most of the screen and horizontally there is about one cycle of the input waveform on the screen. Use the calibration of the VOLTS/DIV control and the horizontal grid lines on the screen to determine the peak to peak voltage of the triangular wave. Use the calibration of the TIME/DIV control and the vertical grid lines on the screen to determine the period of the wave.
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HB 08-18-2010 11 trigger pulses to produce a trace. This corresponds to AUTO for control 32 of the BK scope. A difference here that that the TRIG button must be in to freeze the trace with the SWS scope. In using the SWS scope display, use MON rather than REC. An enormous amount of data flows from the scope and if you use REC the storage capacity of SWS will be over taxed. 6.1 Voltage From 750 Interface In this section voltages from the SWS signal generator will be examined with the SWS scope.
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HB 08-18-2010 7 12 Analog versus Digital Scopes In an analog scope the input voltage, after amplification or attenuation, is applied to the vertical deflection plates of the scope. The scope trace is smooth. In a digital scope, such as the SWS scope, the input signal is sampled at the “sampling rate” and the scope trace is these values usually connected by straight lines. You may have noticed this fact in looking at the SWS scope trace.
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HB 08-18-2010 13 The output of the Wavetek function generator can be modulated at 400 Hz by an internally generated sine wave. The modulation can be either AM (amplitude modulation) or FM (frequency modulation). To enable this function push the button marked MOD ON. The knob marked MOD/DEPTH-SWEEP RATE controls whether the modulation is AM (knob in) or FM (knob out). The amount of modulation is controlled by rotating this knob. An output or “ carrier” frequency of about 10 kHz is suggested.
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HB 08-18-2010 14 Figure 1: Figure 2:
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HB 08-18-2010 15 Figure 3: Figure 4:
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HB 08-18-2010 16 Figure 5: