CRO new mannual

7/25/2019 CRO new mannual

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Govt. Engineering College,AjmerDepartment of Electrical Engineering (Lab Manual)

Electrical Measurement Lab(1)

1. AM! Cathode-ray Oscilloscope (CRO) To introduce the basic structure of a cathode-ray Oscilloscope.

To get familiar with the use of different control switches of the device.

To visualize an ac signal measure the amplitude and the fre!uency.

". A##A$A%&'! Cathode-ray Oscilloscope (CRO)"unction #enerator

. %E*$+Cathode-ray Oscilloscope

The device consists mainly of a vacuum tube which contains a cathode anode grid $%&-

plates and a fluorescent screen (see "igure below). 'hen the cathode is heated (by applying

a small potential difference across its terminals) it emits electrons. aving a potential

difference between the cathode and the anode (electrodes) accelerate the emitted electrons

towards the anode forming an electron beam which passes to fall on nthe screen. 'hen the

fast electron beam stries the fluorescent screen a bright visible spot is produced. The grid

which is situated between the electrodes controls the amount of electrons passing through it

thereby controlling the intensity of the electron beam.

igure 1! The basic structure of a CRO

The $%&-plates are responsible for deflecting the electron beam horizontally and vertically.

* sweep generator is connected to the $-plates which moves the bright spot horizontallyacross the screen and repeats that at a certain fre!uency as the source of the signal. The


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voltage to be studied is applied to the &-plates. The combined sweep and &voltages produce

a graph showing the variation of voltage with time as shown in "ig. +.

Front panel

The front panel of the CRO is shown in "ig. +.

igure "! The front panel of the CRO

Alternating current (ac)

*n ac signal can be of different forms, sinusoidal s!uare or triangular. The sinusoidal

is the most popular type which is the natural output of the rotary electricity generators.

*n ac voltage source can be represented by

(t) m sin(t+

) ()

where m is the ma/imum output voltage value =+ f ( f is the fre!uency) and is the

phase shift. The root-mean-s!uare value rms of the signal given by 0!.() can be

written as

(+)


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which is reduced to

CRO 1RO203,-

1robe is a physical device used to connect an electronics test e!uipment to the device under test.

i.e. connecting an electronic device to cro for testing with help of long wire which is attached with

an banannalugs tips to cro end and crocodile clips on other end. 4easuring an electrical signal

inevitably affects that signal. This applies to all measurements including the display of an

oscilloscope waveform. *ffecting the signal cannot be totally eliminated but it can be minimized

sufficiently that the effect is unimportant. Then the measured result is a sufficiently accurate

representation of the real signal. 5t is therefore critical for the measurement engineer to understand

the effect of the instrument on the signal.

* /6 scope probe (figure ) is useful in several applications,

7 To reduce loading effect on the circuit under test

7 To compensate for the effect of test cable capacitance

7 To permit the measurement of large voltages

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The compensation capacitance (Cp in figure 8) of the 76 probe must be ad9usted to suit the input

capacitance of the oscilloscope. This re!uires ad9usting the compensation each time the probe is

moved to a different scope. To compensate the probe it is driven with a s!uare wave:. The resultant

scope display will appear as one of the three waveforms in figure

The upper display is under-compensated; the compensation capacitance is too small. 5n the lower

display it is overcompensated the compensation capacitance is too large. The centre display is the

#oldilocs version, 9ust right.


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These s!uare waves indicate the response of the probe over a range of fre!uencies. The upper

waveform with its undershoot corresponds to a depressed high-fre!uency response. The lower

waveform corresponds to an emphasized high-fre!uency response. The centre waveform

corresponds to a flat fre!uency response.

The compensation capacitance may typically be found in one of three locations,

7 *s a screwdriver ad9ustment in the body of the probe (visible as the small dot on the probe body in

figure ).

7 *s a screwdriver ad9ustment in the base of the probe cable where the cable plugs into the scope.

7 *s a coa/ial capacitor ad9ustment in the probe. 5n this case the probe body includes a locing ring

which is rotated to unloc the compensation ad9ustment. 1art of the probe is then rotated to ad9ust

the probe compensation. 'hen the compensation is correct the locing ring is re-tightened.

-. #$*CED&$E!

#art one

. Turn on the Oscilloscope wait a couple of seconds to warm up then the trace will

show up on the screen.

+. *d9ust the intensity and the focus of the trace.

. * signal will appear on the screen.

?. 4ae sure that the inner red nobs of the @oltABiv and the TimeABiv are loced

Cloc wise.

. 3et the fre!uency of the generator to +66 z.


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D. *d9ust the @oltABiv and the TimeABiv nobs so that you get a suitable size signal

(from -+ wavelengths filling most of the screen vertically).

6. Count the number of vertical s!uares lying within the signal then calculate the pea

to pea value as, Vp-p Eo. vertical Biv. @oltABiv

. Calculate @rms value record in Table 5,

+. 4easure Vrms using the multimeter (connect the probes of the multimeter to the 35E

and the #EB connectors).

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