MEASUREMENTS USING CATHODE RAY OSCILLOSCOPE

 

AIM:

  1. To get familiar with the various functions of a cathode ray Oscilloscope (CRO)
  2. To measure AC/DC voltages
  3. To measure frequency of a sinusoidal signal
  4. To measure the phase difference between two signals
  5. To display the diode characteristics
  6. To study the resonance condition of an LCR circuit   

 

INSTRUMENTS REQUIRED:

       An oscilloscope, signal generators, ac/dc sources, Resistors, capacitors, diodes, transformer, etc.

 

PROCEDURE:

I. VARIOUS FUNCTIONS OF CRO

         Carefully look at the front panel of the CRO and try to understand the various control knobs (you may use the manual supplied by the manufacturer) like vertical gain switch (volts/div), the horizontal sweep speed selector (time/div), CAL, VAR switch, Intensity and Focussing knobs, Vertical and Horizontal shifting knobs, X and Y inputs, Traces (single or double beam), ALT, chop, x10 (magnification), component test etc.

 

II. VOLTAGE MEASUREMENTS

  1. DC voltage:

        The trace (horizontal) is adjusted to lie along the X-axis passing through the origin (0,0) of the screen. The d.c input to be measure is fed to the Y-input of the CRO in the d.c mode. The vertical shift of the trace is a measure of the magnitude of the d.c voltage. Care should be taken to keep the vertical gain switch at a suitable position depending on the magnitude of the input voltage such that the trace lies with the screen. The measurement can be repeated for various values of the input voltages (both +ve or –ve), at different vertical gain etc. Compare the voltage measured by the CRO with the voltage measured by a multimeter. Are they equal?

  1. AC VOLTAGE

      The ac signal from a signal generator is fed to the Y-input of the CRO in the ac mode and the peak to peak voltage of the signal is measured by noting the height of the signal on the screen and the vertical gain position of the Y input. This can be repeated on signals of different frequencies and magnitude Check peak to peak voltage= 2 Vrms.

 

III. FREQUENCY OF SINUSOIDAL SIGNAL

  1. Using horizontal time base (time div)

       Connect the signal from the function generator (FG) to the Y-input and adjust the horizontal sweep speed selector (time/div) to get a steady pattern of the signal on the CRO screen. Measure the time interval between two peaks, say ‘t’ secs. Hence determine the frequency of the signal f=l/t. It is better to note down the time integral for 5 to 6 peaks then calculate the average time between two adjacent peaks.

B.     USING LISSAJOUS FIGURES

        Set the CRO to XY mode and the function switch at XY position. Preferably adjust (V/divn) of both the channels (X and Y) to be the same. The test signal (of frequency ‘f’) is fed to one of the channels (say Y) and a reference signal (of frequency ‘fr’) to the other channel (x). Adjust the frequency fr until you get a circle or ellipse (Lissajous figures) as steady as possible. In the condition of single loop f:fr = 1:1. Knowing the frequency of the reference signal fr, the frequency of the test signal can be obtained. Repeat the measurement for Different ratio f : fr = 1:2, 1:3, 2:1, 3:1, 2:3 etc. and the obtain the frequency f.

 

IV. PHASE MEASUREMENT

Using Dual trace (channels Y1 and Y2):-

One can approximately measure the phase difference θ between two signals (same frequency) by feeding the signal to two inputs Y1 and Y2 of a dual trace CRO and noting the shift in the peak positions. The shift is measured on the time scale (div/sec) and then converted into the phase difference assuming one period T= 2π radians or 360o. A simple way of producing a difference between two signals is to pass one of the signals through a capacitor C, a inductance L or a combination of R and C. (see fig1)

ELLIPSE METHOD

     Feed the signals (same frequency) channel 1 and channel 2 to X and Y plates of CRO. You will get a circle (Lissajjous figure) on the CRO screen if the two signals are of the same amplitude and same phase. Any phase difference between the signals will distort the circle into an ellipse (fig.2).

 

         You can measure the phase difference 0 by noting down the intercepts of ellipse on the X-Y axis on the screen. The phase difference θ is obtained from the relation sin θ= B/A. The experiment is repeated for different θ. Compare θ obtained by the experiment with the theoretical value θ = tan-1 (1/ω).

 

 

 

 

DISPLAY I-V CHARACTERISTICS OF A DIODE

Try the given circuit (fig.3) and explain the nature of the display you get on the screen. Repeat for different types of diodes (Zener, PN junction etc.)

 

 

F G hghgGG

 

CRO

 
       

                                                            Fig. 3

 

RESONANCE BY LCR CIRCUIT

Ch. 1

 

Ch. 2

 
           

                                                            Fig. 4


Make a simple LCR circuit (fig.4) and study its behaviour at resonance. Feed the signals 1 and 2 to the two traces of the CRO and observe the amplitude as function of the frequency of the signal generator. As the frequency approaches the resonant frequency fo the amplitude of the signal grows fast and pass through a maximum (why?). Compare the theoretical resonant frequency (for LCR values) with the observed frequency fo.

            fo = l/(2π√LC).