TOPIC-3.UNIT-3 by preddyrk



                Oscilloscope probes and Measurements                                         C.R.O. Probes:
                                                                                          The probe is used to connect the test circuit to the oscilloscope. The probe does not alter ,
                                                                                          load or disturbs the circuit and signal conditions to be analyzed. To achieve this , the probes
                                                                                          should have high impedance, probe bandwidth should be high, at least 10 times the

                                                                                          1. Direct probes(1:1):
                                                                                             These probes are most simplest type.
                                                                                             The input impedance of CRO is high to avoid the loading effects.
                                                                                             When direct probes are used, the stray capacitance of probe adds with the capacitance of
                                                                                             oscilloscope. Hence at High frequencies, the total input reactance becomes very low, this
                                                                                             decreases the input impedance.
                                                                                             So these probes are limited to low frequency only.

                                                                                             The isolation type of probe may be used to deal with high frequency signals with series
                                                                                             carbon resistance in series with the test lead, but there is slight reduction in amplitude
                                                                                             and shape of the input.

          P.R.K.Reddy                                                                                   P.R.K.Reddy

 2. High impedance probes (10X):

This probe also called as Passive voltage probe. The basic function of this probe is to
increase the input impedance and reduce the effective input capacitance of an
The resistance R1 is shunted by an adjustable capacitor C1, This capacitor is called
compensating capacitor.

The limitation of this probe is that the signal
attenuation takes place. To avoid this the
probe capacitance must be adjusted correctly.
This adjustment generally checked by
displaying square wave.

  Figure 3: Typical Probe/Oscilloscope 10-
  to-1 Divider Network
          P.R.K.Reddy                                                                                   P.R.K.Reddy


  3. Active probes:                                                               4. Current probes:
                                                                                   These probes provides method of inductively coupling the signal to the CRO.
For connecting fast rising and high frequency signals these probes are used.       The magnitude of current with frequency range d.c to 50 MHz can be measured.
These probes are used for small signal measurements .

The probe consists of active element like FET source follower and BJT as
emitter follower. FET is used as an active element to amplify the signal and
offers high input impedance. BJT emitter follower solving improper impedance
matching problems.

                                                                                 It works on the principle of electro-magnetic
                                                                                 induction(like transformer).
                                                                                 For D.C signal measurement is done with extra
                                                                                 arrangement, like adding Halleffect sensor and
                                                                                 a feed back amplifier to the probe.

                                                                                 The Hall effect sensor is placed in magnetic
                                                                                 core structure of the current probe.
                                                                                 With this Hall effect sensor any magnetic
                                                                                 field present in hall sensor causes current to
  Needs extra D.C. power supply for probes, so makes more expensive.
                                                                                 be induced in the secondary winding of the
  These probes can be used only for small signals usually less than few volts.   current transformer.
         P.R.K.Reddy                                                                       P.R.K.Reddy

 5. High voltage                                                                   6. Differential probes:

    These are used to measure very high voltages in the range of KV. It          This is an active probe. It has two inputs, positive and negative.
    provides the voltage division ratio of 1000 to 1 or more.                    The output voltage is proportional to the difference between the voltages
                                                                                 appearing at the two input terminals.

         The probe consists of high input resistance of 100 M ohms.

                                                                                   These probes can be used generally for small signals with less than few volts only.

         P.R.K.Reddy                                                                       P.R.K.Reddy


                                                            VOLTAGE MEASUREMENTS

                                                            A.DC voltage:

Fig. How to measure rise time, fall time, and pulse width   ������� 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.

                                                            B.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

    P.R.K.Reddy                                                       P.R.K.Reddy

                                                            FREQUENCY OF SINUSOIDAL SIGNAL

                                                            A.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. LISSAJOUS FIGURES

                                                            ������� Set the CRO to XY mode and the function switch at XY position.
                                                            Preferably adjust (V/div) 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.
    P.R.K.Reddy                                                       P.R.K.Reddy


                                                                                        •Lissajous’ Figures

                                                                                                  Same amplitude but different frequencies


Using Dual trace (channels Y 1 and Y 2):-
One can approximately measure the phase difference θ between two signals (same
frequency) by feeding the signal to two inputs Y 1 and Y 2 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.
���� 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 90 phase. Any phase difference between the
signals will distort the circle into an ellipse.

������� �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).

            P.R.K.Reddy                                                                       P.R.K.Reddy


                          Fig. Screen patterns formed by in-phase and out-of-phase combinations of sine waves

 P.R.K.Reddy                           P.R.K.Reddy

Diode testing using CRO

 P.R.K.Reddy                           P.R.K.Reddy


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