De los Puntos de Casos de Uso a la estimación del esfuerzo

The oscilloscope (Fig. 3.9) is probably the most versatile, informative and useful electronic test instrument. It gives a visual indication of what a circuit is doing and often it can show what is going wrong quicker than any other instrument. Besides, some faults a r e often virtually impossible to pinpoint without using an oscilloscope. Pulse and digital circuits are extremely difficult to troubleshoot without this instrument because steady voltage readings are meaningless in pulse circuits as all one can check is whether an IC has a voltage supply or not.

tFig. 3.9 Oscilloscope in use

A cathode ray oscilloscope is the most versatile of the test instruments. It can be used for wave-form analysis, signal frequency measurement, peak-to-peak voltage measure ment and the most important for signal tracing. It is usually necessary for fault-finding on digital circuits, when correct operation depends not on voltage levels but on the presence (or absence) of a fixed level pulse which is too fast to register on a multimeter. A transistor curve tracer facility with an oscilloscope helps to test the semiconductor devices used in the equipment.

Broadly speaking, there are two types of oscilloscopes in the market today: Analog and Digital. Traditionally, oscilloscopes have been analog instruments, but as digital electronics has become cheaper and more powerful, digital scopes have increased in popularity. In fact, digital scopes now out-sell analog scopes as they offer many advantages to the users at an affordable cost.

In analog scopes, an electron beam sweeps across a phosphorescent screen, lighting up the screen wherever the beam hits. Circuits in the scope deflect the beam horizontally and vertically, thereby displaying a signal continuously. Digital scopes, which are often called digital storage oscilloscopes or DSOs, work very differently from analog scopes.

A digital scope measures the voltage of the input signal at discrete time intervals. Using an analogto-digital converter, the DSO converts a waveform into a series of numbers, which it stores in a table in its memory. The scope then uses the table of numbers to create the waveform display since a true display would contain only a series of dots.

The sensitivity offered by an oscilloscope is usually high, typically 10 mV/div, and in some cases 2 mV/division. Its impedance is generally greater than 1 Mohm. An oscilloscope with frequency response of DC to 15 MHz (preferably DC to 50 MHz) and a deflection factor of 5-volts/division is necessary for most of the troubleshooting requirements. A 10 X probe is generally used to reduce circuit loading.

Just to refresh the memory, the heart of an oscilloscope is the cathode ray tube (CRT).

The working of a CRT depends upon the generation of electrons by a heated cathode, focusing it to a thin beam and making it travel towards a positively charged anode. The electrons strike on a glass screen, coated with phosphor which gives off light, making a spot of light on the screen.

The brightness of the spot can be controlled and so also its position. The spot can be deflected (guided) to any part of the screen by applying a varying electric field to the deflection plates-altogether four of them arranged in pairs called the X-plates and the Yplates (Fig. 3.10). The Y-plates deflect the spot vertically, up or down, while the X-plates move it from side to side. Unlike the needle of a meter, a beam of electrons has practically no mass, so it can be moved around to trace out complicated patterns at very high speeds.

Fig. 3.10 Block diagram of an oscilloscope

Every oscilloscope has a built-in oscillator, the time base or horizontal sweep circuit.

This circuitry generates a voltage waveform with a saw-tooth shape and feeds it to the Xplates. This results in moving the spot on the screen at a steady speed, from left to right. The speed can be controlled and measured conveniently and its value can be read in time per centimetre (time/cm, sometimes time/div.) control on the front panel of the oscilloscope.

During the time the spot is moving across the screen, a voltage fed to the Y-plates will make the spot move vertically showing the wave shape of the voltage in the light which is being fed to the Y-plates. An amplifier (Y-amplifier) is provided in the circuit whose gain control is calibrated in volts per centimetre of vertical movement so that the peak-to-peak voltage of the waveform can be measured. Thus, an oscilloscope can be routinely used to:

(a) Display the wave shape;

(b) Measure its frequency; and

(c) Measure the peak-to-peak amplitude.

3.2.1 Unde rstanding an Oscilloscope

If you are not very conversant with the oscilloscope and how to use it, proceed as follows:

(a) Carefully observe all the controls (Fig. 3.11) on the front panel. They may not be the same or designated was the same on all the instruments, but some of them have to be there somewhere and in some form. The essential controls are:

t Fig. 3.11 Controls on an oscilloscope (i) Intensity or Brilliance control;

(ii) Focus control;

(iii) X and Y position controls;

(iv) Trigger, Sync or Level control, auto-mode; and

(v) On/off control; it may be a separate control or combined with Brilliance/

Intensity control.

(b) Before switching on the instrument, make the following settings:

(i) Intensity control fully anti-clockwise (off);

(ii) Stability control to auto;

(iii) Vertical and horizontal position controls to midway round;

(iv) Volts/cm control to highest value of the range; and (v) Time/cm control to 1 ms/cm or its nearest value.

(c) Plug in the instrument to the mains supply and switch on the instrument. Wait for a minute or two so that the CRT heater warms up. Then gradually advance (clockwise) the setting of the brilliance control until you observe the horizontal line of the trace on the screen. Sometimes, the trace may not appear on the screen.

In some oscilloscopes, a push button control `TRACE LOCATE' is provided which, on pressing, produces a spot at the centre of the screen. When the switch is released, the spot slowly moves off to wherever it was before. This indicates that position controls are not properly set. If this control is not provided on the oscilloscope, proceed as follows:

(i) Turn the Brilliance control right up to the fully clockwise position.

(ii) Time/cm control to the slowest speed, but not the off position. A light spot should appear on the screen moving slowly from left to right.

If still nothing is seen:

(iii) Adjust the Trig/Level control in the clockwise direction. Observe if something seems to be happening.

(iv) Operate the vertical position control until the trace appears. Some adjustment of the vertical gain and horizontal position control may be necessary.

If these steps do not result in showing a trace on the screen, there is some problem with the instrument. Unplug the mains and check the fuses before

attempting anything else.

Assuming that the above-mentioned steps have produced the trace on the screen, the following additional steps will help in making various measurements:

(v) The first step is to centre the trace with the help of horizontal and vertical position controls. The trace should start at the left hand side of the screen and lie along the centre-line. If there is a control labelled X Gain or TRACE EXPANSION, set it so that the trace is just enough to stretch across the screen, but no more.

(vi) Reduce the Brilliance setting to a comfortable viewing level and adjust the focus control so that the line is as thin as possible. It is usually difficult to obtain a fine line if the Brilliance control is set too high.

Quite often the waveform does not appear to be stationary on the screen. If the whole wave is moving, adjust the control labelled Sync or Trig level. This control is used to start the time base at the same part of the waveform on each sweep, so that the sweep appears stationary (Fig. 3.12).

Fig. 3.12 Effect of LEVEL control and SLOPE switch on CRT display

If the trace is not still locked, check if there is a switch labelled TRIG INT-EXT (or SYNC INT or INT-EXT). In case it is present, place it on the INT (internal) setting. In this position, the time base is locked to the signal into the Y-input (TRIG, EXT or the Xinput). Sometimes, on a few oscilloscopes, a FINE TIME/cm control may have to be adjusted to obtain a perfect lock.

It is usual for oscilloscopes to provide a choice of AC or DC coupling (Fig. 3.13) by means of an AC/DC switch at the Y-input. In the AC position, the signal on the Y-input i s passed via a coupling capacitor and, therefore, any DC voltage also present in the signal is blocked. With the switch in the DC position, however, the Y-amplifier is completely DCcoupled from the input all the way to the CRT plates.

Fig. 3.13 AC/DC coupling-the display of the same waveform on two different coupling positions

When making oscilloscope measurements, a pair of probes is very valuable which facilitates making a contact on the point of measurement in a convenient manner. For inexpensive instruments, two lengths of ordinary insulated wire are sufficient, though a set of ordinary test leads, the black or earth lead having a crocodile clip on one end and the red or signal lead with a test prod at its end are preferred.

The ordinary test leads are adequate for the measurement of signal voltages of low frequencies. However, for high frequencies, it is essential to use a fully screened probe s o as to avoid the possibility of signal degradation by way of signal amplitude attenuation and phase distortion occurring in a coaxial cable due to its large capacitance. The use of a compensated probe unit will, however, reduce these effects considerably.

3.2.2 Making Me asure me nts with Oscilloscope

All oscilloscopes enable measurements to be made on the displayed waveform. The most common method is to have an engraved plastic sheet, called the `graticule', which

is fitted over the screen. The graticule is engraved with parallel lines, 1 cm apart, with small divisions on the centre lines to show 0.2 cm. Both horizontal and vertical lines are engraved, so that both time and voltage measurements are possible.

Amplitude (Voltage) Measurement: For voltage measurement, count the number of centimetres on the vertical scale from the negative peak to the positive peak and then multiply this number by the setting of the volts per centimetre switch. For example, if the volts/cm switch is set to 5 V/cm, and the waveform measures 4.8 cm from peak-to-peak, the waveform voltage is 4.8 x 5 = 24.0 V peak-to-peak (Fig. 3.14).

t Fig. 3.14 Voltage measurement with an oscilloscope

Frequency Measurement: For frequency measurement, the method is to measure the time (period) of one complete cycle (Fig. 3.15) on the screen, i.e. the horizontal distance between two identical points on the neighbouring waves. This distance is then multiplied by the setting of the time/cm switch to calculate the period of one cycle. The reciprocal of this time (i.e., 1/time) is the frequency of the wave. For example, if the peaks of the waveform are 5 cm apart and the time/cm switch is set to 200 ps/cm, the time of one complete cycle is 5 x 200 = 1000 µs = 1 ms and the frequency is 1/1000 µs

= 1 kHz.

t Fig. 3.15 Frequency measurement with an oscilloscope

An oscilloscope can be used to compare frequencies which are to be adjusted so as to be equal or in some simple relation to each other. The method is an old one called

`Lissajous Figures' which are obtained by feeding two different signals into the scope at the same time, one into the vertical input and the other into the horizontal input. Under these circumstances, the internal time base is switched off, i.e. it is set to the external horizontal time base.

If the two signals are sine waves, and are synchronized, the pattern produced by this arrangement will be stationary. For equal frequency sine waves, the pattern can vary from a diagonal line to a circle due to phase difference between the waves-a difference of 90 degrees produces a circle whereas a 0 degree or 180 degree difference produces a straight line. If the frequencies are not identical, then the pattern will change and the number of complete cycles of change per second is equal to the difference in frequency of the two signals. Using this method, minute frequency differences between two sources can be measured with an accuracy of better than 0.01 Hz. It is an excellent way of testing the frequency stability of one crystal, in an oscillator, as compared to another.

Waveform Analysis: An oscilloscope is an excellent tool to see what is going on in a circuit and with experience, much can be gained from the correct interpretation of what i s displayed. For example, if you are feeding in a pure sine wave signal into an amplifier and the oscilloscope displays a flat-topped waveform when connected at its output, it means that clipping is taking place in the amplifier as a result of over-driving one of its stages.

Similarly, when working with fast repetitive pulses (TTL or CMOS), it is often necessary to look at the leading or trailing edge. In order to facilitate this measurement, oscilloscopes incorporate a pulse delay (Fig. 3.16) facility. The delay is needed because a triggered time base cannot be started instantaneously. By the time a normal triggered time base has started, the pulse you want to see is just about finished so that all you ever see even with a fast time base, is the end of the pulse. To make use of the pulse delay facility, the input signal is also used to trigger a monostable circuit which produces a delayed pulse which, in turn, operates the EXT TRIG circuit of the oscilloscope. The delay and time base controls are adjusted until the edge of the pulse can be seen, which enables one to estimate the rise or fall of time.

3.2.3 Double Be am vs Dual Trace

A double beam oscilloscope is helpful in comparing waveform. In a double beam instrument, two traces appear on the screen, each using the same time base but with separate Yinput controls. The two traces are separate and different waveforms can be displayed. Double beam arrangements are very useful when looking at circuits which make use of pulse triggering and synchronization.

Fig. 3.16 Delay facility for viewing rising (and trailing) edge of a pulse Several different techniques are used to obtain displays. Some of these are:

(a) Separate Guns: In this, two separate electron guns, with the X-plates connected together but with separate Y-plates, are used.

(b) Beam-Splitting In this, the beam from an electron gun is split into two after it has

passed the X-plates but before reaching the Y-plates.

(c) Beam Switching: This makes use of DC coupling into the scope's Y-amplifier. At the start of sweep, one of the two input signals is applied to the Y-input of the amplifier, with some DC level determined by a Y-shift control. On the next sweep, the other signal is applied to the Y-input but at a different DC level, so that the traces are at a different vertical position.

When this is done at a fast rate, it appears as if two traces are present simultaneously.

Obviously, the method cannot work at slow sweep rates wherein you see first one trace, then the other, but never both at once.

An alternative method is to use the `chopping' technique in which input 1 is displayed for a short time, then the trace is shifted up (or down) so that input 2 can be displayed.

Each trace would appear continuous if the beam and the inputs are switched at a frequency many times that of the time base sweep. At speeds approaching the chopping frequency, the trace, however, will appear as a dashed line.

It may be remembered that nothing quite beats the satisfaction of using an oscilloscope for yourself. The more practice you get in using an oscilloscope, the more useful the instrument would prove to be.

3.2.4 Pre cautions in the Use of an Oscilloscope

(a) Keep the beam intensity down to the minimum required for a particular setting.

Take care to turn down the glare on slow sweep speeds.

(b) When using the oscilloscope in the external horizontal time base mode, avoid displaying a stationary bright dot for any length of time. This can result in burning the phosphor on the screen.

(c) While making measurements, it should be ensured that the time base and vertical amplifier controls are in their calibrated positions.

(d) Ensure that the vertical gain control is set above the voltage of the signal to be measured. If in any doubt, start with maximum attenuation (highest voltage setting, minimum sensitivity) and work down the range until the correct setting is reached.