The oscilloscope is an instrument that lets you view transient waveforms in electrical and electronic circuits. For example in the case of televisions, the waveforms found in the various points of the circuits are well defined, and through analysis we can easily diagnose what the problems of performance. Oscilloscopes are the most versatile instruments that exist and have been using television repair technicians to doctors. An oscilloscope can measure a large number of phenomena, provided the appropriate transducer (an element that converts a physical quantity into an electrical signal) will be able to give us the value of a pressure, heart rate, power, sound, vibration level in a car, etc. It is important that the oscilloscope used to display signals that allow for at least 4.5 cycles per second, allowing the verification stages of video, vertical and horizontal sweep to power supply. While the most common is the simple trace oscilloscope, is one stroke better double in more than one phenomenon or waveform can be displayed simultaneously. Oscilloscope operation is based on the possibility of deflecting an electron beam through the creation of electric and magnetic fields. In most oscilloscopes, electronic diversion, call deflection is achieved by electric fields. This is the electrostatic deflection. A minority of specialized equipment oscilloscope display response curves, using the electromagnetic deflection system, as the used in television. This latter type of oscilloscope has no control of exploration time.

The process of electron beam deflection occurs in the vacuum created within the cathode ray tube called (TRC). In this screen is where information is displayed implemented. The cathode ray tube electrostatic deflection is equipped with two pairs of deflection plates horizontal and vertical respectively, that allow properly controlled on-screen representation of the phenomena that you want to analyze.

This representation can be seen inscribed on Cartesian coordinates where the horizontal and vertical axes represent time and voltage respectively. The scale of each of the Cartesian axes recorded on the screen can be changed independently of one another, to give the signal representation is most suitable for measurement and analysis.

The dimensions of the CRT screen are now standard in most instruments, to 10 cm in the horizontal axis (X) by 8 cm in the vertical (Y). On the screen are recorded divisions of 1 cm square, directly on the CRT or on a part superimposed on it, which is printed a grid of 80 cm square. This grid is made where the representation of the signal applied to the oscilloscope. The oscilloscope, as a device that is widely used, is represented in the market for instruments under many forms, not only as to the purely physical aspect but in terms of its internal characteristics and therefore their performance and potential application of same. However, despite the possible differences, all scopes have common operating principles. The most widely used are those that could define as "basic oscilloscopes. With the oscilloscope, one can display signal waveforms alternating voltage by measuring the peak to peak, medium and rms. The basic circuits are:

* Vertical input attenuator

* Vertical Amplifier

* Step vertical deflection

* Amplifier trigger sample (trigger)

* Shooting Mode Selector (interior or exterior)

* Amplifier trigger pulse

* Time Base

* Erase pulse amplifier

* Step horizontal deflection

* Cathode ray tube

* Circuit power.

An alternating current is constantly changing that value and reverses its direction at regular intervals. In the case of an alternator, these changes result from the rotation of the armature or induced, as each turn of the winding wire cuts the lines of magnetic field in one direction and then in the opposite direction, causing the electrons to move alternately in one direction and then in the opposite direction. Accordingly, an alternation of intensity change is suffering an alternating current as it moves in one direction, growing in intensity from zero to its maximum and back to zero. Two alternatives, one in one direction and the other in the opposite direction or not, form a cycle. In a two-pole alternator, when the armature has made a complete revolution will have traveled 360 electric and a cycle will have occurred. The number of cycles that occur during a second is the frequency of the alternating current, which is symbolized by the letter f. Another important parameter is the alternating current period, which is symbolized by the letter T, the period and frequency are reciprocals of each other, fulfilling the following equation: The frequency is usually measured in cycles per second or Hertz (Hz). In the next figure we can get a clearer idea of the period and frequency of a wave:

Types of Oscilloscopes

Electronic equipment is divided into two types: analog and digital. Early work with continuous variables while the latter do so with discrete variables. The first work directly with the applied signal is amplified once deflects an electron beam vertically in proportion to its value. In contrast digital oscilloscopes used a prior analog-digital converter (A / D) for digitally storing the input signal, then reconstructing this information on the screen. Both types have their advantages and disadvantages. The analog is preferable when priority display rapid changes in the input signal in real time. Digital scopes are used when one wants to study events and non-repetitive (voltage spikes that occur at random).

Analog oscilloscopes

When the probe is connected to a circuit, the signal passes through the latter and goes to the vertical section. Depending on where we place the control of the vertical amplifier attenuate or amplify the signal. At the start of this block already has sufficient signal to attack vertical deflection plates and are responsible for diverting the electron beam which emerges from the cathode and hits the fluorescent coating inside the screen, meaning vertical. Up if the voltage is positive with respect to the reference point (GND) or downwards if negative.

The signal also passes the section thus shot to start the horizontal sweep (this is responsible for moving the electron beam from the left of the screen to the right in a given time). The layout (move from left to right) is achieved by applying the upslope of a sawtooth to the horizontal deflection plates, and may be controlled in time acting on the TIME-BASE command. The path (path from right to left) is done much faster with the falling part of the sawtooth.

Thus the combined action of horizontal layout and vertical deflection traces the graph of the signal on the screen. The trigger section is necessary to stabilize the repetitive signals (ensures that the route starts at the same point of the signal-repeating). As a conclusion to properly use an analog oscilloscope we need to make three basic settings: The attenuation or signal amplification needed. Using the AMPL command to adjust the amplitude of the signal before it is applied to vertical deflection plates. The signal should occupy an important part of the screen without going to push the limits. The time base. Use TIME-BASE command to set time which is a division of the screen horizontally. For repetitive signals is desirable that the screen can be observed around a couple of cycles. Triggering of the signal. Using TRIGGER LEVEL control (trigger level) and TRIGGER SELECTOR (type of shot) to stabilize the best possible repetitive signals.

Of course, too, must adjust the controls that affect the display: FOCUS (focus), intensity (intensity) never excessive, Y-POS (vertical beam) and X-POS (horizontal beam). Digital oscilloscopes as also explained in earlier sections are an additional set of data processing which can store and display the signal. When you connect a digital oscilloscope probe to a circuit, the vertical section adjusts the signal amplitude in the same way as into the analog oscilloscope. The analog-digital converter data acquisition system sampling makes the signal to set time intervals and converts the DC voltage signal into a series of digital values called sample. In the horizontal section of a clock signal determines when the A / D converter takes a sample. The speed of this clock is called the sampling rate and is measured in samples per second.

The sampled digital values are stored in a memory as cue points. The number of signal points used to reconstruct the signal on the screen is called registration. The trigger section determines the start and end points of signal in the register. The display section receives these record points, once stored in memory to the signal present on screen. Depending on the capabilities of the oscilloscope, one can have additional processes on the sampled points, you can even have a pre-trigger to observe processes that take place before the shooting. Fundamentally, a digital oscilloscope is operated in a manner similar to an analog one, in order to take the steps needed to adjust the AMPL command, TIME-BASE command and control involved in the shooting.

Terminology

There is a general term to describe a pattern that repeats in time wave. There are sound waves, ocean waves, brain waves and of course, stress waves. An oscilloscope measures the latter. A cycle is the smallest part of the wave that repeats over time. A waveform is a graphical representation of a wave. A voltage waveform is always present over time on the horizontal axis (X) and the amplitude on the vertical axis (Y). The waveform gives us valuable information about the signal. At any moment we can visualize the height reached and, therefore, whether the voltage has changed over time (if we observe, for example, a horizontal line we can conclude that in this time interval the signal is constant). With the slope of the diagonal lines in both rising edge and falling edge in, we know the speed in moving from one level to another could be also seen sudden changes in the signal (acute angles) usually due to transient processes.

Types of waves

Waves can be classified into four types:

* Sine waves

* Square and rectangular waves

* Triangular waves and sawtooth.

* Pulses and edges or steps.

Sine waves: These are the fundamental waves and that for several reasons: They have some very interesting mathematical properties (such as combinations of sinusoidal signals with different amplitude and frequency can be reconstructed any waveform), the signal obtained from the sockets of any home have this form, the test signals produced by a generator oscillator circuits are also sinusoidal signal, most of the sources of power in AC (alternating current) produce sinusoidal signals. The damped sinusoidal signal is a special case of this type occur in waves and oscillation phenomena, but are not sustained over time.

Square and rectangular waves: Square waves are basically waves that pass from one state to another voltage at regular intervals, in a very short time. They are usually used to test amplifiers (this is because these signals contain in themselves all frequencies). Television, radio and computers use a lot of these signals, mainly as clocks and timers. The rectangular waves are different from the square to have no equal intervals in which the voltage remains at high and low. They are particularly important for analyzing digital circuits.

Triangular waves and sawtooth: They occur in circuits designed to control voltages linearly, such as, for example, the horizontal sweep of an analog oscilloscope or the horizontal and vertical scanning of a television. The transitions between the minimum and maximum signal change at a constant rate. These transitions are called ramps. The sawtooth wave is a special case of triangular signal with a ramp down of much steeper than the ascending ramp.

Pulses and flanks or steps: Signals, such as the flanks and pulses that only occur once are called transients. One side or step indicates a sudden change in voltage, for example when connecting a switch. The pulse would indicate, in this instance, the switch is connected in a certain time and was disconnected. Usually the pulse represents one bit of information through a circuit of a digital computer or also a small flaw in a circuit (eg a false momentary contact). It is common to find signs of this type on computers, X-ray equipment and communications.

Voltage: Voltage is the electrical potential difference between two points in a circuit. Normally one of these points is usually ground (GND, 0V), but not always, for example to measure the peak to peak voltage signal (Vpp) as the difference between the maximum and minimum value of this. The word usually means the difference amplitude between the maximum value of a signal and ground. In the range of values experienced by an alternating current or a sinusoidal electromotive force in the course of a cycle, the highest possible is when the inductor cut the maximum number of lines of force. This is called "Maximum Value" and is positive to negative 90 degrees and 270 degrees electrical. Instantaneous value is called the value of current or voltage at any one time. The maximum value is an instantaneous value, as well as the value of zero and any other between these two. From a practical standpoint, is of great importance to the "cash value" or rms, which is the value recorded measurement instruments for alternating current.

The rms value is that which produces the same thermal effect (heat) that the of a direct current. For example, if a direct current of 5 amps heated water from a vessel at a temperature of 90 º C, an alternating current that produces the same temperature rise will have an rms value of 5 amps. The average value of a pure sine wave AC is zero, since the positive half-wave is equal and opposite to the negative half-wave. Hence, when speaking of average value always refers to the average of a half-wave. The average value of a symmetrical sine wave is defined as algebraic average of the instantaneous values during a half period. We can also say that the average value is an ordered such that the area of the resulting rectangle equals the area of half period. It is represented by adding the subscript med to the letter of the magnitude of which concerned, Eave, Imed, Pmed, etcetera. Its mathematical expression: Relations between peak to peak values, maximum and effective.

The maximum value is half the peak to peak value and the rms value is obtained by dividing the peak to peak value by, for example if we get on the measured value of peak to peak voltage of 18 volts and wish to obtain the maximum value and rms value, proceed as follows: Then the maximum voltage in this example is 9 volts, the rms voltage is 6.364 volts and the average voltage is 5.730 volts.

Phase: The phase can be explained much better if we consider the sine waveform. The sine wave can be extracted from the movement of a point on a circle of 360 degrees. A sinusoidal signal cycle covers 360 degrees. When comparing two sinusoidal signals of the same frequency may be that both are not in phase, ie, that do not coincide in time steps equivalent points of both signals. In this case it says that both signals are outdated, being able to measure the gap with a simple rule of three: Where T is the time delay between a signal and another.

Parameters that affect the quality of an oscilloscope

Bandwidth: Specifies the frequency range in which the oscilloscope can measure accurately. By convention the bandwidth is calculated from 0Hz (continuing) to the frequency at which a sinusoidal type signal displayed a 70.7% of the value applied to the input (which corresponds to an attenuation of 3dB).

Rise time: It is one of the parameters that will give us, along with its predecessor, the highest frequency of use of the oscilloscope. It is an important parameter if desired pulses reliably measure and flanks (remember that this kind of signals have transitions between voltage levels very fast). An oscilloscope can not display pulses with rise times faster than your own.

Vertical Sensitivity: Indicates the ease of the oscilloscope to amplify weak signals. Data is provided in mV per vertical division, usually in the order of 5 mV / div (reaching up to 2 mV / div).

Speed: For analog oscilloscopes, this specification indicates the horizontal sweep speed, allowing us to observe events faster. It is usually the order of nanoseconds per division horizontal.

Gain Accuracy: Indicates the precision with which the oscilloscope vertical system attenuates or amplifies the signal. It usually provides maximum percentage of error.

Accuracy of time base: Indicates the precision of the system time base oscilloscope horizontal display time. It is also often given as a percentage of maximum error.

Vertical resolution: It is measured in bits and is a parameter that gives us the resolution of A/D converter digital oscilloscope. It tells us that precision signals are converted into digital values stored in memory. Calculation techniques can increase the effective resolution of the oscilloscope.

Operation of the Oscilloscope

The following are the steps for the proper management of the oscilloscope:

Grounding: A good ground is very important for measurements with an oscilloscope. For safety ground is required to place the oscilloscope. If contact occurs between a high voltage and the housing of a scope not grounded, any part of the case, including managers, can give you a dangerous shock. While a well-placed ground oscilloscope, the current in the previous case the user would go through, is diverted to the ground connection. To ground an oscilloscope is needed to unite the chassis of the oscilloscope with the neutral reference point voltage (usually called ground). This is achieved using power cables with three conductors (two for food and one for ground). The oscilloscope needs, moreover, share the same mass with all circuits under test to which it connects. Some oscilloscopes can operate at different voltages and it is very important to ensure it is set to the same at our disposal on the taking of blood.

Be grounded self: Working in integrated circuits (ICs), especially CMOS, you must register yourself grounded. This is because certain parts of these integrated circuits are susceptible to break down the static tension that holds our own body. To resolve this problem you can use a conductive strap that is properly connected to ground, discharging the static electricity that holds your body.

Initial setting of controls: After connecting the oscilloscope to the outlet and feed by clicking on the switch: You should be familiar with the scope's front panel. All scopes have three basic sections that call: Vertical, Horizontal, and Trigger. Depending on the particular employee oscilloscope, we have other sections. There are some BNC connectors, which are placed the measuring probes. Most of today's oscilloscopes have two channels usually labeled as I and II (or A and B). Having two channels allows us to compare signals from very comfortable. Some advanced oscilloscopes have a switch labeled AUTOSET or PRESET to adjust the controls in a single step to adjust the signal perfectly to the screen. If the oscilloscope does not have this characteristic, it is important to adjust the various controls of the device to its standard position prior to measurement.

These are the recommended steps:

1. Set the oscilloscope to display the channel I. (At the same time be positioned as the trigger channel I).
2. Set to an intermediate scale volts / division of channel I (ie 1v/cm).
3. Send command calibrated position variable volts / division (central pot).
4. Disable any vertical multipliers.
5. Set switch input for channel I in DC coupling.
6. Place the automatic shooting mode.
7. Turn off the shot off or delayed to a minimum.
8. Place the control level to the minimum necessary for assessing the stroke on the screen, and the line of focus adjusted for the cleanest possible display (usually stay with signaling controls close to the vertical).

Probes measure: With the steps above, we are able to connect the probe to measure the channel input connector I. It is very important to use the probes designed to work specifically with the oscilloscope. A probe is not far from a cable with a clamp, but a connector specifically designed to prevent noise that might disturb the measurement. In addition, the probes are constructed to have minimal effect on the measuring circuit. This ability of the probes is called the loading effect, to minimize use a passive attenuator, usually x10. This type of probe is usually provided with the oscilloscope and is an excellent general use probe. For other types of measures are used special probes, such as current probes or activities.

Passive Probes: Most passive probes are marked with an attenuation factor, usually 10X or 100X. By convention attenuation factors appear with the "X factor behind the split. In contrast amplification factor "X" appear in front (X10 or X100). The probe is probably the most used 10X, reducing the signal amplitude by a factor of 10. Its use extends from frequencies above 5 KHz and signal levels greater than 10 mV. The 1X probe is similar to above but introduces more load on the test circuit, but can measure signals with lower levels. For convenience of using special probes have been introduced with a switch that allows use 1X or 10X. When using such probes make sure the position of this switch before making a move.

Probe Compensation: Before you use a 10X attenuator probe is necessary to adjust the frequency on the oscilloscope in particular to be working. This adjustment is called the compensation of the probe and comprises the following steps.

1. Connect the probe channel input I.
2. Connect the probe tip to the point of compensation signal (Most scopes have a power to adjust the probes, otherwise it is necessary to use a square wave generator).
3. Connect the alligator clip of the probe to ground.
4. Note the reference square signal on the screen.
5. With the screwdriver adjustment, acting on the trimmer to see a perfect square wave.

Active Probes: They provide an amplification before applying the signal to the oscilloscope. May be necessary in circuits with a very low output power. This type of sensors needed to operate a power source.

Current Probes: Allow direct measurement of currents in a circuit. They come to measure AC and DC. They have a clip that covers the cable through which you want to measure current. When not placed in series with the circuit cause very little interference in it.

What can we do with an oscilloscope?

1. Measure directly the tension (voltage) of a signal.
2. To measure directly the period of a signal.
3. To determine indirectly the frequency of a signal.
4. Measure the phase difference between two signals.
5. Determine which part of the signal which is DC and AC.
6. Determine which part of the signal is noise and how this varies over time.

Measurement of tension with the oscilloscope

The screens on the oscilloscope are calibrated with a grid so that in terms of gains selected for the internal circuitry, we may use them as reference points to measure tension. So if the key gain selector was in the position of 1V/div, which corresponds to 1 volt per division, to focus the signal enough to get different readings on its intensity from the waveform. Figure for example, is an example of signal voltage of 3 volts or 6 volts peak-peak voltage, if the key is in position selector 1V/div. This procedure applies only to alternating signals.

Also, the voltages can be measured with the oscilloscope. Once the line centered on the screen, we applied in the vertical input we want to measure voltage. The remoteness of the line in the vertical (up or down) will depend on the input voltage. If the signal is analyzed waveform known-sine, triangular, rectangular-plus peak values easy to obtain other values such as average, the rms value. Likewise if it is an audio signal is known, we can calculate power. In each of the vertical positions of the attenuator can be read directly the voltage required to deflect the stroke an inch, vertically.

This allows us to make measurements of voltage on the screen, whether continuous or alternating. In both cases, the switch coupling placed in the proper position. The measure of an alternating voltage is done by counting the inches or tables in the grid it occupies on the screen signal, multiplying by the conversion factor selected with the switch from portrait, bearing in mind that the larger the space occupied by the signal on the screen, the more reliable the measurement made. When performing a voltage measurement, or its component in a waveform, which we will measure will be the vertical displacement experienced by the deflection from a given reference. This shift also will indicate the polarity of the DC voltage measurement, as towards the top of the grid (positive stress) or to the bottom (negative voltage).

Measuring Times with the oscilloscope

The distance over time, between two given points can be calculated from the physical distance in centimeters between those points and multiplying by the factor indicated on the switch time base. In the above example if the key selector was in the time interval 01 seconds, the cycle time would be drawn 1 seconds, i.e., this would be a wave of period equal to 1 seconds.

Frequency measurement

The natural frequency of a given signal can be measured on an oscilloscope under two different methods:

1. Since the measure of a period of that signal by applying the above method and using the formula:
2. By comparing a frequency of known value and we want to know.

In this case the oscilloscope is operated under X / Y (external deflection).

Applying each of the signals to the inputs "X" and "Y" of the oscilloscope and in case there is a complete harmonious relationship between them, the screen enters a so-called to the view which you can find the number of times a frequency contains the other and therefore deduct the value of the unknown frequency.

Measure Phase: The previous system of measurement of frequency by using the "curves, may also be used to determine the lag in degrees between two different signals at the same frequency. We work the oscilloscope with external horizontal deflection by applying to its horizontal and vertical inputs (X/Y) the two signals that you want to compare.    Through this connection will be formed on the screen a "curve" that we will properly interpreted the phase difference between the two waveforms being compared. In the previous drawings, are examples of the application system.

• Written by Rajinder Soni.
• In category Electronics.
• 15 years ago.

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