CHAPTER 3: OSCILLOSCOPE AND SIGNAL GENERATOR 3.1 Introduction to oscilloscope 1. Oscilloscope is basically a graph-displaying device-it draws a graph of an electrical signal. 2. The graph show signal change over time where the vertical (Y) axis represents voltage and the horizontal (X) axis represents time. Sometimes the intensity or brightness of the display was called Z axis. 3. The simple graph from an oscilloscope display can be used to: Determine the time and voltage values of a signal Calculate the frequency of an oscillating signal Determine if there is malfunction component (distortion signal will occurs) 4. Electronic equipments can be divided into two types: analog and digital. 5. Analog equipment works with continuously variable voltages, while digital equipment works with binary numbers (1 and 0 s) that may represent voltage samples. 6. An analog oscilloscope works by direct applying a voltage measured to an electron beam moving across the oscilloscope screen. The voltage deflects the beam up and down propotionally, tracing the waveform on the screen. This will gives an immediate picture of waveform. (see figure 1) 7. While digital oscilloscope samples the waveform and uses an analog-to-digital converter (or ADC) to convert the voltage been measured into digital information. It then uses this digital information to reconstruct the waveform on the screen. (see figure 1) Figure 2 1 O s c i l l o s c o p e & S i g n a l G e n e r a t o r
3.2 Analog oscilloscope Figure 2: Block diagram for analog oscilloscope Depending on how you set the vertical scale (volts/div control), an attenuator reduces the signal voltage or an amplifier increases the signal voltage. Next, the signal travels directly to the vertical deflection plates of the cathode ray tube (CRT). Voltage applied to these deflection plates causes a glowing dot to move. (An electron beam hitting phosphor inside the CRT creates the glowing dot.) A positive voltage causes the dot to move up while a negative voltage causes the dot to move down. The signal also travels to the trigger system to start or trigger a "horizontal sweep." Horizontal sweep is a term referring to the action of the horizontal system causing the glowing dot to move across the screen. Triggering the horizontal system causes the horizontal time base to move the glowing dot across the screen from left to right within a specific time interval. Many sweeps in rapid sequence cause the movement of the glowing dot to blend into a solid line. At higher speeds, the dot may sweep across the screen up to 500,000 times each second. Together, the horizontal sweeping action and the vertical deflection action trace a graph of the signal on the screen. The trigger is necessary to stabilize a repeating signal. It ensures that the sweep begins at the same point of a repeating signal, resulting in a clear picture as shown in following figure. 2 O s c i l l o s c o p e & S i g n a l G e n e r a t o r
Figure 3: Triggered and untriggered display Triggering Stabilizes a Repeating Waveform In conclusion, to use an analog oscilloscope, you need to adjust three basic settings to accommodate an incoming signal: The attenuation or amplification of the signal. Use the volts/div control to adjust the amplitude of the signal before it is applied to the vertical deflection plates. The time base. Use the sec/div control to set the amount of time per division represented horizontally across the screen. The triggering of the oscilloscope. Use the trigger level to stabilize a repeating signal, as well as triggering on a single event. Also, adjusting the focus and intensity controls enables you to create a sharp, visible display. 3.3 Digital oscilloscope Figure 4: Block diagram for digital oscilloscope 3 O s c i l l o s c o p e & S i g n a l G e n e r a t o r
Some of the systems that make up digital oscilloscopes are the same as those in analog oscilloscopes; however, digital oscilloscopes contain additional data processing systems. With the added systems, the digital oscilloscope collects data for the entire waveform and then displays it. When you attach a digital oscilloscope probe to a circuit, the vertical system adjusts the amplitude of the signal, just as in the analog oscilloscope. Next, the analog-to-digital converter (ADC) in the acquisition system samples the signal at discrete points in time and converts the signal's voltage at these points to digital values called sample points. The horizontal system's sample clock determines how often the ADC takes a sample. The rate at which the clock "ticks" is called the sample rate and is measured in samples per second. The sample points from the ADC are stored in memory as waveform points. More than one sample point may make up one waveform point. Together, the waveform points make up one waveform record. The number of waveform points used to make a waveform record is called the record length. The trigger system determines the start and stop points of the record. The display receives these record points after being stored in memory. Depending on the capabilities of your oscilloscope, additional processing of the sample points may take place, enhancing the display. Pre-trigger may be available, allowing you to see events before the trigger point. Fundamentally, with a digital oscilloscope as with an analog oscilloscope, you need to adjust the vertical, horizontal, and trigger settings to take a measurement. 4 O s c i l l o s c o p e & S i g n a l G e n e r a t o r
3.4 Oscilloscope Probe 1. To enable the oscilloscope to connect to the required points, oscilloscope probes or scope probes are required. 2. Although it is possible to use a signal line and earth return connection to form a simple oscilloscope probe, this approach does not provide the optimum performance as both electrical and mechanical aspects need to be considered to meet the necessary requirements. 3. A whole variety of scope probes can be bought and used. Fortunately, there is a high degree of inter-changeability between scopes and scope probes. However it is necessary to know which types to use, and what the scope probe specifications may be when choosing the correct type to use for a given application. 4. Oscilloscope probes may be categorised, and they can fall into one of two main areas: Passive oscilloscope probes Active oscilloscope probes 3.4.1 Passive oscilloscope probes 1. The great majority of test scope probes used with oscilloscopes are the passive variety. They enable a wide range of measurements to be made, and cover most applications. In addition to this, passive test probes are far cheaper than active ones as would be expected. 2. Scope probes are generally classified according to the level of attenuation of the signal they provide. Types including 1X (giving a 1 : 1 attenuation ratio), 10X (giving a 10 : 1 attenuation ratio) and 100X (giving a 100 : 1 attenuation ratio) are available: a. 1X scope probes The most basic form of oscilloscope probe, or scope probe, is what is often termed the 1X probe. It is so called because this type of scope probe does not attenuate the incoming voltage as many other probes do. It consists of a connector to interface to the oscilloscope (generally a BNC connector), and a length of coax which is connected to the probe itself. This comprises a mechanical clip arrangement so that the probe can be attached to the appropriate test point, and an earth or ground clip to be attached to the appropriate ground point on the circuit under test. The 1X probes are suitable for many low frequency applications. They typically offer the same input impedance of the oscilloscope which is normally 1 M Ohm. However for applications where better 5 O s c i l l o s c o p e & S i g n a l G e n e r a t o r
accuracy is needed and as frequencies start to rise, other test probes are needed. b. 10X scope probes To enable better accuracy to be achieved higher levels of impedance are required. To achieve this attenuators are built into the end of the probe that connects with the circuit under test. The most common type of probe with a built in attenuator gives an attenuation of ten, and it is known as a 10X oscilloscope probe. The attenuation enables the impedance presented to the circuit under test to be increased by a factor of ten, and this enables more accurate measurements to be made. In particular the level of capacitance seen by the circuit is reduced and this is reduces the high frequency loading of the circuit by the probe. As the 10X probe attenuates the signal by a factor of ten, this obviously means that the signal entering the scope itself is reduced. This has to be taken into account. Some oscilloscopes automatically adjust the scales according to the probe present, although not all are able to do this. It is worth checking before making a reading. The 10X scope probe uses a series resistor (9 M Ohms) to provide a 10 : 1 attenuation when it is used with the 1 M Ohm input impedance of the scope itself. A 1 M Ohm impedance is the standard impedance used for oscilloscope inputs and therefore this enables scope probes to be interchanged between oscilloscopes of different manufacturers. 10X oscilloscope probes also allow some compensation for frequency variations present. A small variable capacitor placed across the series resistor to compensate for the small capacitance across the scope input terminals. The series variable capacitor in the scope probe enables adjustment to be made to equalise the frequency response. Most oscilloscopes have a small square wave oscillator output. By attaching the oscilloscope probe to this a quick adjustment can be made. As the square wave requires all the harmonics to be present in the correct proportions to provide a "square" wave, the probe can be quickly adjusted accordingly. If the leading edges of square wave, when viewed on the screen has rounded corners, then the high frequency response of the probe is low and an adjustment can be made. However if the leading edges have spikes and rise too high, falling back to the required level, then the high frequency response has been enhanced and this needs to be adjusted. Only when the square wave is truly square is the frequency response correct. Many modern scope probes adopt a slightly different approach, 6 O s c i l l o s c o p e & S i g n a l G e n e r a t o r
although the end result is the same. A laser-trimmed thick-film electronic circuit is sued in the head of the probe. This combines the 9 M Ohm series resistor with a fixed-value bypass capacitor. A small value adjustable capacitor is then placed in parallel with the input capacitance of the oscilloscope. It is this capacitor that is adjusted to provide the compensation. c. 100X scope probes Although they are not as common as the 1X and 10X scope probes, 100X probes and other values including 20X and 1000X are also available. These oscilloscope probes tend to be used very high voltages need to be monitored and a high degree of attenuation is required or if very low levels of loading are needed. These probes are not common and tend to be quite specialised. If they were used for normal applications, the 100X attenuation would result in very small signal levels being presented to the input of the oscilloscope and as a result, noise on the input amplifiers of the scope would tend to be visible. 3. Often the choice of scope probe that is used will depend on what is available, in the laboratory however for most applications a 10X probe is the best all round type of probe, and as a result, these are the most commonly found and purchased. Switchable probes that can switch between 1X and 10X may be another solution. 3.4.2 Active oscilloscope probes 1. Although 10X probes are widely used because of their superior response, they are not able to provide all the performance that may be needed for some applications. By using active electronic circuits in the remote end of the oscilloscope probe it is possible to offer very high levels of performance. 2. Active oscilloscope probes use specially developed integrated circuits. By placing these chips right at the point at which the signal is probed, it enables the signal to be preserved during its transition from the point at which it is sampled to the input of the oscilloscope, in some instances using differential techniques. In this way the signal integrity it maintained, despite the fact that it may have a fast rise time, may have a low signal level, or require a high input impedance at the point at which it is sampled. Not only is the input resistance very high, but more importantly the input capacitance is very low. With capacitance normally providing the limiting factor, the reduction in capacitance enables sensitive waveforms to be measured far more accurately. 3. Although active probes are more expensive than their passive cousins, they offer a better level of performance that may be essential in some circumstances. 7 O s c i l l o s c o p e & S i g n a l G e n e r a t o r
8 O s c i l l o s c o p e & S i g n a l G e n e r a t o r [EE101]