CONTENTS MULTIMETER OSCILLOSCOPE PROBES SIGNAL GENERATOR

1. MULTIMETER A meter is a measuring instrument. An ammeter measures current, a voltmeter measures the potential difference (voltage) between two points, and an ohmmeter measures resistance. A multimeter combines these functions, and possibly some additional ones as well, into a single instrument.

Principles - Ammeter Before going in to detail about multimeters, it is important for you to have a clear idea of how meters are connected into circuits. Diagrams A and B below show a circuit before and after connecting an ammeter: A B to measure current, the circuit must be broken to allow the ammeter to be connected in series ammeters must have a LOW resistance

Principles - Ammeter (cont’d) To start with, you need to break the circuit so that the ammeter can be connected in series. All the current flowing in the circuit must pass through the ammeter. Meters are not supposed to alter the behavior of the circuit, or at least not significantly, and it follows that an ammeter must have a very LOW resistance.

Principles - Voltmeter Diagram C shows the same circuit after connecting a voltmeter: A C to measure potential difference (voltage), the circuit is not changed: the voltmeter is connected in parallel voltmeters must have a HIGH resistance

Principles - Voltmeter (cont’d) This time, you do not need to break the circuit. The voltmeter is connected in parallel between the two points where the measurement is to be made. Since the voltmeter provides a parallel pathway, it should take as little current as possible. In other words, a voltmeter should have a very HIGH resistance. Which measurement technique do you think will be the more useful? In fact, voltage measurements are used much more often than current measurements.

Principles - Voltmeter (cont’d) The processing of electronic signals is usually thought of in voltage terms. It is an added advantage that a voltage measurement is easier to make. The original circuit does not need to be changed. Often, the meter probes are connected simply by touching them to the points of interest.

Principles - Ohmmeter An ohmmeter does not function with a circuit connected to a power supply. If you want to measure the resistance of a particular component, you must take it out of the circuit altogether and test it separately, as shown in diagram D: A D to measure resistance, the component must be removed from the circuit altogether ohmmeters work by passing a current through the component being tested

Principles – Ohmmeter (cont’d) Ohmmeters work by passing a small current through the component and measuring the voltage produced. If you try this with the component connected into a circuit with a power supply, the most likely result is that the meter will be damaged. Most multimeters have a fuse to help protect against misuse.

Digital Multimeter Multimeters are designed and mass produced for electronics engineers. Even the simplest and cheapest types may include features which you are not likely to use. Digital meters give an output in numbers, usually on a liquid crystal display.

Digital Multimeter (cont’d) The central knob has lots of positions and you must choose which one is appropriate for the measurement you want to make. If the meter is switched to 20 V DC, for example, then 20 V is the maximum voltage which can be measured. This is sometimes called 20 V fsd, where fsd is short for full scale deflection. For circuits with power supplies of up to 20 V, which includes all the circuits you are likely to build, the 20 V DC voltage range is the most useful.

Digital Multimeter (cont’d) Sometimes, you will want to measure smaller voltages, and in this case, the 2 V or 200 mV ranges are used. What does DC mean? DC means direct current. In any circuit which operates from a steady voltage source, such as a battery, current flow is always in the same direction. Every constructional project descirbed in Design Electronics works in this way. AC means alternating current. In an electric lamp connected to the domestic mains electricity, current flows first one way, then the other. That is, the current reverses, or alternates, in direction. With UK mains, the current reverses 50 times per second.

Digital Multimeter (cont’d) For safety reasons, you must NEVER connect a multimeter to the mains supply.

Digital Multimeter (cont’d) An alternative style of multimeter is the autoranging multimeter. The central knob has fewer positions and all you need to do is to switch it to the quantity you want to measure. Once switched to V, the meter automatically adjusts its range to give a meaningful reading, and the display includes the unit of measurement, V or mV. This type of meter is more expensive, but obviously much easier to use. Where are the two meter probes connected? The black lead is always connected into the socket marked COM, short for COMMON. The red lead is connected into the socket labelled V mA. The 10A socket is very rarely used.

Analogue Multimeter An analogue meter moves a needle along a scale. Switched range analogue multimeters are very cheap but are difficult for beginners to read accurately, especially on resistance scales. The meter movement is delicate and dropping the meter is likely to damage it! Each type of meter has its advantages. Used as a voltmeter, a digital meter is usually better because its resistance is much higher, 1 MΩ or 10 MΩ , compared to 200 kΩ for a analogue multimeter on a similar range. On the other hand, it is easier to follow a slowly changing voltage by watching the needle on an analogue display.

Analogue Multimeter (cont’d)

Analogue Multimeter (cont’d) Used as an ammeter, an analogue multimeter has a very low resistance and is very sensitive, with scales down to 50 µA. More expensive digital multimeters can equal or better this performance. Most modern multimeters are digital and traditional analogue types are destined to become obsolete.

Example 1 – voltage measurements

Example 2 – voltage measurements

Example 3 – resistance measurements

Example 4 – current measurements

2. Oscilloscope An oscilloscope is easily the most useful instrument available for testing circuits because it allows you to see the signals at different points in the circuit. The best way of investigating an electronic system is to monitor signals at the input and output of each system block, checking that each block is operating as expected and is correctly linked to the next. With a little practice, you will be able to find and correct faults quickly and accurately.

The Interface An oscilloscope is an impressive piece of kit. The diagrams show a Hameg HM 203-6 and a Tektronix model 475A portable analogue oscilloscope, a popular instrument in UK schools. Your oscilloscope may look different but will have similar controls. Faced with an instrument like this, students typically respond either by twiddling every knob and pressing every button in sight, or by adopting a glazed expression. Neither approach is specially helpful. Following the systematic description below will give you a clear idea of what an oscilloscope is and what it can do.

The Display The function of an oscilloscope is extremely simple: it draws a V/t graph, a graph of voltage against time, voltage on the vertical or Y-axis, and time on the horizontal or X-axis. As you can see, the screen of this oscilloscope has 8 squares or divisions on the vertical axis, and 10 squares or divisions on the horizontal axis. Usually, these squares are 1 cm in each direction.

The Display (cont’d) Many of the controls of the oscilloscope allow you to change the vertical or horizontal scales of the V/t graph, so that you can display a clear picture of the signal you want to investigate. 'Dual trace' oscilloscopes display two V/t graphs at the same time, so that simultaneous signals from different parts of an electronic system can be compared.

Working Principles

Working Principles (cont’d) Like a television screen, the screen of an oscilloscope consists of a cathode ray tube. Although the size and shape are different, the operating principle is the same. Inside the tube is a vacuum. The electron beam emitted by the heated cathode at the rear end of the tube is accelerated and focused by one or more anodes, and strikes the front of the tube, producing a bright spot on the phosphorescent screen. The electron beam is bent, or deflected, by voltages applied to two sets of plates fixed in the tube. The horizontal deflection plates, or X-plates produce side to side movement. As you can see, they are linked to a system block called the time base. This produces a sawtooth waveform. During the rising phase of the sawtooth, the spot is driven at a uniform rate from left to right across the front of the screen. During the falling phase, the electron beam returns rapidly from right ot left, but the spot is 'blanked out' so that nothing appears on the screen.

Working Principles (cont’d) In this way, the time base generates the X-axis of the V/t graph. The slope of the rising phase varies with the frequency of the sawtooth and can be adjusted, using the TIME/DIV control, to change the scale of the X-axis. Dividing the oscilloscope screen into squares allows the horizontal scale to be expressed in seconds, milliseconds or microseconds per division (s/DIV, ms/DIV, µs/DIV). Alternatively, if the squares are 1 cm apart, the scale may be given as s/cm, ms/cm or µs/cm. The signal to be displayed is connected to the input. The AC/DC switch is usually kept in the DC position (switch closed) so that there is a direct connection to the Y-amplifier. In the AC position (switch open) a capacitor is placed in the signal path. As will be explained in Chapter 5, the capacitor blocks DC signals but allows AC signals to pass.

Working Principles (cont’d) The Y-amplifier is linked in turn to a pair of Y-plates so that it provides the Y-axis of the the V/t graph. The overall gain of the Y-amplifier can be adjusted, using the VOLTS/DIV control, so that the resulting display is neither too small or too large, but fits the screen and can be seen clearly. The vertical scale is usually given in V/DIV or mV/DIV. The trigger circuit is used to delay the time base waveform so that the same section of the input signal is displayed on the screen each time the spot moves across. The effect of this is to give a stable picture on the oscilloscope screen, making it easier to measure and interpret the signal.

Working Principles (cont’d) Changing the scales of the X-axis and Y-axis allows many different signals to be displayed. Sometimes, it is also useful to be able to change the positions of the axes. This is possible using the X-POS and Y-POS controls. For example, with no signal applied, the normal trace is a straight line across the centre of the screen. Adjusting Y-POS allows the zero level on the Y-axis to be changed, moving the whole trace up or down on the screen to give an effective display of signals like pulse waveforms which do not alternate between positive and negative values.

Connectors

3. LOGIC PROBES Logic probes, as shown in figure opposite, are extremely simple and useful devices that are designed to help you detect the logic state of an IC. Logic probes can show you immediately whether a specific point in the circuit is low, high, open, or pulsing.

Logic Probe A high is indicated when the light at the end of the probe is lit and a low is indicated when the light is extinguished. Some probes have a feature that detects and displays high-speed transient pulses as small as 5 nanoseconds wide. These probes are usually connected directly to the power supply of the device being tested, although a few also have internal batteries.

Logic Probe (cont’d) Since most IC failures show up as a point in the circuit stuck either at a high or low level, these probes provide a quick, inexpensive way for you to locate the fault. They can also display that single, short-duration pulse that is so hard to catch on an oscilloscope.

Characteristics The ideal logic probe will have the following characteristics: 1. Be able to detect a steady logic level 2. Be able to detect a train of logic levels 3. Be able to detect an open circuit 4. Be able to detect a high-speed transient pulse 5. Have over voltage protection 6. Be small, light, and easy to handle 7. Have a high input impedance to protect against circuit loading

Logic Pulser Another extremely useful device for troubleshooting logic circuits is the logic pulser. It is similar in shape to the logic probe and is designed to inject a logic pulse into the circuit under test. Logic pursers are generally used in conjunction with a logic clip or a logic probe to help you trace the pulse through the circuit under test or verify the proper operation of an IC.

Logic Pulser (cont’d) Some logic pursers have a feature that allows a single pulse injection or a train of pulses. Logic pursers are usually powered by an external dc power supply but may, in some cases, be connected directly to the power supply of the device under test.

Logic Pulser (cont’d) Figure on the left below shows a typical logic pulser. Figure on the right shows a logic pulser (right) used with a logic probe (left).

SIGNAL GENERATOR 4. SIGNAL GENERATOR The signal generator is a device used to generate a variety of electrical signal waveforms that are used as inputs to various electronic circuits during testing and/or development activities. Useful piece of equipment in the signal generator family is function generator. Signal Generator Function Generator

Signal Generator (cont’d) It contains an electronic oscillator, an electronic circuit that is capable of creating a repetitive waveform. The most common waveform is a sine wave, but sawtooth, step (pulse), square, and triangular waveform oscillators are commonly available as are arbitrary waveform generators (AWGs). If the oscillator operates above the audio frequency range (>20KHz), the signal generator will often include some sort of modulation including one or more of amplitude modulation (AM), frequency modulation (FM), or phase modulation (PM) as well as a second oscillator that provides an audio frequency modulation waveform.

Signal Generated SIGNAL GENERATOR A bandlimited sawtooth wave pictured in the time domain (top) and frequency domain (bottom). The fundamental is at 220 Hz. Sine and cosine wave

SIGNAL GENERATOR Function Generator A function generator is a piece of electronic test equipment used to generate repetitive waveforms. These waveforms can then be injected into a device under test and analyzed as they progress through the device, confirming the proper operation of the device or pinpointing a fault in the device. Function generators usually generate a triangle waveform as their basic output.

Function Generator (cont’d) SIGNAL GENERATOR Function Generator (cont’d) The triangle is generated by repeatedly charging and discharging a capacitor from a constant current source. This produces a linearly-ascending or descending voltage ramp. As the output voltage reaches upper and lower limits, the charging and discharging is reversed, producing the linear triangle wave. By varying the current and the size of the capacitor, different frequencies may be obtained.

Function Generator (cont’d) SIGNAL GENERATOR Function Generator (cont’d) Most function generators also contain a diode shaping circuit that can convert the triangle wave into a reasonably-accurate sine wave. Function generators, like most signal generators, may also contain an attenuator, various means of modulating the output waveform, and often contain the ability to automatically and repetitively "sweep" the frequency of the output waveform between two operator-determined limits. This capability makes it very easy to evaluate the frequency response of a given electronic circuit.

Schematic Diagram for Function Generator SIGNAL GENERATOR Schematic Diagram for Function Generator

Function Generator (FG) – Principle SIGNAL GENERATOR Function Generator (FG) – Principle Built around a single 8038 waveform generator IC, this circuit produces sine, square or triangle waves from 20Hz to 200kHz in four switched ranges. There are both high and low level outputs which may be adjusted with the level control. All of the waveform generation is produced by IC1. This versatile IC even has a sweep input, but is not used in this circuit.

FG – Principle (cont’d) SIGNAL GENERATOR FG – Principle (cont’d) The IC contains an internal square wave oscillator, the frequency of which is controlled by timing capacitors C1 - C4 and the 10k potentiometer. The tolerance of the capacitors should be 10% or better for stability. The square wave is differentiated to produce a triangular wave, which in turn is shaped to produce a sine wave. All this is done internally, with a minimum of external components. The purity of the sine wave is adjusted by the two 100k preset resistors.

FG – Principle (cont’d) SIGNAL GENERATOR FG – Principle (cont’d) The wave shape switch is a single pole 3 way rotary switch, the wiper arm selects the wave shape and is connected to a 10k potentiometer which controls the amplitude of all waveforms. IC2 is an LF351 op-amp wired as a standard direct coupled non-inverting buffer, providing isolation between the waveform generator, and also increasing output current. The 2.2k and 47 ohm resistors form the output attenuator. At the high output, the maximum amplitude is about 8V pk-pk with the square wave.

FG – Principle (cont’d) SIGNAL GENERATOR FG – Principle (cont’d) The maximum for the triangle and sine waves is around 6V and 4V respectively. The low amplitude controls is useful for testing amplifiers, as amplitudes of 20mV and 50mV are easily achievable.

SIGNAL GENERATOR Type of Signal

Summary This week we have looked at the operation of: MULTIMETER SIGNAL GENERATOR Summary This week we have looked at the operation of: MULTIMETER OSCILLOSCOPE PROBES SIGNAL GENERATOR