ELECTRICAL FUNDAMENTALS
3 STARTER GENERATORS
4.5 MEASURING AC USING OSCILLOSCOPES
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4.4 ADDITION OF AC & DC
It is possible for both ac and dc to exist in the same circuit or conductor. In such cases the ac is said to be superimposed on the dc, or the dc has an ac ripple.
The resultant waveform depends on the relative values of ac and dc, as shown in the diagrams above.
4.5 MEASURING AC USING OSCILLOSCOPES
4.5.1 THE CATHODE RAY OSCILLOSCOPE
Cathode ray oscilloscopes are analogue-graphical instruments which enable electrical waveforms to be displayed for analysis and measurement purposes. A typical instrument is represented in the diagram below.
JAR 66 CATEGORY B1 tube (CRT) form an electron gun which projects a stream of electrons between deflecting plates onto the screen. The screen is coated with a phosphorescent material so that a luminous spot is produced on the screen. A property of the screen coating material allows the spot to persist for a period of time when the stream of electrons is moved or interrupted. The amount of illumination depends on the quantity of electrons in the stream and their velocity on impact with the screen.
The potential at grid g1, which is negative with respect to the cathode, controls the quantity of electrons emitted from the cathode. Adjusting R1 varies the potential at g1, hence R1 controls the brightness of the illuminated spot. Positive potentials at g2 and g3 accelerate the electrons towards the screen. The potential difference between g2 and g3, varied by adjusting R2, sets up an electrostatic field which enables the electron stream to be focused at the screen.
The position of the spot on the screen is determined by the simultaneous effect of voltages applied to the X and Y deflecting plates. A potential difference between the X deflecting plates causes the spot to move across the screen in the
horizontal direction, through a distance proportional to the potential difference. A potential difference between the Y deflecting plates exerts a similar control over the vertical movement of the spot.
The outputs of the X and Y amplifiers establish the potential differences between corresponding pairs of deflecting plates. If these voltages vary in magnitude the spot moves over the screen to produce a continuous trace. Since one voltage controls horizontal deflection and the other controls vertical deflection, the trace forms a graphical representation of one voltage as a function of the other.
4.5.1.1 The Time Base
Most applications require that a signal waveform is displayed as a function of time. To meet this requirement a time base circuit supplies a voltage which varies linearly with time, usually, to the
horizontal (X) deflecting plates whilst the signal to be
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The period t1, is the sweep, that is the time the spot takes to move linearly from left to right across the screen. During the much shorter period t2, called the flyback time, the spot returns rapidly to the left of the screen to start a new cycle.
During flyback the screen may blacked out by a negative pulse generated by the time base circuit and applied to g1, the control grid.
If the sweep period (T) of the time base is equal to, or is a multiple of, the periodic time of the signal applied to the Y deflecting plates, a stationary display of the signal voltage variations with time will be obtained. In the diagram above, the sweep period (T) equals the periodic time 1
f of the signal waveform. In practice the time base is adjusted so that signals over a wide frequency range may be displayed against a convenient time scale.
4.5.1.2 Synchronisation
The time base and the displayed waveform may be synchronised by employing a trigger circuit actuated by the signal itself, that is, by using the output of the Y amplifier. Alternatively, an external signal source or the mains supply may be used for this purpose.
The trigger circuit generates a pulse to initiate one sweep of the time base when the voltage applied to the circuit reaches a predetermined value. The circuit is adjustable so that a particular trigger point on either the positive or negative half cycle of the displayed waveform may be selected.
Where the signal to be observed is non-periodic, or when the signal appears
infrequently, the time base is triggered by the signal, performs one sweep and then waits for the next signal to appear. In order that the beginning of a non-periodic signal can also be examined, the vertical deflecting voltage is delayed relative to the trigger pulse so that the time base is started before the signal to be observed appears on the screen. The time relationship is shown in the diagram.
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4.5.1.4 Amplifiers & Attenuators
The X and Y amplifiers and attenuators provide the voltage scaling required to ensure that the instrument and the measured signal are compatible. Since the oscilloscope is required to display complex voltage waveforms, it is essential that fundamental and harmonic frequencies must undergo the same amplification or attenuation, and that the time relationships between different frequencies must be maintained. It therefore follows that both the amplifier and the attenuators, must have flat amplitude against frequency and transit time against frequency,
characteristics.
4.5.2 TYPES OF OSCILLOSCOPES
4.5.2.1 Sampling Oscilloscopes
At very high frequencies, say above 300MHz, it is not possible using existing techniques to produce a continuous display on an oscilloscope. To obtain a satisfactory display a sampling technique must be used.
As shown in the diagram below, in a sampling oscilloscope the time base circuit produces a stepped voltage waveform to deflect the electron beam in the
horizontal direction. Prior to each step, a pulse is generated which initiates the sampling process.
JAR 66 CATEGORY B1 MODULE 3 (part B)
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4.5.2.2 Multiple Trace Display
Oscilloscopes equipped with multiple trace facilities enable two or more signals to be displayed simultaneously. Essential features of these instruments are a
separate input channel for each signal and a means of separating the electron beams for display. The most widely used instruments enable two signals to be compared, although four beam instruments are quite common.
Cathode ray tubes equipped with two electron guns and two sets of deflecting plates, so that each channel is completely independent, are employed in instruments known as Dual Beam Oscilloscopes. Alternatively, a single gun may be used to produce two traces by switching the Y deflecting plates from one input signal to the other for alternate sweeps of the screen. Although the signals are sampled, the display appears to the eye as a continuous, simultaneous, display of both signals. Oscilloscopes employing this techniques, which is called the alternate mode, can only be used as single channel instruments to
investigate transient waveforms.
4.5.2.3 Dual Trace CRO
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4.5.2.3.2 Chopped Mode
The electronic switch is free running at 100 - 500KHz and is independent of the frequency of the sweep generator. The switch successively connects small segments of the 1 and 2 waveforms to the vertical amplifier. If the chopping rate is much faster than the horizontal sweep rate, the individual little segments fed to the vertical amplifier reconstitute the original 1 and 2 waveforms on the screen, without visible interruptions in the two images.
4.5.2.4 Delayed Sweep Both time bases in operation.
A - delaying sweep B - delayed sweep
Either or both (alternate) signals can be fed to X plates. This allows a closer examination of part of the waveform. CRO contains two linear calibrated sweeps, a main sweep and a delayed sweep. The main sweep is initiated by its trigger pulse at time t0. The delayed sweep will be triggered at time t1, intensifying the original display.
If the CRO sweep control is now set to delay position, the intensified portion will be shown expanded on the screen.
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4.5.2.5 Direct Viewing Storage C.R.T.
The dielectric storage sheet consists of a layer of scattered phosphor particles capable of having any portion of its surface area written to. This dielectric sheet is deposited on a conductive coated glass faceplate called the "storage target backplate".
The flood electrons are distributed evenly over the entire surface area of the storage target.
After the write gun has written a charge image on the storage target, the flood guns will store the image. The written portions of the target are bombarded by flood electrons that transfer energy to the phosphor layer in the form of visible light. This light pattern can be viewed through the glass faceplate.
4.5.2.6 The Digital Storage CRO
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The digital storage CRO stores the data representing the waveforms in a digital memory. The input signal is "digitised", i.e. it is sampled and then converted into binary numbers by the A/D converter.
The resolution of the system depends on the number of bits used by the converter. Converters are said to have a resolution of 1 part in 2 or 'n bit
resolution' where n is the number of bits, i.e. 10 bit resolution would digitise to 210 (1024) discrete levels: the resolution would be 1 part in 1024 or 0.098%.
This digitised input is then converted back to an analogue signal for display by the D/A converter.
(1-2MHz which may be extended to 200MHz using sampling techniques).
4.5.3 USING THE OSCILLOSCOPE
An oscilloscope is an extremely comprehensive and versatile item of test equipment which can be used in a variety of measuring applications, the most important of which is the display of time related voltage waveforms. Such an item probably represents the single most costly item in the average service shop and it is therefore important that full benefit is derived from it.
The oscilloscope display is provided by a cathode ray tube (CRT) which has a typical screen area of 8cm 10cm. The CRT is fitted with a graticule which may be an integral part of the tube face or on a separate translucent sheet. The graticule is usually ruled with a 1cm grid to which further bold lines may be added to mark the major axes on the central viewing area. Accurate voltage and time measurements may be made with reference to the graticule, applying a scale factor derived from the appropriate range switch.
A word of caution is appropriate at this stage. Before taking meaningful
measurements from the CRT screen it is absolutely essential to ensure that the front panel variable controls are set in the calibrate (CAL) position. Results will almost certainly be inaccurate if this is not the case!
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The use of the graticule is illustrated by the following example:
An oscilloscope screen is depicted below. This diagram is reproduced actual size and the fine graticule markings are shown every 2mm along the central vertical and horizontal axes. The oscilloscope is operated with all relevant controls in the 'CAL' position. The timebase (horizontal deflection) is switched to the 1ms/cm range and the vertical attenuator (vertical deflection) is switched to the 1V/cm range. The overall height of the trace is 5cm 1V = 5V. The time for one complete cycle (period) is 4 1ms = 4ms. One further important piece of information is the shape of the waveform, which in this case is sinusoidal.
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4.5.3.1 Layout of Controls
Layouts of the controls and display provided by a typical dual-channel
oscilloscope are shown in the diagrams above and below. The majority of the controls identified in the above diagram are those associated with the position and appearance of the display (e.g. vertical shift horizontal shift, intensity and focus) whilst those shown in the diagram below include the vertical gain and attenuator controls.
JAR 66 CATEGORY B1 MODULE 3 (part B)
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The dual-channel oscilloscope has three BNC coaxial input connectors:
Channel 1. This is the primary vertical input, but it is also used for the horizontal (X) input when the mode switch is set to the 'X-Y' position.
Channel 2. This is the second vertical input which is also used for the vertical input (Y) when the mode switch is set to the 'X-Y' position.
External trigger. This input is only used when the trace is to be locked to an external trigger signal (both 'CH1' and 'CH2' trigger selector buttons must be depressed on the trigger selector).
In addition, a voltage calibrator test point is provided (marked 'CAL 1V' on the front panel). This connector provides an accurate 1V square wave signal which may be used to calibrate the two vertical deflection channels.
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4.5.3.2 Basic Adjustments
The basic adjustments for single-channel waveform measurements are shown in the diagram below. The sequence of adjustments is as follows:
1. The input signal is applied, via a suitable probe, to the Channel 1 (CH1) input connector.
2. The intensity and focus controls are adjusted for a satisfactory display.
3. The display is centred on the graticule using the vertical and horizontal shift controls.
4. The variable gain (Var) and variable sweep (Var Sweep) controls are set to the calibrate (Cal) positions.
5. The trigger selector (TRIGGER) is set the Channel 1 (CH1).
6. Positive edge trigger is selected '+' (note that negative edge trigger may also be selected - in practice the sharpest edge of the waveform will produce the most effective triggering).
7. The display mode switch (MODE) is set to Channel 1 (CH1).
8. The Channel 1 input selector is set to 'AC'.
9. The vertical attenuator (VOLTS/CM) control is adjusted to produce a suitable height display.
10. The trigger level control (Trig Level) is adjusted to obtain a stable (locked) display.
11. The timebase selector (TIME/CM) control is adjusted to produce a suitable number of cycles on the display (usually two to five cycles).
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The basic adjustments for dual-channel waveform measurements are shown in the diagram below. The sequence of adjustments is as follows:
1. The first input signal is applied, via a suitable probe, to the Channel 1 (CH1) input connector.
2. The second input signal is applied, via a suitable probe, to the Channel 2 (CH2) input connector.
3. The intensity and focus controls are adjusted for a satisfactory display.
4. The displays are centred using the horizontal shift control.
5. The displays are adjusted (vertically separated into the upper and lower parts of the display) using the two vertical shift controls.
6. The two variable gain (Var) and variable sweep (Var Sweep) controls are set to the calibrate (Cal) positions.
7. The trigger selector (TRIGGER) is set to either Channel 1 (CH1), or Channel 2 (CH2), as necessary.
8. Positive or negative edge triggering is selected as required.
9. The display mode switch (MODE) is set to dual-channel (Dual).
10. Both input selectors are set to 'AC'.
11. The vertical attenuator (VOLTS/CM) controls are adjusted to produce displays of a suitable height.
12. The trigger level control (Trig Level) is adjusted to obtain a stable (locked) display.
13. The timebase selector (TIME/CM) control is adjusted to produce a suitable number of cycles on the display (usually two to five cycles).
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The basic adjustments for measurement of DC offset voltages are shown in the diagram below. The sequence of adjustments is as follows:
1. The input signal is applied, via a suitable probe, to the Channel 1 (CH1) input connector.
2. The intensity and focus controls are adjusted for a satisfactory display.
3. The display is centred on the graticule using the horizontal shift control.
4. The variable gain (Var) and variable sweep (Var Sweep) controls are set to the calibrate (Cal) positions.
5. The trigger selector (TRIGGER) is set to Channel 1 (CH1).
6. Positive edge trigger is selected '+' (note that negative edge trigger may be also be selected - in practice the sharpest edge of the waveform will produce the most effective triggering).
7. The display mode switch (MODE) is set to Channel 1 (CH1).
8. The Channel 1 input selector is set to 'GND'.
9. The vertical shift control is adjusted so that the trace is exactly aligned with the horizontal axis of the graticule (this line will then correspond to 0V) 10. The Channel 1 input selector is set to 'DC'.
11. The vertical attenuator (VOLTS/CM) control is adjusted to produce a suitable height display.
12. The trigger level control (Trig Level) is adjusted to obtain a stable (locked) display.
13. The timebase selector (TIME/CM) control is adjusted to produce a suitable number of cycles on the display (usually two to five cycles).
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4.5.3.3 Waveform Measurements Examples of some basic waveform measurements using an oscilloscope are shown in the diagram to the left. In (a), a square wave is displayed. One complete cycle of this waveform occupies 2cm on the display. Since the timebase range selector (TIME/CM) is set to 1ms/cm, the time for one complete cycle of the
waveform is 2 1ms = 2ms. The vertical size of the waveform (i.e. its peak-peak value) measures 2cm on the graticule.
Since the vertical attenuator (VOLTS/CM) is set to 1V/cm the peak-peak voltage is 2
1V = 2V.
A sine wave is shown in (b). One
complete cycle of this waveform occupies 2.5cm on the display. Since the timebase range selector (TIME/CM) is set to
2ms/cm, the time for one complete cycle of the waveform is 2.5 2ms = 5ms. The vertical size of the waveform (i.e. its peak-peak value) measures 3cm on the
graticule. Since the vertical attenuator (VOLTS/CM) is set to 50mV/cm the peak-peak voltage is 3 50mV = 150mV.
An irregular pulse is shown in (c). The display is 'low' for 3.4cm measured on the
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4.5.3.4 Pulse Rise and Fall Times The rise and fall of a pulse can be easily measured using the
techniques previously described (note that this measurement is only valid if the oscilloscope is fitted with a properly compensated probe). The diagram shows the parameters of a pulse including:
Rise time (10% to 90%) Fall time (90% to 10%) On time (time above 50%) Off time (time below 50%)
4.5.3.5 Pulse Delay
A dual-channel oscilloscope can be easily used to measure pulse delay (see diagram below). Note that this measurement should be performed with the timebase mode switch set to 'CHOP' rather than 'ALTERNATE' on oscilloscopes that offer an alternate sweep facility.
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4.5.3.6 Sine Wave Performance checks An oscilloscope can provide a very rapid assessment of the performance of an amplifier. A pure sinewave (of appropriate frequency and amplitude) is applied to the input of the amplifier (or other system under test) and the output is displayed on the screen of the oscilloscope. The effects of non-linearity, clipping noise, distortion, etc.
and be easily seen (see diagram).
4.5.3.7 Square Wave Performance Checks
An alternative, but equally revealing assessment of an amplifier can be made using a square wave test. An accurate square wave (of appropriate frequency and amplitude) is applied to the input of the amplifier (or other system under test) and the output is once again displayed on the screen. The effects of poor
frequency response, 'ringing', etc. can be easily detected (see diagram).
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4.5.3.8 Phase Measurement
A number of useful measurements can be made with an oscilloscope in X-Y mode. It is possible to carry out reasonably accurate measurements of phase angle using Lissajous figures (see diagram to the left).
In order to obtain these displays, the two signals must be applied with identical gain/attenuation and it is usually necessary to calibrate the instrument by applying the same sine wave signal to the X and Y inputs and adjust the gain controls to obtain a straight line at exactly 45º (see diagram).
Thereafter, the signal to be measured is applied to vertical channel (Y) whilst the reference signal is applied to the horizontal channel (X). The shape of the display indicates the phase shift between the two signals. This technique is ideal for rapidly checking the phase shift produced by a network, filter or amplifier.
4.5.3.9 Frequency Measurement Lissajous figures can also be used to determine the frequency relationship between two signals (see diagram). The frequency ratio is given by the ratio of the number of 'peaks' produced in the
horizontal direction to the number of
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4.5.3.10 Modulation Measurement
4.5.3.10 Modulation Measurement