19 oscilloscopes and chart recorders rev 3 080524

19 Oscilloscopes & Chart Recorders 0908341 Measurements & Instrumentation

© Copyright held by the author 2008: Dr. Lutfi R. Al-Sharif of 11

Chapter 19 Oscilloscopes and Chart Recorders

(Revision 3.0, 24/5/2008) 1. Introduction Having discussed the measurement of the main seven electrical quantities in the previous chapters (voltage, current, resistance, capacitance, inductance, frequency, phase shift), we now examine how we can display and store a time varying electrical signal on a display. This is generally done by using oscilloscopes and chart recorders. 2. Oscilloscopes In many applications, there is a requirement to display a time varying electrical quantity (e.g., voltage) against time. A multi-meter is not suitable for such an application as it cannot display a time history of the variable, and it cannot respond to a fast changing signal. In general, conventional oscilloscopes use CRT (cathode ray tube) technology with luminescent screen and do not have any means of signal storage. Most modern oscilloscopes on the other hand use an LCD display, process the signal digitally and have means of signal storage and further processing (digital storage oscilloscopes). ‘Conventional’ oscilloscopes can display a periodic signal against time. This allows the user to do the following:

1. By placing the oscilloscope in the Yt mode (whereby the external signal is applied to the Y axis, and the time base signal is applied to the X axis), the shape of the waveform can be displayed.

2. Measure the amplitude of the signal.

3. Measure the frequency and the period of the signal.

4. Display two signals simultaneously on the same display against time and compare their amplitude and relative frequency and phase shift.

5. By placing the oscilloscope in the XY mode, two signals can

be displayed simultaneously applied to the two different axes (X and Y). This allows the use of the so-called Lissajous patterns to find the relative phase shift and relative frequencies of the two signals1.

‘Modern’ oscilloscopes however allow the user to do much more, such as:

1 Lissajous patterns were discussed in depth in the Chapter on frequency and phase measurement.



6. Store the waveform in order to be later retrieved.

7. Complex forms of triggering can be used to capture the waveform. Post triggering and pre-triggering are possible. This allows the user to see events that took place prior to the trigger point.

8. Carry out analysis on the waveform. Automatic calculations

are possible in some devices such as peak to peak voltage calculation, frequency measurement, period measurement, rms voltage, phase shift.

9. In some advanced devices, harmonic analysis can be carried

out on the waveform using FFT (Fast Fourier Transform).

10. The user can set the scope to capture glitches in the waveform.

11. Most devices will have a serial link that can be connected to

a PC for data download, logging and further analysis. Figure 1 shows a conventional CRT analogue oscilloscope with a 20 MHz bandwidth. Figure 2 shows a hybrid (analogue/digital) storage oscilloscope of 60 MHz bandwidth. Figure 3 shows a portable digital storage scope with an LCD display.

Figure 1: CRT based 20 MHz anaogue osciloscope CS-1425A (courtesy of Kenwood).



Figure 2: CRT analogue/digital hybrid oscilloscope, Metrix OX8050 (Courtesy of Metrix).

Figure 3: Digital Storage colour LCD based Fluke 190 Series Scope meters (Courtesy

of Global Spec.).

The main disadvantage of oscilloscopes is that they are fragile and relatively expensive. 2.1 Principle of Operation In a conventional CRT oscilloscope a heated filament is used to generate electrons and an anode is used to accelerate and focus the electron beam. An acceleration voltage is used to accelerate the beam and the higher the value of the voltage, the higher the intensity of the resultant beam (typical values are 2 kV and 12 kV). The electron beam is then made to impact on luminescent screen, which in turn glows.

Along the path of the electron beam, two sets of deflection plates (horizontal and vertical) are used to electrostatically deflect the electron beam horizontally (producing the X channel) and vertically (producing the Y channel).



The oscilloscope can display the signal as a trace against time by applying an internally generated saw-tooth signal to the x plates (horizontal) and the signal to be displayed to the y plates (vertical). By varying the frequency of the sawtooth signal signals of varying frequencies can be displayed.

Figure 4: Front panel of the Hameg 203-6 Oscilloscope (source: http://www.doctronics.co.uk/scope.htm#what).

2.2 Dual Mode (chop & alternate) Although any conventional CRT oscilloscope has only one trace, most oscilloscopes can display two or more signals simultaneously. These are referred as channels and are usually called Channel 1 and 2.

Two signals can to be displayed simultaneously on the screen at the same time by using the ‘alternate’ and ‘chop’ modes. In the ‘alternate’ mode each signal is displayed once on the full screen and then the other signal is displayed for a full screen, and so on (this suits the higher frequency signals). In the chop mode the beam moves very quickly between the two signals (this suits the lower frequency signals).

The scope can also be put in the X-Y mode, by which two different signals are entered into the Ch1 and Ch2. this allows the derivation of the phase and frequency relationships between the two signals. 2.3 Oscilloscope parameters The most important aspects of an oscilloscope are: bandwidth, rise-time and accuracy.

The oscilloscope generally has low accuracy (inaccuracy ranges from ±1% to ±10%) and is not used in cases where high accuracy is required. It has relatively high input impedance (1 MΩ to 10 MΩ).

The Frequency response of oscilloscopes ranges from 20 MHz for the cheaper types to 500 MHz to the more expensive types.

The bandwidth is defined as frequencies at which the gain drops to -3dB (0.707) of the mid range gain. The rise time is the time it takes for the trace to



rise from 10% to 90% of the final value in response to a step input. As a rule of thumb the product of the bandwidth and rise-time is equal to 0.35 (this is derived in Appendix A). This rule can be used to find the rise time of the oscilloscope based on the bandwidth or vice versa. It is important to ensure that the signal that is being measured does not have a rise time near the rise-time of the oscilloscope, to avoid incorrect measurements. 2.4 Triggering Each cycle of the sweep is initiated by a pulse from an internal pulse generator. This is called triggering. Triggering is used to ensure that the signal to be displayed is traced in the same place on the screen to make it clear to see. Figure 5 shows what a signal will look like when it is not triggered correctly.

Figure 5: Signal cannot be 'fixed' on the screen without correct triggering.

The triggering circuitry extracts the phase information from the signal to be displayed and uses it to synchronise the sawtooth time-base signal. This is an example of internal triggering (i.e., the triggering information is extracted from the signal itself).

Triggering can be set to any one of three sources: internal, external and line (i.e., the 50 Hz mains). Internal mode triggers from one of the channels (1 or 2), external allows the user to enter an external signal (this is needed when the input signal is too weak to be used for triggering) and the line triggering triggers based on the phase information from the 50 Hz power supply (which is useful when measuring signals from drives that control the power from a single phase or three phase source.



Triggering can also be set to trigger on the positive slope of the signal (Figure 6) or the negative slope of the signal (Figure 7). It can also be set to trigger at a different signal level, as shown in Figure 8.

Figure 6: Positive Edge Triggering. Figure 7: Negative Edge triggering.

Figure 8: Negative edge triggering at a higher level.

2.5 Coupling Each input signal can be coupled via AC, DC or Ground. AC coupling removes any DC content in the signal. DC coupling does not alter the signal. Ground coupling grounds the input signal to allow the user to find the trace. 2.6 Differential Inputs When a channel is entered to the scope, the common line is connected to the common of the other channel and to the chassis and the earth from the power supply. These are called single ended inputs. Great care has to be taken when measuring signals that have different earths, in order to avoid making short circuits between different points on the circuit through the oscilloscope.

To solve this problem, special oscilloscopes are available that have differential inputs (as opposed to single ended inputs). The common points for different channels are isolated from each other. These are more expensive that the single ended input scopes. 3. Chart recorders



Chart recorders allow the user to have a permanent record of the waveform captured. These are becoming less popular now with the increased use of digital storage scopes and PC based virtual instrumentation. There are seven types of chart recorders (material generally based on reference [1]):

1. Mechanical chart recorders: These are simple, cheap and reliable, but

their dynamic response is limited to 30 Hz. There are two types:

• Galvanometric Recorders: These work on the principle of the permanent magnet moving coil (inaccuracy of +/- 0.2%).

• Potentiometric Recorders: These have a better specification than the galvanometric recorders (+/- 0.1% of full scale) but suffer from a very slow response time (0.2 to 2 seconds). See Appendix B for an example of this type.

One example of mechanical recorders is the circular chart recorder (usually covering 24 hours per disk). Examples include the speed of a truck, and the humidity over one day in an office.

2. Ultra-violet recorders: These following the same principle as the moving coil recorders but use ultra-violet light reflected on thin mirrors, thus reducing the inertia of the system and improving the frequency response (up to 13 KHz). The narrow mirror reflects a beam of ultraviolet light onto sensitive paper.

They have an accuracy of ±2%, but are more expensive and fragile.

3. Fibre optic recorders (recording oscilloscopes): These are similar to

oscilloscopes and they direct an electron beam onto a fluorescent screen and then onto photo-sensitive paper. They are more expensive but have a bandwidth of 1 MHz.

4. Hybrid chart recorders: These are a mixture of analogue and digital

and comprise a potentiometric recorder with a microprocessor.

5. Magnetic tape recorders: These record the signal onto a magnetic tape. Analogue signals of upto 80 kHz in frequency can be recorded. Signals can be recorded at one speed and played at another. They usually have between 4 and 10 channels, and suffer from an inaccuracy of ±5%.

6. Digital recorders: Digital recorders are multi-channel recorders, with 10

bit A/D converters (offering a resolution of 0.1%) or 12 bits giving 0.025% resolution. They have a frequency response of 25 kHz, a maximum sampling of 200 MHz and data storage of 4000 data points per channel. Some devices have an integrated CRT or LCD display to view data.



7. Digital storage scopes are effectively conventional scopes with A/D converters and a storage facility. They are very effective in capturing transient signals when set to single sweep mode (e.g., mechanical bounce of switch). They can be ‘armed’ in order to trigger under certain signal conditions (slope, level). Once armed, they wait for the trigger conditions to take place and will record a certain period of time before and after the trigger point. They can output analogue signals to devices like chart recorders and digital signals to IEE488 interfaces, RS232 interfaces. Some have floppy drives to export data. Some devices have computation facilities such as peak value, mean value, r.m.s., maximum/minimum values, rise time, frequency, phase shift and multiplication.

References [1] “Measurement & Instrumentation Principles”, Alan S. Morris, Elsevier,

2001. Further Reading [1] “XYZs of Oscilloscopes: Primer”, 2001, Tektronix, 05/01 HB/PG 03W-

8605-2. An excellent modern introduction to the state of the art in oscilloscopes from one of the leading device manufacturers.

[2] “Modern Electronic Instrumentation and Measurement Techniques”, Albert D. Helfrick & William D. Cooper, Prentice Hall International Edition, 1990. Chapter 7 contains a detailed description of the oscilloscope design, especially electronic circuitry.

[3] “Experimental Methods for Engineers”, J.P. Holman, Seventh Edition, McGraw Hill International Edition. Contains a good section (4.16) on oscilloscope selection.

[4] “Elements of Electronic Instrumentation and Measurement”, Joseph J. Carr, Third Edition, Prentice Hall, 1996. Detailed discussion on oscilloscopes, including medical types in Chapter 8.

[5] “Measurement Systems: Application and Design”, Ernest O. Doebelin, Fifth International Edition, McGraw Hill, 2003. Section 12.6 (pp 968-974).

[6] “An Introduction to Electrical Instrumentation and Measurement Systems”, B.A. Gregory, Second Edition, 1981, McMillan and English Language Book Society. Section 2.6 (p115-134).

[7] “Applied Electronic Instrumentation and Measurement”, David Buchla and Wayne McLachlan, 1992, McMillan Publishing Company. Chapter 8.

[8] “Guide to Electronic Measurements and Laboratory Practice”, Stanley Wolf, Second Edition, Prentice Hall, 1983. Chapter 6 contains a detailed discussion on Oscilloscopes and introduces the use of an oscilloscope as a curve tracer for electronic components characteristics.

[9] “Instruments and Automatic Test Equipment”, K.F. Ibrahim, Longman Group UK Limited, 1988.



Appendix A Product of Oscilloscope Bandwidth and its Rise Time

We can think of the oscilloscope as an RC circuit, with a time constant τ equal to the product of R and C. When rising between 10% of final value and 90% of final value, it will take the following time:

0.1=1-exp(-t0.1/τ) t0.1=0.105·τ

0.9=1-exp(-t0.9/τ)

t0.9=2.303·τ

trise=2.2·τ Based on the bandwidth of the oscilloscope, this would be the frequency at which the frequency would be -3dB of the mid-band value (or at which the voltage would be 0.707 of the mid-band value). So the frequency at this point of -3dB attenuation will be 1/(2π·R·C) or 1/(2·τ·π). So multiplying the -3dB frequency by the rise time gives:

trise ·f-3dB=(2.2·τ)· 1/(2·τ·π)=0.35 So this shows that the product of the bandwidth of the oscilloscope and its rise time is 0.35. This allows us to find out the limit on the rise time of a signal that we want to measure using the oscilloscope, by dividing 0.35 by the bandwidth of the oscilloscope. It is no point trying to measure a rise time of s signal that is smaller than that of the oscilloscope. In effect we would be measuring the rise time of the oscilloscope not the signal!



Appendix B Example of a Potentio-metric servo Chart Recorder (BD11/BD12)

19 oscilloscopes and chart recorders rev 3 080524

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