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In this short series, we will investigate those most useful electronic instruments, Cathode Ray Oscilloscopes. In this first part we will look at analog oscilloscopes and delve into their basic operation. By BRYAN MAHER T HE CATHODE RAY Oscilloscope, commonly referred to as a CRO or scope, is an extremely useful instrument for experimenters and designers, and for servicing. The purpose of any oscilloscope is to enable us to observe a light pattern in the shape of a graph of whatever electrical signal is applied to the instrument, as depicted in Fig.1. Many details of voltage waveforms can be inspected, such as peak values, rise and fall times, frequency, period, glitches, interferences, oscillation or instability. Also, we can trace signals through circuits for the source of gross distortion, if present, as shown in the photo next to Fig.1. The pure sinewave is the input voltage to an audio power amplifier which is faulty, while the distorted signal is the output of that amplifier resulting from severe crossover distortion. A CRO can be used with radio transmitters or receivers to display amplitude modulated (AM) signals and show clearly the modulation percentage and any over-modulation, if present. With suitable probes, current waveforms can be displayed. Also, we can display the magnetic properties of iron or ferrite materials, draw the B/H curve and illustrate hysteresis. In fact, the range of device parameters which can be measured and displayed on an oscilloscope is virtually unlimited. The heart of any oscilloscope is the 12  Silicon Chip cathode ray tube, sometimes called a CRT. A simplified cross section of an oscillo­scope tube is shown in Fig.2. The long glass vacuum tube has a screen at one end, the inside surface of which is coated with a fluorescent phosphor materi­ al. Also the inside surface of the glass side walls, near the screen, is coated with a conductive material called Aquadag which is connected to an external terminal. At the opposite (socket) end is a heater filament and a coated cathode which emits elec­trons. A high voltage DC source has its positive output connected to the aquadag coating near the screen while the negative termi­nal is connected to the cathode. Electrons emitted from the cathode are attracted and accel­erated to the front screen by the high positive voltage. The electrons arrive at the The Hitachi V223A is a modern dual-channel oscilloscope. This portable model, intended for field service as well as laboratory work, offers DC to 20MHz bandwidth, 1mV/div sensitivity and numerous "creature comforts". Above: most oscilloscopes can display two separate signals simul­taneously. In this off-screen photo, a dual input CRO is being used to signal trace through an amplifier under repair, to find the point at which the signal becomes distorted. By comparing the input (sinewave) signal with the signals found at different points along the circuit, the faulty section can be identified. Fig.1: by moving a spot of light on its front screen, a cathode ray oscilloscope (CRO) can draw a graph of any voltage signal applied to its vertical deflection plates. screen with sufficient energy to cause the sensitised material on the inside surface of the front screen glass to fluoresce, or to emit light, at point L. This material, or phosphor, consists of extremely fine grained compounds of specially select­ed light metals. Screen persistence Any point on the CRO screen will give off some light for a little time after the electron beam has moved away. The time taken for this lingering light to fade away to 1% of its initial value is called the persistence time. A typical value for screen persistence in the phosphors used in oscilloscopes is 250 microseconds. Imagine that the frequency of the vertical deflec­tion signal applied to the Y1-Y2 plates in Fig.2 is increased – so that the spot moves up and down the screen faster in less than 250 microseconds. The light spot will be moving faster than screen persistence time and so the spot will trace the whole vertical pathway before any one point can fade away. As a result, we will see a complete vertical line drawn on the screen. When the emitted light ceases al- Fig.2: simplified part diagram of a CRO tube, showing only the evacuated hard glass envelope; the heater and cathode at the lefthand end; the vertical deflection plates Y1, Y2; and the fluorescent phosphor screen at right. The heated cathode emits electrons. A conduc­tive coating called aquadag (AQD) is deposited on the inside surface of the tube near the righthand end. This is connected to the positive end of high voltage supply. March 1996  13 Fig.3: cutaway drawing of a simple CRO tube showing the heater h, cathode K, control grid G1, focus grid G2, accelerating grid G3, vertical deflection plates Y1 & Y2, and horizontal deflection plates X1 & X2. This example shows a 5kV acceleration potential between G3/ screen and cathode K. A1-A5 form the vertical deflection amplifier system, while A6-A10 make up the timebase gen­erator which provides the sawtooth horizontal sweep voltage. Oscilloscopes are such useful instruments that two or more are often used simultaneously on an electronic workbench, as in the scene above. The scope at the right is actually a spectrum analyser and is showing the harmonics of the waveform on the scope at left. 14  Silicon Chip tence time. Some of the common screen phosphors and their specific uses are listed in Table 1. Vertical deflection In Fig.2, a pair of metal plates, Y1 and Y2, are placed above and below the beam of electrons. If a voltage is applied between these plates, with Y1 more positive than Y2, then the resulting electric field will attract the electron beam upwards in the direction of Y1. Thus the electrons will strike the screen material at point M and cause light to be Fig.4: simplified horizontal sweep voltage which deflects the electron beam across emitted there. the CRO tube screen. The rising ramp voltage from time t1 to t3 sweeps the beam Similarly, if the potentials on forward from left to right of screen. During the short time t3 to t5 the beam is swept Y1 and Y2 are reversed, the elecback (retrace or flyback) from right to left of screen. In very simple systems the next tric field will deflect the electron forward sweep then commences. beam downwards, striking the screen material at point P, where most immediately after the elec­tron removed (ie, a long persistence time), light will be emitted. Y1 and Y2 are irradiation has been removed (ie, a we call that screen phosphorescent. called the vertical deflection plates. very short persistence time), we say If a very low frequency repetitive Phosphor numbers that the screen is fluorescent. voltage, which swings through both Conversely, in cases where light conpositive and negative values, is These days oscilloscope tube mantinues to be emitted for a considerable ufacturers can produce screens with applied between plates Y1 and Y2, time after the electron beam has been the electron beam will follow this almost any desired colour and persis- Table 1: Commonly Used Phosphor Numbers & Screen Properties Phosphor Number Screen Colour Persistence Time to 1% Uses and comments P1 Green 50ms Cathode Ray Oscilloscopes and RADAR P2 Yellow/Green 200us to 4% CRO tubes and RADAR P4 White Blue 150us Yellow 480us TV B/W Px tube. Blue component dominates the yellow component; giving daylight white. P5 Blue 52us High speed CRO, for off-screen photography P7 B/G/Y Blue 500us Yellow >3 sec. RADAR cascade screens. Blue image fades fast leaving lasting yellow record. P11 Blue 500us CRO off-screen photography P12 Orange 420ms RADAR receivers P14 (B+R)/Y Purple 200us Yellow 120ms RADAR two-layer cascade screens P15 UV/Violet/G (time to 10%) Violet 3us UV 0.05us Flying-Spot scanning TV camera tube. Fastest screen made P16 UV/Violet 0.12us (10%) Flying-Spot scanning TV camera tube. Fastest visible screen made P22 Blue/Green/Red Blue 5ms Green/Red 50ms Colour TV P28 Yellow/Green Yellow 7 sec RADAR P31 Green 250 microsec Preferred phosphor for Oscilloscopes. P33 Orange 8 seconds RADAR P34 B/G/Y 400 seconds RADAR long persistence March 1996  15 Fig.5: a sinewave signal (a) applied to the vertical input terminal of an oscilloscope deflects the beam (and the consequent spot of light on the screen) in a vertical direction in propor­tion to the voltage value of (a) at any time. At the same time, the electron beam is deflected horizontally by the ramp voltage (b) generated by the sweep system and applied to the horizontal deflection plates. The combined action of both voltages (a) and (b) draws a graph on the screen of voltage (a) as a function of time. changing Y1-Y2 field up and down. Observing the screen, we would see the light spot travel slowly up and down, following a straight line. Horizontal deflection When you draw a voltage waveform 16  Silicon Chip on paper, for instance a sinewave, you use a vertical scale of volts to represent the signal and a linear horizontal scale to represent time. To show the same waveform on the screen of the CRO, the spot of light is moved horizontally at constant speed (X input) and at the same time moved vertically, corresponding to the vertical input signal. Fig.3 depicts a cutaway view of a simple oscilloscope tube, with vertical deflection plates Y1 & Y2. In addition, there are a pair of horizontal deflection plates, X1 & X2, one each side of the electron beam. Any voltage waveform applied to these plates will deflect the electron beam sideways. For the electron beam to move horizontally at constant speed, the voltage applied to the horizontal deflection plates must increase in a straight line with respect to time. So a linear ramp voltage signal (or sawtooth) is applied to the horizon­tal plate. This waveform is shown in Fig.4. This horizontal deflection voltage must run from negative values, through zero, to positive values, to take the spot from far left to far right of screen. In Fig.4, this horizontal deflection voltage is at its most negative at time t1. Therefore, the spot of light will be at the left of the CRO screen. Below: in research laboratories, oscilloscopes are often dedicated to specific tasks. The scopes in this photo are permanently connected in a measurement setup. As the voltage rises towards zero, the light spot moves horizontally to the right, reaching centre screen at time t2. Continuing on, the trace reaches extreme right of screen at time t3. Now let us start again but this time with the vertical input signal applied to the Y1-Y2 plates. The light spot on the screen will trace out a graph of the vertical signal, as depicted in Fig.5, as its voltage values change with time. Notice that in Fig.5 we have arranged for the horizontal signal to start at time t1, just as the signal applied to the verti­cal plates passes through zero. This is called synchronisation, a topic we will go into a little later. This is an old 100mm CRO tube made by Cossor. The black aquadag conductive coating, extending from about the middle to near the screen end, can be seen on the inside of the glass envelope. These days, all but the cheapest CRO tubes have a rectangular screen. Flyback & blanking We have drawn the first trace on the screen, from time t1, through t2 to t3. Usually, to obtain a bright picture, we repeatedly redraw this trace many times, superimposed. To do this, the electron beam must return from the t3 position (at far right of screen) to the starting point at far left of screen as quickly as possible, so that it is ready to draw the trace over again, restarting at time t1. We cannot change the voltage of the horizontal deflection signal in Fig.4 from maximum positive to maximum negative instan­taneously (ie, it cannot be done in zero time). Therefore, in Fig.4, t3 (RHS of screen), t4 (mid screen) and t5 (LHS of screen) are not simultaneous. But they can occur in a very short interval of time. This fast return of the horizontal signal is called the “retrace” or “flyback” because the electron beam has to fly back to its initial starting position. To prevent a confusing trace being drawn on the screen by the spot of light flying back at high speed, the electron beam is turned off during retrace. This is called fly­back blanking. Vertical & horizontal stages The vertical amplifier is also shown in schematic form on Fig.3. There are five amplifier stages shown although typical scopes may have more or less amplifier stages. A1 accepts whatev­er input signal you want to view on your oscilloscope, reduced if too large by attenuator VR1. A3 provides a phase change action so that A4 and A5 can deliver a push-pull or complementary drive to the vertical deflection plates Y1 and Y2. The basic essentials of a timebase generator and X or hori­zontal sweep amplifiers are also shown in Fig.3. A6 is an oscil­lator which produces the linear ramp voltage signal. It is also referred to as a sawtooth waveform generator. CX indicates that capacitors can be switched in the A6 circuit to produce different rates of rise of voltage; ie, different amounts of time to get from t1 to t3. This is called changing the sweep rate. A9 provides a phase change for the drive to A10. Thus, A8 and A10 put out a complementary signal sufficient for the horizontal deflection plates X1 and X2 to deflect the electron beam across the full width of the CRO screen. Beam current In Fig.3, the CRO tube heater heats the cathode which emits copious quantities of electrons. The conductive coating (aquadag) and grid G3 are connected to the positive end of a high voltage supply, shown in this example as 5kV. The relatively positive G3 grid and screen end attract the electrons emitted from the cathode, K. The voltage applied to grid G1 is even more negative than that on the cathode. This allows G1 to control the quantity of electrons in the electron stream (the beam cur­ rent), by the voltage difference between G1 and the cathode. Yes, that stream of electrons is an electric current. Its value may be 20 microamps for some simple CRO tubes, or 50 milliamps or more in some high brightness top performance tubes. However, electron beam current does not obey Ohm’s Law. Instead, it is proportional to the square root of the acceleration voltage which causes it to flow! Hence, the G1-K potential decides the brightness of the trace on the CRO screen. G2 is called the focus grid. The mass of electrons is focused, by the potential difference between G2 and G3, into a stream, to arrive at the screen at a fine point. G3 is a hollow metal cylinder called the accelerating grid. Being more positive than the cathode, G3 attracts electrons away from the cathode. The electron stream passes straight through G3 without touching it and continues on to the screen. In this example of a simple CRO tube, G3 and the screen are at the same poten­ tial. In more complex tubes this is not so, as we shall see in a future article. Grounding of 5kV supply To prevent deceleration of the electrons, everything to the right of G3 must be either at the same potential as G3, or more positive. Because all deflection plates are part of the vertical (Y) or horizontal (X) amplifier circuits, their voltage levels are at amplifier potentials: usually no more than a few hundred volts above or below ground. The above two statements together imply that G3 and the CRO tube screen must be no more than a couple of hundred volts above ground, about the same potential as Y1, Y2 and X1, continued on page 83 March 1996  17 For more information, contact Sealcorp, PO Box 670, Lane Cove, NSW 2066. Phone (02) 418 9099; fax (02) 418 9313. Function generators from Yokogawa Traditionally, high-performance function generators have been difficult to operate, involving the manipulation of many front panel keys. A new generation of 2-channel, compact function generators from Yokogawa, which feature a large LCD display and touch screen, has addressed this difficulty. The FG200/FG300 series function generators offer 2 channels in a compact, lightweight package and feature sweep and modulation capabilities. The new generators provide sine and square outputs up to +/-10V over a frequency range of 1uHz to 15MHz, and triangle, pulse and arbitrary (on the FG300) outputs from 1uHz to 200kHz. Frequency resolution is 1uHz or a maximum nine digits. Operation of the FG200/FG300 series has been simplified by virtue of the large LCD touch screen. The setup and display or arbitrary sweep patterns and simple arbitrary waveforms can be defined by entering points within the scaled ranges on the X and Y axes, and can be generated using linear, step or spline interpolations between the points. Alternatively, the data may be loaded in ASCII format via the internal floppy disc drive. This interface may also be used to load waveforms created with Yokogawa AG series waveform generators or captured with the company's digital oscilloscopes. Sweeps may be made in frequency, KITS-R-US Cathode Ray Oscilloscopes – from page 17 X2. But for good luminescent efficiency, thousands of volts acceleration voltage is necessary to produce bright sharp traces on the CRO screen. In this example, we have shown 5kV, which is relatively standard for a CRO. Therefore, in oscilloscopes using the simple CRO tubes shown, the high voltage supply is grounded (or nearly so) at the CRO screen end. Consequently, the heater, cathode, control grid G1 and focus grid G2 are all at high negative voltages with respect to ground. As a consequence, lethal voltages exist on the heater, cathode, grid and other wiring and terminals inside an oscilloscope. Next month we will dig further into how analog oscilloscopes are designed to reproduce high frequencies, up to 1000MHz (1GHz) and how the trace on the screen can be so bright, sharp, clear, calibrated and accurate. Acknowledgements Thanks to Philips Scientific & Industrial and to Tektronix Australia for data and illustrations; also to Ian Hartshorn, Jack Sandell, Professor David Curtis, Ian Marx and Dennis Cobley. References "ABCs of Oscilloscopes" – Philips/Fluke USA "Solid State Physical Electronics" – Van der Ziel A; Prentice Hall NJ, USA. "Basic Television" – McGraw-Hill NY, USA. Tektronix Aust. Application Notes. SC phase, amplitude, offset voltage or duty cycle, in linear, log linear step, log step or arbitrary sweep patterns. The sweep parameters may be controlled by an external analog or digital signal. Output amplitude and duty cycle are continuously variable and by linking multiple generators together, three or more channels of phase synchronised signals may be obtained, with even the sweep synchronised if required. For further information, contact Yokogawa Australia, 25-27 Paul St North, North Ryde, NSW 2113. Phone SC (02) 888 1844. PO Box 314 Blackwood SA 5051 Ph 018 806794 TRANSMITTER KITS $49: a simple to build 2.5 watt free running CD level input, FM band runs from 12-24VDC. •• FMTX1 FMTX2B $49: the best transmitter on the market, FM-Band XTAL locked on 100MHz. CD level input 3 stage design, very stable up to 30mW RF output. $49: a universal digital stereo encoder for use on either of our transmitters. XTAL locked. •• FMTX2A FMTX5 $99: both FMTX2A & FMTX2B on one PCB. 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Any of the 24 lines can be used as an input or output. Good value. 19" Rack Mount PC Case: $999. •• Professional All-In-One 486SLC-33 CPU Board $799: includes dual serial, games, printer floppy & IDE hard disk drive interface, up to 4Mb RAM 1/2 size card. PC104 486SLC CPU Board with 2Mb RAM included: 2 serial, printer, floppy & IDE hard disk $999; VGA •PC104 card $399. KIT WARRANTY – CHECK THIS OUT!!! If your kit does not work, provided good workmanship has been applied in assembly and all original parts have been correctly assembled, we will repair your kit FREE if returned within 14 days of purchase. Your only cost is postage both ways. Now, that’s a WARRANTY! KITS-R-US sell the entire range of designs by Graham Dicker. The designer has not extended his agreement with the previous distributor, PC Computers, in Adelaide. All products can be purchased with Visa/Bankcard by phone and shipped overnight via Australia EXPRESS POST for $6.80 per order. You can speak to the designer Mon-Fri direct from 6-7pm or place orders 24 hours a day on: PH 018 80 6794; FAX 08 270 3175. March 1996  83