Oscilloscopes 2012 ppt

With 20Ms/s 6bit signal capture, displays can alternatively be downloaded to a PC via a D9 serial port reproduced by courtesy of Pico Technology Ltd Figure 1.5 The 200MHz PM3394B is th

Oscilloscopes

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Oscilloscopes

H o w to use them, h o w they w o r k

Fifth Edition

I a n H i c k m a n BSc (Hons), CEng, MIEE, MIEEE

E L S E V I E R

B ~ W O R T I - I

A M S T E R D A M 9 B O S T O N ~ H E I D E L B E R G ~ L O N D O N ~ N E W Y O R K 9 O X F O R D

P A R I S ~ S A N D I E G O ~ S A N F R A N C I S C O ~ S I N G A P O R E ~ S Y D N E Y ~ T O K Y O

Newnes

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9 H o w oscilloscopes w o r k (1): the c.r.t 176

10 H o w oscilloscopes w o r k (2): circuitry 188

11 H o w oscilloscopes w o r k (3): storage c.r.t.s

A p p e n d i x 1 Cathode ray tube p h o s p h o r data

A p p e n d i x 2 Oscilloscope manufacturers a n d agents

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b o o k explains briefly h o w these i n s t r u m e n t s work, on the basis that the best drivers have at least some idea of w h a t goes on

u n d e r the b o n n e t This takes us into electron physics and circuit

t h e o r y - b u t n o t too far F o r m u l a e a n d results are simply stated, not derived or proved, and those w i t h only the haziest k n o w l e d g e

of m a t h e m a t i c s will find n o t h i n g to alarm t h e m in this book Consequently, readers in their earliest teens will be able to learn

a lot from it; Chapter 1 is w r i t t e n especially for a n y o n e w i t h no prior k n o w l e d g e of the subject Sixth-formers and students on ONC and HNC courses should all find the b o o k useful Even

m a n y degree students will find it of considerable help ( t h o u g h

t h e y m a y choose to skip Chapter 1!); electronic engineering

u n d e r g r a d u a t e s have plenty of o p p o r t u n i t y to learn a b o u t oscilloscopes, but m a n y graduates come into electronic engineer- ing from a physics degree course, a n d will w e l c o m e a practical

i n t r o d u c t i o n to oscilloscope techniques

Technicians and technician engineers in the electronics field will of course be used to oscilloscopes, b u t the following chapters should enlarge their u n d e r s t a n d i n g a n d enable t h e m to use the facilities of an oscilloscope to the full Finally, I h o p e that those

w h o s e interest in electronics is as a hobby, including m a n y

a m a t e u r radio h a m s a n d r a d i o - c o n t r o l l e d - m o d e l enthusiasts, will find the b o o k valuable, especially if t h e y are considering b u y i n g

or even constructing their o w n oscilloscopes

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Preface to fifth e d i t i o n

K e e p i n g this b o o k up to date is r a t h e r like p a i n t i n g E d i n b u r g h ' s

f a m o u s bridge over the Firth of F o r t h - n o s o o n e r do t h e y get to t h e

e n d t h a n it's time to start all over again at the beginning In t h e

s a m e way, n o s o o n e r does a n e w edition of this b o o k c o m e o u t t h a n

o n e or o t h e r of the oscilloscopes illustrated or f e a t u r e d will go out

of p r o d u c t i o n , usually to be r e p l a c e d by a later, i m p r o v e d m o d e l

A n d as for A p p e n d i x 2, o n e can m o r e or less g u a r a n t e e t h a t by t h e

t i m e a n e w edition is in t h e offing, at least 50 per cent of t h e

m a n u f a c t u r e r s or their agents will h a v e c h a n g e d their address or

M y first scope, h o m e built for c h e a p n e s s of course, w a s a h o m e -

b r e w c o n v e r s i o n of a n ex-RAF I n d i c a t o r Unit Type 182A, w h i c h

i n c o r p o r a t e d a VCR517C c a t h o d e ray tube The u n i t w a s available

o n t h e p o s t - w a r military e q u i p m e n t surplus m a r k e t for a f e w

p o u n d s , a lot of m o n e y in those days - especially for a lad still at school E v e n so, it w a s considerably c h e a p e r t h a n units c o n t a i n i n g

t h e m o r e p o p u l a r VCR97 c a t h o d e ray tube, w i t h its short- persistence g r e e n p h o s p h o r So, for r e a s o n s of financial stringency,

m y first oscilloscope h a d a long persistence c a t h o d e ray t u b e w i t h a blue 'flash' a n d y e l l o w 'afterglow' In its original role as a r a d a r display, a glass filter t i n t e d deep y e l l o w in front of t h e screen

s u p p r e s s e d the flash, b u t I r e m o v e d this, m a k i n g t h e t u b e r a t h e r less i n a p p r o p r i a t e for oscilloscope duty Nevertheless, t h e a f t e r g l o w

w a s always a n u i s a n c e e x c e p t for single shot applications or d u r i n g

e x t e n d e d o b s e r v a t i o n of a stable triggered w a v e f o r m - u n f o r t u -

n a t e l y I n e v e r t h o u g h t of p u t t i n g a d e e p blue filter in front of t h e screen (A s u b s e q u e n t c o n v e r s i o n to TV use w a s e v e n less

x Preface to fifth edition

curved yellow tail looked like a comet, and when thc camera panned from onc end of thc ground to the ot.her, confusion reigned supreme.)

A scopc with a long-persist.erice screen is still very ~ i ~ c f ~ i l in

place of a standard one are by now unobtainable, but many long- persistence scopes are still in regular use Thus in the world of the oscilloscope, the old and the new both continue to be useful, each

in its appropriate sphere

Another example of this is the ‘second user market’, an area of steadily growing importance As Government Departments and Agencies and large firms re-equip themselves with the latest and best in oscilloscopes, large quantities of used but perfectly

offer it f o r resale The riiore reputable dealers will have had the

c q u i p ~ i c n l ovcrha u l c ~ l and rccalibrated to good-as-new condition,

I r i !his way, an cxccllcnt oscilloscope, sprct i - i i n i analyser or other instrumcnt (adtnillcdly of a rnodcl often no 1ongc.t i t 1 production)

repaired a n d maintained as necessary, giving many years of faithful

j list before the period of support expires

This f i l t h edition of the hook, which was lirst published in 1981 and has never been o u t of print since, ha\ been extensively revised

storage scopes (i.e those using direct-view storage c.r.t.s) were

available from a n u m b e r of m a n u f a c t u r e r s This is no longer the case, so perhaps the logical m o v e m i g h t seem to be the omission of the chapter in its entirety But it has b e e n retained, for a n u m b e r of reasons Firstly, the description of the operation of storage c.r.t.s illustrates some interesting aspects of electron optics, a b r a n c h of physics on w h i c h all c.r.t.s d e p e n d for their operation Secondly,

w i t h the m a r c h of time, sources of i n f o r m a t i o n on the m o d u s operandi of storage cathode ray tubes will b e c o m e rarer a n d rarer Thirdly and m o r e importantly, m a n y analogue storage scopes are still in use, a n d some guidance on their advantages, limitations a n d quirks m a y not come amiss A n d while oscilloscopes using a storage cathode ray tube no longer s e e m to be available (except on the second user market), one of the m a j o r oscilloscope m a n u f a c t u r e r s still produces analogue storage oscilloscopes, using a 'scan converter tube' The principle of operation of these is also t o u c h e d

on in Chapter 11 The chapter has therefore b e e n retained, but

w i t h the substantial p r u n i n g carried out at the previous (fourth) edition, while still covering all the f u n d a m e n t a l s of the subject The book n o w includes p h o t o g r a p h s of later models of some of the i n s t r u m e n t s w h i c h w e r e illustrated in the fourth edition, plus details and p h o t o g r a p h s of i n s t r u m e n t s from various m a n u - facturers w h o s e p r o d u c t lines w e r e not previously r e p r e s e n t e d in these pages, whilst illustrations of models no longer available have,

w i t h but one or t w o exceptions, b e e n r e m o v e d

The a u t h o r gratefully acknowledges the m a n y m a n u f a c t u r e r s and their agents w h o have assisted by providing i n f o r m a t i o n on,

a n d pictures of, their products F r o m these, a selection of photographs has b e e n included illustrating real-time oscilloscopes,

b o t h storage a n d non-storage, sampling a n d digital storage oscilloscopes a n d their accessories In each case, the caption at least gives brief details of the p e r f o r m a n c e of the i n s t r u m e n t , whilst in several cases it has b e e n possible to give a m o r e extensive a c c o u n t

of its p e r f o r m a n c e in the text M y special t h a n k s are due to Tektronix UK Ltd for providing material u p o n w h i c h I h a v e d r a w n freely in Chapters 6 a n d 11 a n d elsewhere, a n d for o t h e r v a l u e d assistance

I.H

October 2000

~

An advanced orcillowrpc of the 1940s Thc Cossor niodcl 1 0 3 5 M k l 1A was a true dual beam oscilloscope with a ~ ~ i a x i r i i ~ ~ i i i bandwidth of 7 MHr ( Y I amplifier),

100 kHz j Y 2 anqilifirrj and a fasrcst sweep rate of 1 5 p per scan, with repetitive,

triggered and single-stroke operation (courtesy Coscor Electronics Lid)

1 Introduction

The cathode ray oscilloscope is an i n s t r u m e n t designed to display the voltage variations, periodic or otherwise, that are m e t w i t h in electronic circuits a n d elsewhere

The w o r d is an etymological hybrid The first part derives from the Latin, to swing backwards and forwards; this in t u r n

is from oscillum, a little m a s k of Bacchus h u n g from the trees, especially in vineyards, a n d thus easily m o v e d by the wind The second part comes from the Classical Greek skopein, to observe, aim at, examine, from w h i c h developed the Latin ending-

scopium, w h i c h has b e e n used to form n a m e s for i n s t r u m e n t s that enable the eye or ear to m a k e observations For some reason the subject of the design and use of oscilloscopes is generally not called oscilloscopy but oscillography, from oscillo-

a n d graphein, to write

There are other types of oscilloscope besides those using cathode ray tubes For example, p e n recorders, ultra-violet chart recorders a n d XY plotters are all oscilloscopes or oscillographs of

a sort, as indeed is 'Fletcher's Trolley' of school physics fame However, this b o o k is c o n c e r n e d mainly with cathode ray oscilloscopes, together w i t h the increasing n u m b e r of similar

i n s t r u m e n t s using LCD (liquid crystal display) technology

R e p r e s e n t i n g a v a r y i n g voltage

The basic principle of oscillography is the representation, by graphical means, of a voltage that is varying The voltage is plotted or traced out in t w o - d i m e n s i o n a l Cartesian coordinates,

n a m e d after Descartes, the famous French s e v e n t e e n t h - c e n t u r y philosopher a n d m a t h e m a t i c i a n

Figure 1.1 shows the general scheme for the r e p r e s e n t a t i o n of

a n y two related variables Both positive a n d negative values of each variable can be represented The vertical axis is called the Y axis, and the horizontal the X axis The point w h e r e the axes cross, w h e r e b o t h X = 0 a n d Y = 0, is called the 'origin'

may he two different scales, even diilcrrnt units, Cartesian or graphical cnordinarcs The horizontal and vertical axes for graphical purposes

Any point is defined by its X a n d Y coordinates Thus the point

P in t h e top right-hand quadrant is t h e point ( 3 , 2 ) , because its distance to the right (called its 'abscissa' o r X coordinate) is 3 units a n d its distance up (called its 'ordinate' or Y coordinate) is

2 units

Figtirt 1.2 is a n r x a n ~ p l t ~ of a graph ploltcd o n Carlesian

Figure 1.2 Fictional plot of teniperatiire in tirst wcck ot January A n exarnple of

a graph where the horironral and vertiral axcs arc to different scales and in different units

during the first w e e k of January Quantities t h a t v a r y w i t h time, like t e m p e r a t u r e a n d voltage, are very i m p o r t a n t in e n g i n e e r i n g

a n d are f r e q u e n t l y r e p r e s e n t e d in graphical form As we d o n ' t usually attribute m u c h m e a n i n g to the concept of negative time, the Y axis (the vertical line corresponding to the point w h e r e X =

0, or the start of 1 J a n u a r y in this case) has b e e n s h o w n at the

e x t r e m e left The X axis n o w represents time, s h o w n in this case

in days, t h o u g h for o t h e r purposes it m i g h t be m i n u t e s , seconds

or microseconds (usually w r i t t e n ~s a n d m e a n i n g millionths of a second) Negative t e m p e r a t u r e s are plotted b e l o w the axis a n d positive ones above it Time is t a k e n as increasing (getting later) from left to right, starting at zero at the origin Thus the X axis is

a 'timebase', above a n d b e l o w w h i c h the related variable (in this case, t e m p e r a t u r e ) is plotted

Voltages can be positive or negative, just like t e m p e r a t u r e s The usual reference p o i n t for voltages is t a k e n as e a r t h or ground This is called zero volts, 0 V, just as 0~ the m e l t i n g point of ice,

is t a k e n as reference for t e m p e r a t u r e s

What the oscilloscope shows

W h e r e y o u or I m i g h t d r a w a graph like Figure 1.2 w i t h a pencil,

a n oscilloscope draws its 'trace' w i t h a m o v i n g spot of light on the screen of a cathode ray tube The screen is a p p r o x i m a t e l y flat a n d coated on the inside w i t h a p o w d e r t h a t emits light w h e r e it is struck by a b e a m of electrons M o r e a b o u t the o p e r a t i o n of the cathode ray tube can be f o u n d in C h a p t e r 9; here it is sufficient to note that i n t e r n a l circuitry in the oscilloscope causes the spot of light to travel from left to right across the 'screen' of the tube at

a steady rate, until on reaching the r i g h t - h a n d side it r e t u r n s rapidly to the left r e a d y to start a n o t h e r traverse, usually called a 'trace', ' s w e e p ' or 'scan' As n o t e d above, some oscilloscopes use

an LCD display This is a t r e n d w h i c h will continue; in future

m o r e a n d m o r e models, especially portable a n d h a n d h e l d oscilloscopes a n d digital storage oscilloscopes, will opt for this display technology

Figure 1.3 shows the picture t h a t m i g h t a p p e a r on the screen of

an oscilloscope if it w e r e used to display the w a v e f o r m of the 240 V a.c (alternating current) domestic m a i n s electricity supply This

or ' d e f l e c t i o n factor' is 100 V p e r division On oscilloscopes w i t h a

Figure 1.4 The OsziFOX handheld oscilloscope operates from a 9 V d.c supply This plugs into the rear end, and may be the matching mains power supply unit,

or a PP3/6F22 miniature 'transistor' battery With 20Ms/s 6bit signal capture, displays can alternatively be downloaded to a PC via a D9 serial port (reproduced

by courtesy of Pico Technology Ltd)

Figure 1.5 The 200MHz PM3394B is the top model in the PM33xxB range of Fluke 'Combiscopes'| These provide both real-time and digital storage modes The least expensive PM3370B, pictured above, features 60MHz bandwidth in either mode, a 5.8 ~s risetime and a 200Ms/s single shot sample rate, 10Gs/s

6 Oscilloscopes

(Hz is s h o r t for h e r t z a n d m e a n s 'cycles p e r s e c o n d ' ) ; t h u s it takes

2 0 m s to c o m p l e t e e a c h cycle As t h e full t e n s q u a r e s of t h e

g r a t i c u l e r e p r e s e n t 50 m s in t h e h o r i z o n t a l d i r e c t i o n , t w o a n d a half c o m p l e t e cycles a r e t r a c e d o u t as t h e spot scans across t h e

s c r e e n D u r i n g t h e n e x t half cycle t h e spot r e t u r n s r a p i d l y to t h e left of t h e screen This r e t u r n j o u r n e y is called t h e 'flyback' or ' r e t r a c e ' , b u t n o trace of it is s e e n , as t h e spot is s u p p r e s s e d b y a 'flyback b l a n k i n g ' circuit

T h e n e x t trace t h u s starts t h r e e cycles after t h e start of t h e

p r e v i o u s o n e , so 16~ i d e n t i c a l traces are d r a w n e v e r y s e c o n d This is n o t fast e n o u g h for t h e e y e to see a single s t e a d y picture,

so t h e r e is p r o n o u n c e d flicker ( u n l e s s t h e c a t h o d e r a y t u b e uses

a l o n g - p e r s i s t e n c e p h o s p h o r , see A p p e n d i x 1) If t h e scan or

Figure 1.6 Ttle DL708E, with built-in hardcopy printer, provides up to eight isolated input channels with a maximum input of 850 V d.c + a.c peak Input modules are plug-in, with a choice of 10Ms/s 10 bit resolution, 100ks/s 16 bit resolution, and various other options (reproduced by courtesy Yokogawa Martron Ltd)

Introduction 7

Figure 1.7 The 8835-01 'MEMORY HiCORDER' from HIOKI provides four or eight input channels and displays these on a 6.4 inch colour TFT display screen and records them onto 110 mm thermal paper roll and into memory Versatile trigger functions include pre-trigger storage (reproduced by courtesy of ASM Automation Sensors Limited)

s w e e p r a t e w e r e c h a n g e d f r o m 5 m s / d i v to 2 0 m s / d i v , t e n

c o m p l e t e cycles w o u l d a p p e a r p e r s c a n a n d t h e m o v i n g spot of light w o u l d be s e e n b o b b i n g u p a n d d o w n as it c r o s s e d t h e s c r e e n

O n t h e o t h e r h a n d , if a 500 Hz w a v e f o r m w e r e v i e w e d at 0.5 m s / div ( t h e s a m e as 500 ~ s / d i v ) , t h e r e w o u l d b e 166 i d e n t i c a l t r a c e s

H a v i n g l e a r n t a little of w h a t a n oscilloscope is a n d w h a t it c a n

do, in C h a p t e r 2 w e l o o k in m o r e d e t a i l at t h e facilities p r o v i d e d

b y a basic oscilloscope

2 The basic oscilloscope

C h a p t e r 1 briefly described h o w a n oscilloscope d r a w s its trace

w i t h a spot of light ( p r o d u c e d by a deflectable b e a m of electrons)

m o v i n g across t h e screen of its c.r.t (cathode ray tube) At its

m o s t basic, t h e r e f o r e , a c a t h o d e ray oscilloscope (further details

of c a t h o d e ray t u b e s can be f o u n d in C h a p t e r 9), consists of a ' t i m e b a s e ' circuit to m o v e t h e spot steadily f r o m left to right across the screen at t h e a p p r o p r i a t e time a n d speed, a n d s o m e

m e a n s (usually a 'Y' deflection amplifier) of e n a b l i n g the signal

w e wish to e x a m i n e to deflect the spot in the vertical or Y direction A l t e r n a t i v e l y s o m e o t h e r display t e c h n o l o g y such as LCD m a y be used, t h o u g h in this case the i n s t r u m e n t is usually a digital storage type of oscilloscope

In addition, of course, t h e r e are a few f u r t h e r h u m b l e essentials like p o w e r supplies to r u n the c.r.t, or LCD display and circuitry,

a case to k e e p it all together, a n d a Y input socket plus a f e w controls on the front panel Figure 2.1 is a block d i a g r a m of s u c h

an i n s t r u m e n t

This type of oscillosc()pe, m o r e or less sophisticated as the case

m a y be, belongs to w h a t was traditionally by far the c o m m o n e s t

a n d most i m p o r t a n t category: the ' r e a l - t i m e ' oscilloscope This

m e a n s simply t h a t the vertical deflection of the spot on the screen

at a n y i n s t a n t is d e t e r m i n e d by the Y i n p u t voltage at that instant Not all oscilloscopes are r e a l - t i m e i n s t r u m e n t s : Figure 2.2

a t t e m p t s to categorise the various types available The distinction

b e t w e e n r e a l - t i m e i n s t r u m e n t s a n d others is not absolute a n d clear cut, but the fine distinctions need not w o r r y us here

A really basic oscilloscope t h e n is one w i t h the necessary facilities for e x a m i n i n g a repetitive w a v e f o r m An i n s t r u m e n t

w i t h but a single Y input, c o r r e s p o n d i n g to Figure 2.1 a n d t h e

e x t r e m e l e f t - h a n d b r a n c h of Figure 2.2, m e e t s this description With such an i n s t r u m e n t , the relative timing b e t w e e n t h e

w a v e f o r m s at different points in a circuit can be established, albeit indirectly, by using t h e e x t e r n a l trigger i n p u t a n d viewing t h e

w a v e f o r m s one after the other The a d v a n t a g e of being able to see

c rt X c rt Y

trigger I ,nt l,,~slicer 'og,\ ~,,~,~- ~,og~ I I'~HI

,~e, / I I -'1 T I t S t Xir I to c.,-t gr,d 1

F i g u r e 2.1 Block diagram of basic oscilloscope Note: It is n o w c o m m o n to fit a

t w o pole m a i n ON/OFF switch, b o t h for safety reasons a n d to c o m p l y w i t h

n a t i o n a l electrical e q u i p m e n t regulations

relative timing directly by viewing t w o w a v e f o r m s simultane- ously is so great that, increasingly, even inexpensive basic oscilloscopes offer this facility Most of the i n s t r u m e n t s illustrated

t h r o u g h o u t this b o o k have t w o such i n d e p e n d e n t channels, a n d some have three or even four channels

However, even a basic single c h a n n e l oscilloscope is an inestimable help in viewing the action of electronic circuits, a n d the n e x t section describes such an i n s t r u m e n t , the Metrix OX71

A l t h o u g h to some readers the facilities it provides m a y seem entirely self-explanatory, t h e y are in fact w o r t h a closer look, a n d

a few c o m m e n t s on the characteristics a n d o p e r a t i o n of scopes in general have b e e n t h r o w n in for good measure

I

real- time scopes

basic

scopes

sIng le- trace dual-trace,

scopes dual beam

scopes

1

advanced scopes multiple timeboses, timebose delay facilities

signal delay etc

Figure 2.2 Types of cathode ray oscilloscope

c a t h o d e - r o y osc, Iloscopes

! non-real-time scopes i

persistence screens

'traditional' storage scopes with storage c.r.t

i

digital storage scopes

c.rt.-based instruments not used solely for

di splaying woveforms e.g logic analysers

Basic oscilloscope controls

The M e t r i x OX71, illustrated in Figure 2.3, is also k n o w n as t h e 'Didascope', f r o m its i n t e n d e d didactic or e d u c a t i o n a l role Unlike

s o m e low priced i n s t r u m e n t s , w h e r e t h e O N / O F F s w i t c h is

c o m b i n e d w i t h t h e brilliance or i n t e n s i t y control, t h e OX71 is

p r o v i d e d w i t h a s e p a r a t e p u s h b u t t o n m a i n s switch, IN for ON, OUT for OFF T h e r e is also a n LED m a i n s i n d i c a t o r light, w h i c h

i n t e r e s t i n g l y is red This is or w a s the t r a d i t i o n a l colour for a

m a i n s indicator light in t h e UK, b u t c o n t i n e n t a l practice is to use

g r e e n for m a i n s indicators, r e s e r v i n g red for a n a l a r m or

m a l f u n c t i o n indication

Of course, a light is n o t u s u a l l y n e e d e d as a w a r n i n g t h a t o n e has left the oscilloscope s w i t c h e d on; after all, t h e trace o n t h e

s c r e e n does t h a t quite effectively The indicator's m a i n f u n c t i o n is

to assure t h e u s e r that, o n plugging in a n d s w i t c h i n g on, t h e

m a i n s socket is live a n d h e n c e t h e oscilloscope will be o p e r a t i o n a l

as soon as t h e c.r.t, has w a r m e d up

A n oscilloscope's i n t e n s i t y control, in this case fitted j u s t to t h e right of the c.r.t, s c r e e n at t h e top of t h e panel, s h o u l d n o r m a l l y

F i g u r e 2.3 The Metrix OX71 Educational Oscilloscope - see text (reproduced by

12 Oscilloscopes

be used at the lowest setting that gives an adequately bright trace

In particular, if the external X input is selected and no X and Y signals are applied, the spot will r e m a i n stationary; if the intensity control w e r e t h e n left at too high a setting for a long period,

p e r m a n e n t damage to the screen could occur in the form of a ' b u r n m a r k ' (an area of reduced screen sensitivity) On the other hand, if e x a m i n i n g in detail say a 10 ~s long pulse occurring once every 500 ~s, it w o u l d be necessary to advance the intensity control This is because, w i t h a suitable timebase setting such as

2 us/division, the spot w o u l d spend only o n e - t w e n t y - f i f t h of the time writing the trace, and the rest of the time waiting to trigger from the next pulse But it will be f o u n d that, on advancing the intensity control, the trace becomes not only brighter, but thicker This coarsening of the trace can be largely corrected by

a d j u s t m e n t of the focus control, the o p t i m u m setting of which depends therefore to some extent on the setting of the intensity control There is a limit to just h o w m u c h the intensity can be increased to compensate for low repetition rate of the trace For example, in the case m e n t i o n e d above, if the 10 Us pulse occurred once every 20 ms it w o u l d not be possible to e x a m i n e it on a basic oscilloscope One would require an i n s t r u m e n t with a m u c h higher 'writing speed', a concept m o r e fully explained in later chapters

Below the intensity control to the right of the screen is the focus control, just above the ON/OFF Indicator and Switch This control should be adjusted to give the smallest spot size, resulting

in the sharpest possible trace It m a y need r e a d j u s t m e n t w h e n viewing low duty cycle waveforms, as explained above The graticule has the usual ten divisions in the horizontal direction by eight in the vertical, each division being one centimetre

To the right of the intensity control knob is a hole providing access to a preset control This is the trace rotation control, w h i c h enables the trace (which should of course be a horizontal straight line in the absence of a Y input) to be set exactly horizontal At the top of the front panel, to the right of the trace rotation control access hole, is the vertical shift control, labelled POSITION with a vertical double e n d e d arrow To the right of that again is the horizontal shift control, labelled POSITION with a horizontal

double e n d e d arrow The shift controls enable the trace to be centred horizontally a n d adjusted vertically so that, for example, zero i n p u t voltage corresponds to the centre horizontal graticule line This can c o n v e n i e n t l y be done w i t h the i n p u t coupling switch

in the GND (ground) position, as the Y c h a n n e l i n p u t is t h e n disconnected from the i n p u t socket, a n d g r o u n d e d This avoids the

n e e d to disconnect the signal being e x a m i n e d from the input For e x a m i n i n g voltage variations as a function of t i m e - the

m a i n purpose of a n y oscilloscope - the user m u s t select a suitable timebase speed w i t h the ' t i m e / d i v ' switch On the OX71, a 20 w a y rotary switch provides 19 timebase speeds of 0.5 ~s/div to 0.5 s/div,

in a 1, 2, 5 sequence The t w e n t i e t h position selects the XY mode,

in w h i c h the X deflection is no longer provided by the timebase, but by a signal applied to the red X i n p u t t e r m i n a l on the front panel The use of an oscilloscope's XY m o d e is covered in a later chapter B e t w e e n the y e l l o w Y i n p u t t e r m i n a l a n d the red X i n p u t

t e r m i n a l is situated the black g r o u n d or reference terminal, used for the low or r e t u r n connections of i n p u t signals

For m o s t signal viewing tasks, a timebase is required, a n d one

w o u l d n o r m a l l y select a speed w h i c h results in b e t w e e n t w o a n d three complete cycles of the w a v e f o r m being displayed Too slow

a timebase speed results in so m a n y cycles being displayed t h a t the detailed shape of each c a n n o t be distinguished: too fast a speed results in the display of only a part of one cycle Likewise,

a suitable setting of the volts/div sensitivity switch, w i t h a.c or d.c coupling, should be chosen as required, so t h a t the w a v e f o r m occupies b e t w e e n half a n d full screen height The Y sensitivity switch is located on the front p a n e l b e l o w the vertical position control a n d above the y e l l o w Y i n p u t terminal To the left of the yellow Y i n p u t t e r m i n a l is the i n p u t coupling switch The i n p u t coupling switch provides a choice of a.c or d.c coupling a n d also,

as m e n t i o n e d earlier, a GND (ground) position The seven position volts/div switch provides a sensitivity of 5 0 m V / d i v to

5 V/div in a 1, 2, 5 sequence

The last control function to be m e n t i o n e d is in m a n y w a y s the

m o s t i m p o r t a n t : triggering This topic looms large in later chapters, b u t on the OX71 it is very simply h a n d l e d by a single knob The trigger level, i.e the vertical level up the positive-going

14 Oscilloscopes

Figure 2.4 The V- 1565 is a two channel 100 MHz analogue real-time oscilloscope with delayed sweep, cursor measurements and frequency counter Maximum sensitivity is 2 m V/division and fastest sweep speed, with x 10 magnifier on, is 5 ns/ division (reproduced by courtesy of Hitach Denshi (UK) Ltd)

Figure 2.6 The dual m o d e two channel HM407 provides a 40 MHz b a n d w i d t h in analogue m o d e and 100Ms/s in digital The latter m o d e offers Refresh, Roll, Single, XY, Average and Envelope modes (reproduced by courtesy of Hameg Ltd)

anticlockwise click-stop position In this case, the trigger level is fixed at m i d screen height In the AUTO position, the timebase runs, giving a horizontal straight line, e v e n if the signal is too small (less t h a n half a vertical division) to operate the trigger circuit, or there is no input signal at all W h e n not in the AUTO

Figure 2.7 Not an oscilloscope, but an advanced Scope Calibration Workstation, the model 9500 can calibrate analogue and digital oscilloscopes with bandwidths

up to 1.1 GHz Active Head Technology T M delivers calibration waveforms directly

to the oscilloscope's input connectors, w i t h o u t the n e e d for connecting leads, for the ultimate in accuracy (reproduced by courtesy of Wavetek Ltd)

16 Oscilloscopes

Figure 2.8 The VDS2152 Virtual Digital Scope from CELL, with its 20Ms/s sample rate provides a 150 MHz bandwidth (5 MHz single shot) With a maximum sensitivity of 2 mV/division and trigger facilities including TV, the instrument interfaces with a PC via a serial port, leaving the parallel LPT unencumbered

m o d e , t r i g g e r i n g will n o t t a k e place if t h e t r i g g e r level is set too

h i g h or t o o low, so t h a t t h e t r a c e does n o t cross t h e trigger level

In this case, t h e s c r e e n will s i m p l y a p p e a r b l a n k

By c o n t r a s t , in AUTO t r i g g e r m o d e , t h e display will a u t o m a t i - cally r e t r i g g e r on c o m p l e t i o n of a s l o w scan or at a c e r t a i n

m i n i m u m r e p e t i t i o n r a t e at faster scan speeds, so as a l w a y s to display a t r a c e e v e n in t h e a b s e n c e of a Y i n p u t This u n i v e r s a l l y useful f e a t u r e , o f t e n called ' a u t o b r i g h t l i n e ' , is i n c o r p o r a t e d in

The f r o n t p a n e l also carries t h e b l u e Z m o d u l a t i o n i n p u t

t e r m i n a l : a n e x t e r n a l signal applied to this s o c k e t m o d u l a t e s t h e

internal graticule, ensuring freedom from parallax w h e n viewing the trace from any angle

The OX71 is only one of the Metrix range of oscilloscopes but

it is fairly typical of a wide range of basic oscilloscopes available from a n u m b e r of manufacturers Some m a y have one or two facilities not found on the OX71 and vice versa, and like the OX71 most are (within the limits of this basic class of instrument) very good value for money

Figure 2.9 The Unigraf UDS-2020 is another PC add-on based instrument; this one offering two input channels each with a 20GHz bandwidth and 17.Sps risetime Fastest timebase speed is 10ps/division, with record lengths up to 4K, and resolution up to 14 bit (with averaging) Display types include variable- and infinite-persistence Also incorporated is a fast step generator, permitting TDR measurements with a resolution of about 8 m m (reproduced by courtesy of

Unigraf Oy)

3 Advanced real-time

oscilloscopes

Entirely at the other end of the price range from the basic type of

This typically has a host of features not found on a basic scope and may be a mainframe plus plug-in system or a stand-alone scope The latter is often dcscrihcd as a ‘portablc’, to distingiiish it from

as parasitic oscillations, that would otherwise pass unnoticed

is taken as an example, a n d its facilities discussed in detaiI The

ins Its facilirics are comprchensive, and the tollowing description

made to mainfmrrit plus pl~ig-irt systerns These art potentially

o u t rriory c.xpt.tisivc for 11ir sariic‘ iwrformancc

Advanced real-time oscilloscopes 19

Figure 3.1 The microprocessor-controlled PM3094 advanced analogue real- time oscilloscope has a bandwidth of 200MHz and a wide range of features covered in this chapter (reproduced by courtesy of Fluke Europe BV)

featured i n p u t channels, while the o t h e r t w o m o r e e c o n o m i c a l l y priced i n s t r u m e n t s provide t w o such c h a n n e l s plus t w o supple-

m e n t a r y channels The latter, w i t h just t w o i n p u t sensitivity settings of 0.1 V/division a n d 0.5V/division (I V/division a n d

5 V/division w h e n used w i t h a x l 0 probe), are ideal for use in logic circuit testing All four i n s t r u m e n t s h a v e the controls a r r a n g e d in 'functional groups', in designated s u b - p a n e l areas of the front panel Thus all the C h a n n e l 1 controls are g r o u p e d together, as are those for the o t h e r channels, for the M a i n Timebase a n d etc The rest of this chapter describes the PM3094, b u t m o s t of the following applies to all four i n s t r u m e n t s in the range

The P M 3 0 9 4 will first be described in its basic form, t h a t is

w i t h o u t a n y of the various options available Like m a n y m o d e r n

i n s t r u m e n t s , the P M 3 0 9 4 incorporates a tapless switching m o d e

p o w e r supply, obviating the n e e d for a m a i n s voltage a d j u s t m e n t switch It operates from a n y a.c supply of 100 V to 240 V rms, 50

to 400 Hz

As one w o u l d expect, the P M 3 0 9 4 has all the facilities f o u n d in the Metrix OX71 described in C h a p t e r 2, t h o u g h s o m e t i m e s differently labelled The facilities offered by the P M 3 0 9 4 are so extensive t h a t it is not possible in the confines of this c h a p t e r to describe t h e m all in full detail: t h e y greatly surpass the capabil- ities of the Tektronix 475A described in the first edition of this book in 1981, a l t h o u g h t h a t early model's b a n d w i d t h was 25 per

20 Oscilloscopes

cent greater t h a n the 2 0 0 M H z b a n d w i d t h of the PM3094 However, the PM3094 does have the great virtue that the trigger sensitivity is specified right up to 300 MHz

Power and display controls

On the e x t r e m e left-hand side of the i n s t r u m e n t , beside the screen of the c.r.t, with its 8 • 1 0 c m graticule, is a group of controls mainly concerned with the c.r.t, display The topmost of these is the Trace Intensity knob, which controls the brightness of the trace(s), but not of the r e a d o u t display of scale calibration factors This is controlled by the next knob down Below this again is the Trace Rotation control This screwdriver-adjusted preset control can be used by the o p e r a t o r to align the c.r.t, trace with the horizontal graticule lines Once adjusted, it does not require r e a d j u s t m e n t during normal operation of the instrument Below the Trace Rotation control is the Focus knob, w h i c h adjusts the focus of both traces and readout text Astigmatism is pre-adjusted and set during m a n u f a c t u r e ; consequently a user operated Astigmatism control is not provided The lowest knob in this group is the Graticule Illumination control, whilst below that

is the ON/OFF latching push button Pressing this b u t t o n turns on the power, and the oscilloscope automatically enters a self-test routine covering the i n s t r u m e n t ' s internal control bus, front panel to microcontroller c o m m u n i c a t i o n s and the i n s t r u m e n t settings stored in m e m o r y (if back-up batteries have been installed) This self-lest routine lakes less than a second, and any fault found would flag a corresponding err{~r message on the screen Thereafter, w h e r e back-up batteries are installed, the settings stored in m e m o r y become active The stored settings are those which applied w h e n the i n s t r u m e n t was last switched off, while in the absence of back-up batteries, a set of standard default settings apply

Like the scope described in the last chapter, the PM3094 has an internal graticule for freedom from parallax errors The graticule includes dotted lines at 2~ divisions above and below the centreline, to facilitate rise and fall time m e a s u r e m e n t s as illustrated in Figure 10.4(c) In addition to the internal graticule,

a blue tinted filter is fitted in front of the c.r.t

Advanced real-time oscilloscopes 21

Vertical controls

The Y amplifier controls are located to the right of the c.r.t screen, occupying the w h o l e of the l o w e r half of the front panel

At the b o t t o m of the front panel, b e l o w the C h a n n e l 1 controls,

is the C h a n n e l 1 i n p u t connector This specially modified BNC connector has a contact w h i c h senses w h e n the lead c o n n e c t e d to

it is one of the x l 0 divider probes supplied w i t h the i n s t r u m e n t , automatically adjusting the deflection factor displayed on the c.r.t, screen r e a d o u t to indicate the true deflection factor at the probe tip Above the C h a n n e l 1 i n p u t are located a n u m b e r of

p u s h buttons, a n d the rotary Y1 shift control k n o b setting the vertical position of the C h a n n e l 1 trace Two buttons, the u p p e r

w i t h an Up a r r o w and the l o w e r w i t h a D o w n arrow, i n c r e m e n t

or d e c r e m e n t the C h a n n e l 1 Y sensitivity in the usual 1:2:5 sequence, from 2 m V / d i v i s i o n to 5 V/div Pressing b o t h at once enables the VAR (variable gain) function The t w o b u t t o n s n o w provide m u c h finer sensitivity steps t h a n the 1:2:5 sequence Pressing b o t h again turns the VAR function off, a n d the gain reverts to the nearest setting in the 1:2:5 sequence The c u r r e n t deflection factor is indicated by the screen readout, assuming Text

is t u r n e d On, as described later

To the right of the Up b u t t o n is a b u t t o n w h i c h selects the

C h a n n e l 1 i n p u t impedance Two values are available: high

i m p e d a n c e (1 MF~ in parallel w i t h 25pF) or 5 0 ~ Below this

b u t t o n is one labelled ON, w h i c h enables or suppresses display of the Channel 1 trace on the c.r.t, screen To the right of this b u t t o n

is one labelled AC/DC/GND, successive presses of w h i c h cycle

t h r o u g h these three i n p u t coupling conditions In the GND position, the C h a n n e l 1 amplifier is disconnected from the i n p u t socket and c o n n e c t e d instead to ground This allows the Y1 shift control t o set zero signal voltage to any desired level on the screen, such as the centreline Above the AC/DC/GND b u t t o n is one labelled CH1 + CH2 This toggles b e t w e e n displaying just the

C h a n n e l 1 input, or a trace representing the s u m of the C h a n n e l

1 and C h a n n e l 2 inputs Above this b u t t o n again is the TRIG 1 button, pressing this sets C h a n n e l 1 as the timebase trigger source, and r e p e a t e d presses toggle b e t w e e n selecting positive- going or negative-going triggering

22 Oscilloscopes

This c o m p l e t e s the tally of C h a n n e l 1 controls, b u t g r o u p e d

w i t h t h e m , for c o n v e n i e n c e , is t h e BWL b u t t o n This toggles

b e t w e e n the i n s t r u m e n t ' s full 2 0 0 M H z b a n d w i d t h , a n d the

r e d u c e d B a n d W i d t h Limit of 20 MHz To the right of the C h a n n e l

1 i n p u t socket a n d controls are to be f o u n d those of C h a n n e l 2 These are identical e x c e p t for the following The CH1 + CH2

b u t t o n is replaced by a n INV b u t t o n This toggles b e t w e e n the

n o r m a l display m o d e , a n d the i n v e r t e d m o d e w h e r e positive- going e x c u r s i o n s of the i n p u t deflect the trace downwards instead

of u p w a r d s This m e a n s that, w h e n u s e d in c o n j u n c t i o n w i t h CHI + CH2, t h e C h a n n e l 1 trace displays t h e difference of the C h a n n e l

1 a n d C h a n n e l 2 inputs Thus a n y c o m m o n m o d e c o m p o n e n t is rejected, giving in effect a b a l a n c e d floating input The degree of

b a l a n c e is 40 dB at 1 MHz, 28 dB at 50 MHz In practice, this will

be e r o d e d to a s o m e w h a t l o w e r figure w h e n using x l 0 probes But on a n y selected ( c o m m o n ) sensitivity range, the gain of

C h a n n e l 1 or C h a n n e l 2 can be t r i m m e d back slightly, as

a p p r o p r i a t e , using the VAR facility, to restore or e v e n b e t t e r the

e x a m p l e in one 10 ms period w h e n a t i m e b a s e speed of l m s / d i v

is selected The s e g m e n t s follow each o t h e r so closely t h a t to the eye t h e y a p p e a r as c o n t i n u o u s traces

To t h e right again are the i n p u t c o n n e c t o r s a n d controls for

C h a n n e l s 3 and 4 These are the s a m e as for C h a n n e l s 1 a n d 2 respectively, so that trace 3 can display C h a n n e l 3 - C h a n n e l 4 if desired To the right of the C h a n n e l 4 i n p u t c o n n e c t o r is a 4 m m ' b a n a n a ' socket c o n n e c t e d to the i n s t r u m e n t ' s chassis ground,

a n d t h e n c e via the p o w e r cord to m a i n s earth To the left of the

C h a n n e l 1 i n p u t c o n n e c t o r is the CAL (calibration) o u t p u t

c o n n e c t o r , providing a 600 mV p e a k to p e a k s q u a r e w a v e at 2 kHz This is u s e d to set up probes for correct response, as described in detail in the n e x t chapter Note t h a t e a c h probe s h o u l d be set up

Advanced real-time oscilloscopes 23

for the particular c h a n n e l w i t h w h i c h it is to be used, a n d probes

s h o u l d n o t t h e r e a f t e r be needlessly i n t e r c h a n g e d b e t w e e n c h a n - nels O t h e r w i s e t h e y will n e e d setting up again The CAL signal, applied to t w o probes simultaneously, can also be u s e d in

m a i n t i m e b a s e trigger level control This sets the p o i n t o n the

w a v e f o r m selected for triggering at w h i c h the t i m e b a s e triggers,

o n the rising or falling edge as selected by the TRIG slope b u t t o n

o n the C h a n n e l selected as the trigger source This c o n t r o l sets the level, at a n y p o i n t up or d o w n the display, at w h i c h triggering occurs If the level is set above the top or b e l o w the b o t t o m of the

w a v e f o r m selected for triggering, t h e n the t i m e b a s e will n o t r u n (Triggered m o d e selected), or will free r u n u n s y n c h r o n i z e d (Auto free r u n selected) However, following a n AUTOSET (see later), the range c o v e r e d by the LEVEL MTB control n o l o n g e r covers the w h o l e eight vertical display divisions, b u t is c o n s t r a i n e d to a range equal to the w a v e f o r m ' s p e a k to p e a k excursion

The t i m e b a s e speed is c o n t r o l l e d by t w o b u t t o n s labelled MTB/ VAR, to the left of the LEVEL MTB knob The r i g h t - h a n d b u t t o n ,

m a r k e d w i t h a right arrow, increases the t i m e b a s e speed, w h i l e the l e f t - h a n d b u t t o n , m a r k e d w i t h a left arrow, reduces it The range is f r o m 20 n s / d i v to 0.5 s/div, in a 1, 2, 5 sequence Pressing

b o t h b u t t o n s at once toggles to or f r o m the VAR m o d e , w h e r e the timebase speed is c o n t i n u o u s l y variable A 10 x MAGNification

b u t t o n effectively increases the fastest s w e e p speed to 2 ns/div

To the left of the MTB/VAR b u t t o n s is the TRIGGER MTB

b u t t o n , w h i c h activates the various m a i n t i m e b a s e trigger ' m e n u s ' A m e n u is displayed at the r i g h t - h a n d side of the c.r.t screen, as a series of messages adjacent to the c o l u m n of six

24 Oscilloscopes

'softkeys' One m e n u sets t w o of the softkeys to control coupling

m o d e a n d noise The coupling m o d e softkey cycles b e t w e e n a.c., d.c., 1.f reject or h.f reject The reject settings roll off the response

of the trigger circuitry b e l o w or above 30kHz respectively The noise softkey toggles noise rejection on/off W h e n selected, by enlarging the trigger gap (of MTB and DTB), the triggering becomes less sensitive to noise

A n o t h e r TRIGGER MTB m e n u provides a softkey w h i c h toggles b e t w e e n the tv trigger mode, and edge triggering Selecting either calls up an appropriate s u b m e n u In the edge

s u b m e n u , triggering is d e t e r m i n e d by the LEVEL MTB knob and the trigger polarity selected by the TRIG b u t t o n of the channel selected as the trigger source A n o t h e r softkey toggles the trigger polarity of the selected source, and a third toggles b e t w e e n CH and COMP trigger In CH mode, triggering is always from the channel selected as the trigger source, h o w e v e r m a n y channels (traces) are displayed on the screen The COMP m o d e is called the NORMAL m o d e on some o t h e r makes of oscilloscope, and in this mode, each trace is triggered from its corresponding input Thus two or m o r e signals of unrelated frequencies can be stably displayed simultaneously, w h e r e a s in the CH mode, only the trace corrresponding to the channel selected as trigger source would s h o w a stable, locked display

Selecting the tv s u b m e n u gives access to the various TV trigger modes These support HDTV as well as NTSC, PAL and SECAM, and the main timebase can be triggered from line, field 1 or field

2 The delay timebase (see below) can t h e n be used to view any particular line

Above the 10 • MAG b u t t o n is the the TB MODE button, which toggles b e t w e e n the AUTO, TRIG and SINGLE modes The AUTO m o d e causes the timebase to free run in the absence of an input signal, providing the usual 'brightline' display In TRIG mode, the trace is displayed c o m m e n c i n g at the trigger point, as

d e t e r m i n e d by trigger level and slope This m o d e should be used for signals of less than 10 Hz, as otherwise the AUTO function

m a y cause the timebase to run again before the arrival of the next trigger The SINGLE m o d e causes the timebase to run once only, following the next trigger event The RESET b u t t o n resets or 're-

Advanced real-time oscilloscopes 25

Figure 3.2 The TDS694C samples at up to IOGs/s on all four inputs simultaneously, providing 3 GHz bandwidth, with 15 ps delta time measurement accuracy GPIB, RS232 and Centronics interfaces are standard, as is a floppy disk drive and a 7in NuColor T M display, while a hard disk drive is optional (reproduced by courtesy of Tektronix UK Ltd)

Setting the delay timebase speed to a faster setting than the

delayed timebase trigger level control knob, which operates in the

the start of the main timebase sweep that the delayed sweep starts,

if there is some jitter on the signal, as, in st.arts mode, this will appear greater due t o the mace magnification The leftmost conrrol

st.arts o r triggcrcd delay timebase niode, and d.c., a.c., 1.f reject

SEPARATION hutrons in the main timebase group can be used to separate the two traces for clarity

Cursors

The Cursors control group is above the delay timebase control

between them the CURSORS button Pressing this calls up the

softkeys, as indicated beside each, on the display Cursors are on-

againti1 {.he graticulc, hcca~lsc hey a r e no1 allwlctl tiy linearity cotisidel-atioiis in t h c vcrtical atid horizontal deflection amplificrs,

or the tii-nt+astl gent-rator lincharity

tiirasiireni~wts a n d Iir~rizciiital lines = lor voltage nicasurcrnenrs,

o r voltage diffvrence between {lie cursors is showri in tlie cursor display area, see Figure 4.1 1 The vertical time cursors can be

Advanced real-time oscilloscopes 27

located at a n y t w o points of i n t e r e s t o n a w a v e f o r m , t h e m a i n cursor being p o s i t i o n e d w i t h t h e TRACK control, a n d t h e delta cursor w i t h t h e A control The r e a d o u t s h o w s t h e t i m e difference

b e t w e e n t h e t w o points at w h i c h the cursors intersect the

w a v e f o r m The t w o cursors m a y be p o s i t i o n e d i n d e p e n d e n t l y , b u t

t h e m a i n c u r s o r is u s u a l l y set first This is b e c a u s e s u b s e q u e n t l y adjusting t h e m a i n cursor p o s i t i o n w i t h t h e TRACK control 'drags' the A cursor along w i t h it at t h e delta spacing Various delta r e a d o u t f o r m a t s are possible For e x a m p l e , if t h e cursors are set at a spacing c o r r e s p o n d i n g to o n e cycle of t h e w a v e f o r m , t h e

m e n u s a n d softkeys c a n be set to r e a d o u t t h e p e r i o d T of t h e

w a v e f o r m , or t h e f r e q u e n c y f = l i t The p e r i o d T can also be

n o r m a l i z e d to r e a d 100 p e r cent or 360 ~ so t h a t w h e n t h e delta

c u r s o r is m o v e d to a n i n t e r m e d i a t e p o i n t o n t h e w a v e f o r m , t h e distance b e t w e e n t h e cursors can be r e a d o u t as p e r c e n t a g e of a cycle, or p h a s e in degrees In t h e s a m e way, t h e p h a s e lead or lag

of o n e w a v e f o r m w i t h respect to a n o t h e r can be m e a s u r e d The voltage cursors w o r k in t h e s a m e way, a n d again m a y be set for v a r i o u s types of r e a d o u t In a d d i t i o n to setting t h e

h o r i z o n t a l voltage cursors s e p a r a t e l y w i t h t h e TRACK a n d A knobs, t h e y m a y be c o m m a n d e d to set t h e m s e l v e s a u t o m a t i c a l l y

to t h e top a n d b o t t o m p e a k s of t h e w a v e f o r m This defines t h e

p e a k to p e a k v a l u e as 100 p e r cent, a n d 'Trise' r i s e t i m e cursor positions can t h e n be called up The cursors t h e n a u t o m a t i c a l l y position t h e m s e l v e s at 10 p e r cent a n d 90 p e r cent (or, if r e q u i r e d ,

at 20 per cent a n d 80 p e r cent), so t h a t t h e delta t i m e r e a d o u t gives the risetime (or falltime) directly

T e x t

S i t u a t e d b e l o w t h e c o l u m n of softkeys to t h e right of t h e s c r e e n

is t h e TEXT OFF b u t t o n Pressing this suppresses t h e display of

t h e softkey m e n u , t h e n e x t press b l a n k s also t h e display of

i n s t r u m e n t settings (see Figure 4.11 ), w h i l e a t h i r d press restores both I m m e d i a t e l y a b o v e the c o l u m n of six softkeys is t h e STATUS/LOCAL b u t t o n N o r m a l l y a m a x i m u m of four lines of setting i n f o r m a t i o n are given in t h e l o w e r s c r e e n area, referring

j u s t to the c h a n n e l ( s ) in use The STATUS b u t t o n toggles b e t w e e n this a n d a m o r e e x t e n s i v e status display covering, a m o n g o t h e r