Oscilloscopes Episode 13 pptx

This collector voltage limit is called 'writing... collector voltage FP UWL OL WT RT floodgun cathode USP upper stable point FP fade-positive level RP rest potential UWL upper writing

228 Oscilloscopes

c u r v e lies so n e a r t h e first c r o s s o v e r t h a t t h e w h o l e s c r e e n will

s p o n t a n e o u s l y fade positive B o t h t h e s e c o l l e c t o r levels are

s h o w n in Figure 1 1.6

T u r n i n g n o w to t h e c o n s e q u e n c e s of d e c r e a s i n g t h e collector

v o l t a g e b e l o w OL, w e m u s t recall t h a t t h e u p p e r stable p o i n t of

t h e t a r g e t a l w a y s o c c u r s at a v o l t a g e in t h e vicinity of t h e collector

v o l t a g e , since it is t h e failure of t h e c o l l e c t o r to collect w h i c h

c a u s e s t h e a b r u p t d r o p in t h e t a r g e t ' b a l a n c e s h e e t ' c u r v e of

F i g u r e 1 1.3 N o w if t h e c o l l e c t o r is l o w e r e d to t h e vicinity of t h e first c r o s s o v e r v o l t a g e , this will r e s u l t in a c u r v e as s h o w n in

F i g u r e 1 1.7, a n d it is clear t h a t u n d e r t h e s e c o n d i t i o n s t h e r e is

o n l y o n e stable p o i n t , t h e l o w e r stable p o i n t The f l o o d b e a m will

r e t u r n all t a r g e t a r e a s to t h e l o w e r stable point; w r i t t e n

i n f o r m a t i o n is n o l o n g e r r e t a i n e d This c o l l e c t o r v o l t a g e is

t h e r e f o r e called t h e r e t e n t i o n t h r e s h o l d (RT)

N o w w e can d e f i n e t h e stable r a n g e : it is t h e r a n g e of collector

o p e r a t i n g v o l t a g e s b e t w e e n r e t e n t i o n t h r e s h o l d a n d f a d e - p o s - itive A n d it is this stable r a n g e w h i c h is affected by t h e t h i c k n e s s

of t h e t a r g e t in t h e m a n n e r s h o w n in Figure 1 1.5 In itself it will

n o t c o n c e r n us o p e r a t i o n a l l y , since w e w o u l d be u n w i s e to

o p e r a t e t h e c o l l e c t o r n e a r e i t h e r of t h e s e e x t r e m e limits B u t a large stable r a n g e will o b v i o u s l y p r o v i d e a g r e a t e r o p e r a t i n g

-o 8

.m

~

C:

0

o_

(/)

~

E

0 -10

normal V c

retention threshold f/

I

9 i//

0

target voltage (relative to cathode) curve for V c just below retention threshold normal bistable curve

Figure 11.7 If the collector voltage is too low, it becomes impossible to store a trace (courtesy Tektronix UK Ltd)

m a r g i n for the collector voltage This m a r g i n is i m p o r t a n t for several reasons:

9 Setting the collector voltage operationally to the centre of this range is a subjective p r o c e d u r e w h i c h will yield a certain spread from operator to operator

9 In m a n y instances the c.r.t, h e a t e r is u n r e g u l a t e d , a n d varying mains voltages can cause p e r f o r m a n c e changes

9 Storage c.r.t.s are subject to ageing effects w h i c h might, if the operating m a r g i n is too small, require f r e q u e n t recalibrations

9 Even w i t h best m a n u f a c t u r i n g t e c h n i q u e s t h e r e is usually some n o n - u n i f o r m i t y across the target, calling for different

o p t i m u m collector voltage settings, a n d in the presence of a large operating m a r g i n the choice of a suitable c o m p r o m i s e setting is m u c h easier

9 For all these reasons a large stable range is so i m p o r t a n t t h a t w e sacrifice m u c h contrast to obtain it, as suggested by Figure 11.5

W h e n contrast was first m e n t i o n e d as a significant factor in

c o n n e c t i o n w i t h Figure 11.5 y o u m a y h a v e b e e n puzzled since it

is n o r m a l l y t a k e n for g r a n t e d in oscilloscopes t h a t u n w r i t t e n areas of the screen are practically black a n d the contrast therefore practically infinite The discussion of the average rest potential will have e x p l a i n e d why, on p h o s p h o r - t a r g e t storage tubes, the contrast is on the contrary quite limited But a l t h o u g h Figure 11.5 shows a typical contrast figure of only 3:1, some i m p r o v e -

m e n t can in fact be expected after a h u n d r e d operating h o u r s or

so The r e a s o n is t h a t m u c h b a c k g r o u n d light is c o n t r i b u t e d by those dots w h i c h h a v e faded positive, a n d as these p h o s p h o r dots operate c o n t i n u a l l y at full light o u t p u t t h e y will be the first to age

a n d e v e n t u a l l y b u r n out, leaving the u n w r i t t e n part of the screen darker On m o s t tubes the contrast ratio will r e a c h 20:1 after

a b o u t 300 hours

O p e r a t i n g characteristics of the p h o s p h o r - t a r g e t tube

One of the m a i n limitations of a storage tube is its inability to store traces if the b e a m is m o v i n g too fast - if it exceeds the

m a x i m u m writing speed The b u l k of this section will be

230 Oscilloscopes

c o n c e r n e d w i t h t h e d e f i n i t i o n of w r i t i n g speed, w h a t factors

i n f l u e n c e it a n d h o w it c a n be i m p r o v e d T h e n w e shall r e t u r n to

t h e topic of erasing a n d see in detail h o w this is d o n e

In a bistable tube, w r i t i n g is t h e process of raising t h e voltage of

t h o s e p o i n t s o n t h e t a r g e t w h i c h are s c a n n e d by t h e w r i t i n g b e a m

a b o v e t h e first crossover, despite t h e c o n t i n u i n g a t t e m p t s of t h e

f l o o d b e a m to r e t u r n to t h e rest p o t e n t i a l (Once t h e critical first

c r o s s o v e r level has b e e n passed, t h e f l o o d b e a m will carry t h e m to

t h e w r i t t e n level e v e n w i t h o u t a n y f u r t h e r c o n t r i b u t i o n f r o m t h e

w r i t i n g b e a m ) The effect of t h e f l o o d b e a m is to a d d a given

n u m b e r of e l e c t r o n s to u n i t target a r e a in u n i t time B u t this

n u m b e r d e p e n d s o n t h e s e c o n d a r y e m i s s i o n ratio a n d is h i g h e s t

w h e r e t h e ' b a l a n c e s h e e t ' c u r v e of Figure 1 1.2 d e p a r t s m o s t f r o m

t h e 8 = 1 level, trailing off to zero as the first crossover is

a p p r o a c h e d Since w e can n e i t h e r m e a s u r e t h e s e c o n d a r y

e m i s s i o n in an actual c.r.t., n o r e v e n be sure f r o m w h a t rest

p o t e n t i a l t h e target m u s t be lifted, it is impossible to q u a n t i f y t h e

d e m a n d s m a d e on t h e w r i t i n g b e a m if it is to a c h i e v e storage

B u t t h e effect of t h e w r i t i n g b e a m itself is also far f r o m

s t r a i g h t f o r w a r d C o n s i d e r first the s i t u a t i o n of a s t a t i o n a r y b e a m

E v e n t h o u g h it is focused, the spatial d i s t r i b u t i o n of b e a m

i n t e n s i t y follows the n o r m a l G a u s s i a n distribution curve s h o w n

in Figure 1 1.8 At t h e p o i n t on t h e target w h e r e it peaks, t h e

b e a m d e n s i t y per u n i t target area is greatest, h e n c e t h e n u m b e r of

s e c o n d a r y electrons lost in u n i t t i m e is highest If this n u m b e r

e x c e e d s t h e n u m b e r g a i n e d f r o m t h e f l o o d b e a m action the target

, m

e -

E

distance (in all directions) across target

Figure 11.8 Electron density distribution across the beam (courtesy Tektronix

UK Ltd)

will begin to charge up However, the charging process takes time and relies o n the c o n t i n u i n g presence of the writing b e a m if it is

to reach a successful conclusion, n a m e l y that the target voltage passes the first crossover With greater b e a m density, the disparity

b e t w e e n electron loss due to writing b e a m a n d gain due to floodbeam increases and a shorter b e a m dwell time is e n o u g h to achieve storage

Away from the centre of the writing beam, since the b e a m intensity decreases, the n u m b e r of electrons lost per unit time by the target will also decrease As long as it is still greater t h a n the gains m a d e from floodbeam action, the target will still m o v e positive, b u t it will require a longer b e a m dwell time to reach a successful conclusion

So let us r e v i e w the picture given in the last three paragraphs, and assume for simplicity t h a t the target rest potential is at point

B of Figure 11.3 To achieve storage, the r e q u i r e m e n t is that the centre of the writing b e a m (where its intensity is greatest) should cause the target to lose m o r e electrons per unit time t h a n it gains from the floodbeam, and that the writing b e a m should dwell long

e n o u g h at t h a t spot to cause the resulting positive target drift to reach the first crossover We can instinctively feel that s o m e t h i n g like the p r o d u c t of dwell time a n d b e a m intensity is significant here, but t h e r e is a certain m i n i m u m intensity b e l o w w h i c h no

a m o u n t of dwell time will achieve storage because the target gains m o r e electrons from the floodbeam t h a n it loses f r o m the writing b e a m It w o u l d be misleading to try to quantify this complicated situation in a formula, b u t w e will refer to the dwell

t i m e - i n t e n s i t y p r o d u c t in this loose sense later in the text One last consideration: if we start w i t h the m i n i m u m dwell time and b e a m intensity w h i c h will just achieve storage at the

b e a m centre, and t h e n increase either factor, areas a w a y from the centre of the b e a m will also m a n a g e to reach the first crossover

As dwell time or intensity are increased w e therefore obtain a stored dot of increasing diameter

In practice, the b e a m is n o r m a l l y m o v i n g a n d w e m u s t n o w study this situation If a given spot on the target lies in the p a t h

of this beam, t h e n as the b e a m approaches, its intensity will increase in a m a n n e r w h i c h corresponds to the slopes of the

232 Oscilloscopes

distribution curve It will reach a p e a k w h e n the b e a m is centred

on the spot, and t h e n decrease in a similar m a n n e r But w h e t h e r storage will take place depends on the same considerations w h i c h

we e n u m e r a t e d previously: w h e t h e r the m a x i m u m b e a m inten- sity is great e n o u g h a n d the dwell time long enough In this situation quantitative analysis is futile Specifications are verified

by selecting the highest b e a m intensity before defocusing occurs, and increasing the b e a m velocity until the b e a m moves so fast that t h e r e is insufficient dwell time for storage to occur This specification is called 'writing speed' and is typically, for phos-

p h o r - t a r g e t tubes, 0.1 c m / ~ s

If the dwell time is m a d e longer by m o v i n g the b e a m m o r e slowly, areas to the side of the central path of the b e a m will receive a sufficient dwell t i m e - i n t e n s i t y product to b e c o m e written As the b e a m is slowed d o w n we therefore get a progressively wider stored trace

At the end of this discussion we hope that you will have an instinctive feeling for the principal factors affecting dot writing time and writing speed We will n o w consider in w h a t w a y the writing speed, and also the brightness and contrast of the stored display, are affected by the collector operating voltage

The published specifications a s s u m e that the collector operat- ing level (OL) is set normally, let us say to the centre of the stable range in Figure 1 1.6 As we increase the collector w~ltage, leakage increases, the average rest potential increases, and c o n s e q u e n t l y the target rests n e a r e r to the first crossover This m e a n s that a lesser dwell t i m e - i n t e n s i t y product will suffice to achieve writing; holding the intensity constant we can increase the b e a m velocity and still store The writing speed specification has b e e n improved But the i m p r o v e m e n t is not spectacular and the change of collector w~ltage has o t h e r side-effects w h i c h are m o r e

i m p o r t a n t and which we will look at shortly

If the collector w~ltage is decreased the opposite effect takes place The ARP drops and the writing b e a m m u s t linger longer to achieve writing In fact, for a specified b e a m velocity, if the collector w~ltage is decreased sufficiently, a level will be reached

at w h i c h the dwell t i m e - i n t e n s i t y p r o d u c t is no longer e n o u g h to achieve writing This collector voltage limit is called 'writing

How oscilloscopes work (3)" storage c.r.t.s 233

t h r e s h o l d ' (WT) Unlike all o t h e r collector voltage limits (FP, UWL, RT), this one is n o t a limit due to basic constructional features of the tube; it is d e p e n d e n t on the b e a m velocity w h i c h

we specified

For such a specified velocity, the writing t h r e s h o l d represents the l o w e r limit of the collector voltage operating m a r g i n to w h i c h

we referred earlier Neither can we operate successfully above the

u p p e r writing limit since trace s p r e a d i n g occurs This defines the collector operating range a n d is s h o w n in Figure 11.9 A writing speed specification is only realistic if it puts the writing t h r e s h o l d

in a p p r o x i m a t e l y the position s h o w n in Figure 11.9, giving a usefully large operating range

target voltage.- -

e') L_

_ J ,i

lstX over ARP RP r~

]

d i s t a n c e a c r o s s s c r e e n

on same scale collector voltage

FP UWL

OL

WT

RT

floodgun

cathode

USP upper stable point FP fade-positive level

RP rest potential UWL upper writing limit

ARP average rest potential OL operating level

LSP lower stable point W'I- writing threshold

RT retention threshold

Figure 11.9 As Figure 11.6, but showing the writing threshold WT (courtesy Tektronix UK Ltd)

2 3 4 Oscilloscopcs

Now to the other effects of departing from t.he normal collector operating level We said (hat as the collector voltage is

raised, the ARP goes up Thcrcforc the light level or the unwritten area will increase, R u t also, since the upper stable point follows the collector voltage up, the brightness of the written trace increases The converse is true when the collector voltage is decreased We must consider whether, on balance, these effects produce traces with more or less contrast, and whether, if one has the choice, it is more important to get the maximum possible contrast or the maximum possible absolute light output (Contrast, as defined here, means the brightness ratio of written to unwritten areas.) The brightness of the unwritten areas increases more rapidly with increased collector voltage than the brightness o f the written trace, so the contrast becomes poorer On the other hand, with increasing ambient light, the contrast decreases, hut it decreases least if the c,r,t light output is high, because the ambient light cannot then swamp the tube light as easily

Which is prcIchrablc? To see the trace a l all, we need contrast - and the more we have, ihc hcitcr B u t ir turns out rhar for rliffrren I a rnbirn I I jgh t i rig conditions di ffcrcnt collector voltages

will give best contrast, so 1 1 0 hard-ancl-fasr rule i s possible Phorograpliy, of course, takes place in total darkness as the camera shuts out all amhicnt Iight and would therefore benelit from a low collector voltage

Changes in collector volrage, as we have seen, affect writing speed, absolute light output and contrast They also affect tube

life We can summarize by saying that increased collector voltage

will increase writing speed and absolute light output, and wilI

wish t o favour one of these factors you can adjust the collector accordingly But remember that whenever you depart from the normal OL voltage i n either direction y o u are moving away from

t h e centre of thc operating range which we tried t o make large to give long, t roiihlc-frcc periods bctwwn rccalibraiions

It has already been said thai thy improvernrni in wrilirig speed which can be achieved with higher collector voltage is only Inarginal There are two other techniques, howcvcr, which arc

capable of increasing the writing speed by a factor of 10 or more These will n o w be discussed

To u n d e r s t a n d h o w t h e y work, we m u s t first visualize w h a t

h a p p e n s w h e n the b e a m moves faster t h a n the m a x i m u m writing speed and fails to store In such a case, the dwell t i m e - i n t e n s i t y

p r o d u c t is n o t e n o u g h to raise the target voltage above the first crossover, a n d as soon as the writing b e a m is passed, the floodbeam begins the destructive process of m o v i n g the target back to the rest potential Nevertheless, the writing b e a m did raise the target above its rest potential The secret of the t w o techniques is to m a k e use of this charge p a t t e r n before the floodbeam can destroy it

The first t e c h n i q u e is useful on repetitive sweeps, a n d is called the 'integrate' mode By stopping the floodbeam altogether, the destructive process can be halted Any charges laid d o w n by the writing b e a m will r e m a i n on the target, if n o t indefinitely, at a n y rate for minutes If the signal is repetitive, successive b e a m passages will scan the same target areas a n d will add to the charge pattern This is a c u m u l a t i v e process w h i c h m u s t e v e n t u a l l y lead

to the point w h e r e the w r i t t e n target areas cross the first crossover If the f l o o d b e a m is t h e n restored it will m o v e these areas to the w r i t t e n state a n d the trace will be seen

But imagine n o w that we wish to store a single transient, some

u n i q u e event, at a speed exceeding the n o r m a l writing speed Since we c a n n o t repeat the event, the integration t e c h n i q u e is useless Yet e v e n that one sweep did leave s o m e charge behind The second technique, called ' e n h a n c e ' mode, again attempts to salvage the situation A positive pulse is applied to the collector, Figure 11.10, of such a m p l i t u d e that capacitive coupling will lift the w h o l e target by just the a m o u n t n e e d e d to bring the w r i t t e n area above the first crossover The floodbeam will t h e n i m m e - diately set to w o r k separating the w r i t t e n and u n w r i t t e n potential further We m a i n t a i n the positive pulse long e n o u g h to ensure that at its e n d the w r i t t e n areas do not drop back b e l o w the first crossover The curvatures recall the fact that the f l o o d b e a m is

m o s t effective at voltages w h e r e the secondary emission ratio departs most from unity, a n d f l o o d b e a m action slows d o w n as a

of 1 is approached

236 Oscilloscopes

OL

USP

first

crossover

target

ARP-~ !? ~ "'-

T

beam

-~adjustable

.j-"

i " J " J

enhance pulse

J

collector

/

J

s

.J

s

J /

written target

, - ~ ~ , v I

unwritten target /

Figure 11.10 Enhance mode can increase storage writing speed by a factor of ten (courtesy Tektronix UK Ltd)

Figure 1 1.10 also m a k e s t h e p o i n t t h a t i m m e d i a t e l y after t h e

b e a m passage t h e f l o o d b e a m starts r e m o v i n g t h e l a i d - d o w n charge The e n h a n c e pulse m u s t t h e r e f o r e be applied as s o o n as

p o s s i b l e - in o t h e r words, as s o o n as t h e s w e e p is c o m p l e t e d But

o n slow s w e e p speeds, say 5 i~s/div or slower, e v e n this m a y be too late The e n h a n c e pulse will o n l y rescue the later p o r t i o n s of

t h e trace w h i l e t h o s e n e a r t h e b e g i n n i n g of the s w e e p will a l r e a d y

h a v e b e e n partly or w h o l l y d e s t r o y e d by the f l o o d b e a m

N e v e r t h e l e s s , if e n h a n c i n g w e r e t h a t simple o n e w o u l d h a v e to ask w h y t h e t e c h n i q u e is n o t m a d e a p e r m a n e n t f e a t u r e of t h e fast-

s w e e p storage, giving at a stroke a t e n f o l d i m p r o v e m e n t in w r i t i n g speed B u t Figure 1 1.10 is oversimplified in an i m p o r t a n t respect The a v e r a g e rest p o t e n t i a l is a fictitious level, a n d t h e actual target rests o v e r a b r o a d r a n g e of levels W h e n t h e w r i t i n g b e a m s adds a

c h a r g e to this, t h e w r i t t e n areas, too, will e n d up o v e r a b r o a d

r a n g e of levels T h e r e will t h e r e f o r e be n o o n e correct a m p l i t u d e of

e n h a n c e pulse w h i c h can raise all t h e written, a n d n o n e of t h e

u n w r i t t e n , areas a b o v e t h e first crossover

In fact, t h e smaller t h e c h a r g e left b e h i n d by t h e w r i t i n g b e a m ,

t h e m o r e likely it will be t h a t e v e n w i t h o p t i m u m e n h a n c e pulse

amplitude some w r i t t e n parts will r e m a i n unstored, a n d some

u n w r i t t e n parts will b e c o m e stored The exact a m p l i t u d e t h e n becomes a m a t t e r of e x p e r i m e n t a t i o n until the user subjectively feels that he or she has achieved the best compromise, m a k i n g for clearest visibility

W h e n we said that the e n h a n c e t e c h n i q u e allowed a tenfold increase in writing speed, this was m e a n t as a guideline only In any given situation it depends on the kind of c o m p r o m i s e the user still finds acceptable (Luckily, the interpretative p o w e r s of eye and brain far exceed that of any computer.) By contrast, the integrate t e c h n i q u e really has no u p p e r speed limit; it just depends on w h e t h e r y o u can afford e n o u g h time to integrate long e n o u g h to a c c u m u l a t e e n o u g h charges to reach the first crossover In cases w h e r e the signal repetition rate is 1 Hz or so and the r e q u i r e d sweep speed very fast, this can b e c o m e a question of o p e r a t o r patience

The n e x t topic in this section is the erase process used in

p h o s p h o r - t a r g e t tubes Basically, the erase pulse is a negative pulse applied to the collector, w h i c h capacitively m o v e s the

w h o l e target negative The aim is to m o v e the w r i t t e n portions from the u p p e r stable point to b e l o w the first crossover, after

w h i c h the f l o o d b e a m can complete the erasure But there are t w o problems The first arises from the fact that s o o n e r or later w e will have to r e t u r n the collector back to its n o r m a l operating level,

a n d if we do this too fast we will capacitively m o v e the target back up This is true e v e n if the negative pulse was long e n o u g h

to give the f l o o d b e a m a chance to stabilize the target at the rest potential, because the voltage separating rest potential a n d first crossover is m u c h smaller t h a n that b e t w e e n first crossover a n d operating level t h r o u g h w h i c h the collector m u s t m o v e The solution is to m a k e the trailing edge of the erase pulse so slow that any capacitive coupling effects o n the target can be

c o u n t e r e d by floodbeam action

The other p r o b l e m w i t h erasing is that w h e n small w r i t t e n areas are s u r r o u n d e d by large u n w r i t t e n areas, a n d the target is capacitively lowered, the u n w r i t t e n areas will m o v e to a potential

w h i c h is so greatly negative that the f l o o d b e a m is totally repelled from the target The small w r i t t e n areas are in effect t h e n