Oscilloscopes Episode 9 docx

150 Oscilloscopes Figure 8.1 Tile Hitachi V-209 20MHz dual trace portable oscilloscope operates fronl its internal battery pack, external 12 V d.c... With this type, Figure 8.3 The noteb

148 Oscilloscopes

c a p t u r e d p r o v i d e d t h a t it is n o t so n a r r o w t h a t it can slip b e t w e e n samples at the i n s t r u m e n t ' s m a x i m u m digitizing rate

In h i g h speed logic circuitry, n o t a b l y ECL logic, glitches as

n a r r o w as o n e or t w o n a n o s e c o n d s can occur Even o n o n e of the m o r e e x p e n s i v e DSOs capable of digitizing at 5 0 0 M s / s , the digital glitch c a p t u r e m o d e described a b o v e could n o t g u a r a n t e e

to c a p t u r e that, let a l o n e a n oscilloscope w i t h a 1 0 0 M s / s

m a x i m u m digitizing rate However, t h e r e is a n o t h e r approach, using a n a l o g u e p e a k detectors These are i n c o r p o r a t e d in the Tektronix 2430, w i t h its 1 5 0 M H z a n a l o g u e m o d e b a n d w i d t h

a n d 100 Ms/s digital m o d e , enabling it to c a p t u r e a 2 ns spike at

a n y s w e e p speed

Inevitably, d u e to p r e s s u r e of space, special facilities f o u n d in

s o m e DSOs h a v e not b e e n covered, w h i l e s o m e of t h e finer points of the m o d e s w h i c h h a v e b e e n covered h a v e b e e n glossed over However, e n o u g h has b e e n said to c o n v e y the

m e s s a g e t h a t choosing a DSO is a m o r e c o m p l e x task t h a n choosing an a n a l o g u e scope Do not rely on the assurances of the salespeople - h a v i n g m a d e a t e n t a t i v e choice of a n instru-

m e n t to m e e t a p a r t i c u l a r m e a s u r e m e n t need, you s h o u l d insist

on a d e m o n s t r a t i o n of its ability to fit t h a t particular applica- tion If t h e i n s t r u m e n t is for general l a b o r a t o r y use r a t h e r t h a n

a p a r t i c u l a r application, t h e r e is no substitute for close scrutiny

of the specifications including the small print For a general-

p u r p o s e i n s t r u m e n t , m y personal p r e f e r e n c e w o u l d always be for o n e w i t h a r e a l - t i m e a n a l o g u e scope capability as well as digital storage

8 Oscilloscopes for special

purposes

It w o u l d be very difficult, indeed quite impossible, to design an oscilloscope suitable for all the very wide range of uses to w h i c h this most versatile of electronic i n s t r u m e n t s is put C o n s e q u e n t l y there is and always will be a wide variety of oscilloscopes, each aimed primarily at its o w n particular field of application

Of course a m a i n f r a m e plus plug-in approach permits one oscilloscope (plus a cupboard full of plug-ins) to cover a wide variety of uses, but this format is confined to m e d i u m a n d large oscilloscopes The m a i n f r a m e will be either an a n a l o g u e - o n l y scope, or offer storage facilities, n o w a d a y s invariably digital storage as m a n u f a c t u r e r s no longer offer oscilloscopes using the type of storage tube described in Chapter 11 A n o n - s t o r a g e scope

m a y be cheaper t h a n a DSO of comparable single shot b a n d w i d t h ,

t h o u g h the price differential is decreasing steadily But first let us consider the smaller, simpler, specialized instruments

Small p o r t a b l e scopes

Being such versatile instruments, oscilloscopes often get used in inaccessible places, d o w n a hole in the ground, for example, or at the top of a pole Here, a small, light i n s t r u m e n t , p o w e r e d from internal batteries, has obvious advantages Figures 8.1 to 8.4

s h o w a selection of such instruments, some p o w e r e d from internal p r i m a r y ('dry') batteries a n d some from internal second- ary (rechargeable) batteries Often the latter variety incorporates

a m a i n s - p o w e r e d battery charger, a n d depending on the m a k e

a n d model it m a y also be possible w h e n mains is available to use the oscilloscope whilst s i m u l t a n e o u s l y recharging the battery for later portable use

Figure 8.5 shows a n o t h e r e m i n e n t l y portable oscilloscope, the Fluke 'ScopeMeter'| m o d e l 123 w i t h a 20 MHz b a n d w i d t h The

i n s t r u m e n t also doubles as a dual i n p u t recorder, a n d as t w o 5000 counts t r u e - r m s digital multimeters An optically isolated RS-232

150 Oscilloscopes

Figure 8.1 Tile Hitachi V-209 20MHz dual trace portable oscilloscope operates fronl its internal battery pack, external 12 V d.c or 90-260 V a.c mains supply (courtesy Thurlby-Thandar Lid)

i n t e r f a c e is p r o v i d e d , a n d t h e i n s t r u m e n t is s a f e t y certified to

6 0 0 V CAT III level A line p o w e r e d a d a p t e r / b a t t e r y c h a r g e r is

i n c l u d e d , b u t - w h i l e w e i g h i n g in at just 1.2 k g - t h e m o d e l 123

p r o v i d e s 5 h o u r s ' p o r t a b l e m a i n s - f r e e o p e r a t i o n f r o m its i n t e r n a l NiCad b a t t e r i e s O t h e r m o d e l s in t h e r a n g e i n c l u d e t h e S c o p e -

M e t e r @ 199, w i t h t w o i n p u t c h a n n e l s e a c h h a v i n g a m a x i m u m digitizing r a t e of 2.5 Gs/s This p r o v i d e s a 2 0 0 M H z b a n d w i d t h

w i t h o u t r e s o r t to sine i n t e r p o l a t i o n For less d e m a n d i n g applica- tions, t h e r a n g e also i n c l u d e s 100 M H z a n d 60 M H z m o d e l s

Figure 8.2 The ADC200 is a PC-based 'virtual oscilloscope', connecting to the host personal c o m p u t e r via a parallel port A different port (LPT2, say) from the printer's LPT1 is a good idea Advanced trigger modes, such as 'save to disk on trigger, with time and date stamp', help track d o w n intermittent faults Three models, with m a x i m u m sampling rates of 20, 50 and 100Ms/s, are available: all provide 8 bit resolution (Reproduced by courtesy of pico Technology Ltd)

Educational scopes

There is one category of oscilloscope, however, w h e r e high performance is not so i m p o r t a n t a consideration M u c h m o r e

i m p o r t a n t in a scope for the educational m a r k e t are simplicity of operation, low cost and, above all, safety Few oscilloscope

m a n u f a c t u r e r s specifically address this market, being c o n t e n t w i t h the hope (often forlorn) that the lowest price m o d e l in their range will pick up some educational sales One of the few m a n u f a c t u r e r s with a product truly designed from the g r o u n d up for this particular m a r k e t is Metrix Figure 2.3 shows their m o d e l OX71 'Didascope', so n a m e d from its didactic connotations F r o m the point of view of the p a r a m e t e r s most i m p o r t a n t in a high- performance scope, its specification is very m o d e s t - just a single

c h a n n e l with 5 MHz b a n d w i d t h at a highest sensitivity of 50 mV/ division However, in view of its i n t e n d e d sphere of operation, it is double insulated (making it suitable for floating m e a s u r e m e n t s )

a n d meets safety specification EN61010 (IEC 1010-1 ), class II For ease of operation, automatic triggering is available a n d the

i n s t r u m e n t even offers XY operation and Z m o d u l a t i o n

Long-persistence scopes

Traditionally, an i m p o r t a n t category of special-purpose oscillo- scopes was t h a t used for displaying l o w - f r e q u e n c y repetitive

w a v e f o r m s , or fast single shot events With the m e d i u m / s h o r t persistence p h o s p h o r s such as P31 used in the majority of oscilloscopes, flicker of the trace will be noticed w h e n its

r e p e t i t i o n rate is m u c h l o w e r t h a n 50 times per second The l o w e r the repetition, the w o r s e the flicker, a n d at a b o u t 15 traces per second the eye ceases to see a trace at all, seeing only a m o v i n g spot of light bobbing up and d o w n

One solution to this p r o b l e m is to use an oscilloscope fitted

w i t h a c.r.t, h a v i n g a long-persistence p h o s p h o r With this type,

Figure 8.3 The notebook type VC-5430 portable oscilloscope runs from internal batteries, dedicated a.c p o w e r adaptor or external battery pack Its two 30Ms/s input channels each provide a 50 MHz bandwidth, with timebase speeds d o w n to

5 ns/div The instrument's most unusual feature is a backlit colour-TFT liquid crystal display, adding clarity to multi-trace displays (courtesy Hitachi Denshi

Oscilloscopes for special purposes 153

Figure 8.4 The battery operated hand-held THS730A oscilloscope, with its

1 Gs/s samplers, provides a 200 MHz bandwidth on each of its two input channels Cursors ease measurements, while the various trigger modes include, for TV work, odd field, even field and line (courtesy Tektronix UK Ltd)

t h e p a t h t r a c e d o u t b y t h e s p o t c o n t i n u e s to g l o w for s e v e r a l o r

e v e n m a n y s e c o n d s a f t e r its p a s s a g e T h e r e is a w i d e r a n g e of

p h o s p h o r s a v a i l a b l e to t h e c.r.t, m a n u f a c t u r e r , see A p p e n d i x 1,

b u t o n e of t h e c o m m o n e s t l o n g - p e r s i s t e n c e p h o s p h o r s is t y p e P7,

w i t h a b l u e 'flash' ( f l u o r e s c e n c e ) a n d a y e l l o w i s h - g r e e n a f t e r g l o w ( p h o s p h o r e s c e n c e ) w h i c h fades o u t g r a d u a l l y o v e r a b o u t e i g h t

s e c o n d s A d e e p y e l l o w filter glass in f r o n t of t h e c.r.t, s u p p r e s s e s

t h e spot, w h i c h c o u l d o t h e r w i s e b e d i s t r a c t i n g as it is q u i t e b r i g h t ,

154 Oscilloscopes

Figure 8.5 Tile Fluke 123 Industrial ScopeMeter| samples al up to 25Ms/s (1.25Gs/s in equivalent time), providing a bandwidth of 20MHz Features include a t r u e r.m.s, digital multimeler t ' u n c t i o n , a n optically coupled RS232 port and up to five h~urs' ballery life (reproduced by courtesy ~t' Fluke Europe BV)

l e a v i n g o n l y t h e a f t e r g l o w visible With a l o n g - p e r s i s t e n c e scope

u s i n g this p h ~ s p h o r , r e p e t i t i o n rates d o w n to a b o u t o n e trace p e r

s e c o n d or less can be c o m f o r t a b l y v i e w e d : t h e m o v i n g spot of light is visible, b u t leaves its p a t h t r a c e d o u t as a line b e h i n d it

S u c h an i n s t r u m e n t also a l l o w s t h e o b s e r v a t i o n of short, single

o c c u r r e n c e s For e x a m p l e , t h e f e w m i l l i s e c o n d s of c o n t a c t b o u n c e

o n a s w i t c h or relay can be ' f r ~ z e n ' u s i n g single sh~! triggering,

a n d o b s e r v e d for a few s e c o n d s until t h e trace fades away

T h e r e w e r e t w o d i s a d v a n t a g e s to t h e l o n g - p e r s i s t e n c e scope, useful t h o u g h it u n d o u b t e d l y w a s in a p p r o p r i a t e c i r c u m s t a n c e s

Oscilloscopes for special purposes 155

The first is that the trace persists for a few seconds only, and is

t h e n gradually irretrievably lost ( t h o u g h it can of course be

p h o t o g r a p h e d in the m e a n t i m e ) The second is t h a t if the timebase runs repetitively but w i t h o u t being correctly triggered, the screen rapidly fills up w i t h a spaghetti jungle of u n w a n t e d traces that always seem to take ages to fade away

A l t h o u g h long-persistence scopes w e r e c o m m o n l y available at one time, few if a n y m a n u f a c t u r e r s n o w offer the option of a c.r.t

w i t h a long-persistence screen in one of their s t a n d a r d scopes In the past, a long-persistence scope offered a m u c h cheaper solution

to m a n y m e a s u r e m e n t problems t h a n the t h e n only alternative - a storage scope using one of the storage tubes described in Chapter

11 But the function of b o t h these types has n o w b e e n t a k e n over

by the ubiquitous digital storage oscilloscope

Recording oscilloscopes

An alternative to p h o t o g r a p h i n g the trace on the screen of a long-persistence scope or a storage scope is to record it on paper Once u p o n a time this was done w i t h tracing paper held against the graticule of the c.r.t., a pencil and a steady hand

N o w a d a y s there are oscilloscopes w i t h built-in recorders capable

of plotting out the trace s h o w n on the screen Figure 8.7 shows such an i n s t r u m e n t - the chart p a p e r outlet is visible on the left side of the top of the i n s t r u m e n t , a n d can be seen in o p e r a t i o n

in a n o t h e r i n s t r u m e n t from the range in Figure 8.6 Usually, as here, the ' h a r d c o p y p r i n t o u t ' can be fully a n n o t a t e d w i t h the graticule a n d the i n s t r u m e n t ' s settings, a great c o n v e n i e n c e a n d time saving

The recorder n e e d n o t be built in With m a n y DSOs, a

c o n v e n t i o n a l XY recorder or YT (chart) recorder can be pressed into service In this case, the sequence of digital values represent- ing the trace is passed to a DAC (digital-to-analogue converter)

w h i c h reconverts it to a t i m e - v a r y i n g voltage, similar to the original signal b u t suitably slowed d o w n for the benefit of the XY recorder This signal is c o n n e c t e d to the recorder's Y i n p u t while

an appropriate r a m p voltage, representing the original timebase,

is fed to the recorder's X input Both X a n d Y w a v e f o r m s are fed out simultaneously, w h e n the appropriate b u t t o n , labelled

136 Oscilloscopes

Figure 8.6 The DataSYS 7100 is a good example of oscilloscopes for special purposes, in [his case, mains power analysis Measures V, I, W, VA and PF with simultane<>us display of Voltage, Curren[ and Power waveforms The instrument performs specific iesls, such as checking cquipmenis [o EN61000-3-2 (Current Harmonics), equivalent to IEC 1000-3-2, and alsc> d<>ubles as a powerful general- purpose 200 MHz digital si~>rage oscillosccipe (reproduced by courtesy of Gould Nicolet Tcchn<~logies Ltd)

R E C O R D o r w h a t e v e r , is p r e s s e d In t h e c a s e of a c h a r t r e c o r d e r ,

o n l y t h e Y i n p u t is n e e d e d , a s u i t a b l e c h a r t s p e e d b e i n g s e l e c t e d

t o m a t c h t h e d u r a t i o n o n t h e X o u t p u t w a v e f o r m T h u s w h a t m a y

o r i g i n a l l y h a v e b e e n a h i g h - f r e q u e n c y w a v e f o r m o r r a p i d

t r a n s i e n t c a n b e r e p r o d u c e d in h a r d c o p y f o r m o n a n i n e x p e n s i v e ( a n d h e n c e f a i r l y s l o w ) XY o r c h a r t r e c o r d e r

Oscilloscopes for special purposes 157

Figure 8.7 The four input channel Delta 9500A has a m a x i m u m 2 Gs/s sampling rate, providing a 500MHz bandwidth With the optional exceptional memory length of 1 Mbyte/channel, horizontal expansion (X zoom) up to x4000 permits viewing of very fine signal detail (reproduced by courtesy of Gould Nicolet Technologies Ltd)

Figure 8.8 The Yokogawa PZ4000 Power Analyser is a good example of a special- purpose oscilloscope Sampling at up to 5 Ms/s and providing differential inputs, the instrument makes inrush current, power factor and three-phase measurements among many others (reproduced by courtesy of Yokogawa Europe BV)