As expected, th e c o n s ta n t in p u t give s r i s e t o a s t e a d i l y i n c r e a s i n g i n t e g r a t o r o u tp u t v o lta g e (Figure 9 . 1 ) . The q u a n t i s a t i o n n o is e e v i d e n t on th e t r a c e i s due t o th e d i g i t a l s to ra g e o s c i l l o s c o p e . One of the major d i f f i c u l t i e s in a s s e s s i n g th e i n t e g r a t o r performance of the advanced m icro p ro cesso r-b a se d instrum ent i s t h a t t h i s form of response only occurs a t very low s i g n a l l e v e l s . At h ig h e r s i g n a l l e v e l s th e i n t e g r a t o r behaves as a v o lta g e t o frequency c o n v e r t o r , w ith th e i n t e g r a t o r sw itc h in g many tim es w ith i n one c y c le of th e in p u t s i g n a l . This makes l i n e a r i t y measurements of the i n t e g r a t o r alone very d i f f i c u l t . However, as dem onstrated w ith th e f i n a l in s tr u m e n t, o p ti m iz a tio n of the complete instrum ent i s not n e c e s s a r i l y achieved by s e t t i n g th e c o n s t i t u e n t elements in i s o l a t i o n . The l e v e l of performance of an instrum ent i s s t r o n g l y determined by th e s e t t i n g up p ro c e d u re . A good design should in c o rp o r a te a r e p e a t a b l e and r a p i d l y convergent adjustm ent scheme.
Although th e in p u t waveform fo r th e tone b u r s t t e s t (Figure 9.3) i s not re s o lv e d p a r t i c u l a r l y w e ll , i t i n d i c a t e s th e p o s i t i o n of t h e im pulses and se rv e s t o h i g h l i g h t the d i f f i c u l t i e s p re s e n te d t o an ISLM in c a p t u r in g im pulsive s i g n a l s . The t r a c e f o r Figure 9.3 was ta k e n from an instrum ent which had an i n t e n t i o n a l l y slow r e t r a c e , b u t had th e compensation r e s i s t o r f i t t e d in the 21og sta g e ( s e c ti o n 7 . 5 . 3 ) . The uniform s te p s i z e , d e s p i t e an impulse o c c u r r i n g a t sw itc h back, dem onstrates the value of t h i s m o d i f ic a t io n .
The so ftw are package which produced F igure 9.4 was p a r t of a p a r a l l e l development programme w ith th e French company Soeur-Anne. Although
developed fo r combined use w ith th e f i n a l instrum ent as a powerful d a ta a c q u i s i t i o n and a n a l y s i s system, i t a l s o provides a co nvenient t o o l fo r a s s e s s i n g th e performance o f the hardware. The g l i t c h e s in Figure 9.^ are due t o th e a t t e n u a t o r p assin g through i t s -® p o s i t i o n . With the a t t e n u a t o r in t h i s p o s i t i o n th e n o is e f l o o r r e s u l t i n g from the l i m i t s of th e in s tr u m e n ta t io n system i s e v i d e n t . I t i s not com pletely determined whether t h i s i s due t o th e in stru m en t alone or as a r e s u l t of a t t e n u a t o r breakthrough and o th e r e x t e r n a l n o i s e . Under th e se c o n d i t i o n s most of th e measurement system i s o p e r a t in g a t th e extreme of i t s o p e r a t i n g l i m i t s as th e in p u t c o rresponding t o t h i s in stru m ent re a d i n g i s l e s s than yV.
Although th e t r a c e was o b ta in ed manually, t h e r e i s obvious scope t o automate instrum ent e v a l u a t i o n and documentaion. This i s g r e a t l y a s s i s t e d by th e command and d ata s t r u c t u r e a s s o c i a t e d w ith th e DP37 i n t e r f a c e system [1 7 ].
At the upper extreme of the t r a c e , th e overload c o n d i t io n was not trap p e d so t h a t th e l i m i t of th e dynamic range could be r e a d i l y i d e n t i f i e d fo r th e purposes of t h i s t h e s i s . A dynamic range of around 110 dB can be i d e n t i f i e d .
The r e s u l t s p re se n te d i n Table 9.1 r e p r e s e n t four i d e n t i f i a b l e s t a g e s of development of the p r o j e c t . The performance of the e a r l i e s t h y b r id instrum ent i s s t i l l a s i g n i f i c a n t achievement, s e v e r a l y e a rs a f t e r i t s d esign. However, i t did n o t f u l f i l th e c r i t e r i o n , d is c u s s e d e a r l i e r in t h i s c h a p t e r , t h a t th e s e t t i n g up procedure should n o t be to o c r i t i c a l . The performance of th e h y b rid instrum ent o u t l i n e d in Table 9.1 (in stru m e n t number 1) could n o t be m aintained over a wide te m p e ra tu re range. This design was t h e r e f o r e not c o n sid e re d s u i t a b l e f o r m anufacture.
Instrum ent number 2 i s a h y b rid instrum ent which i s a s i m p l i f i e d v e r s io n of th e f i r s t in stru m en t which was designed t o meet th e needs of an Open U n iv e r s it y c o u r s e . S p e c i f i c a l l y th e major s i m p l i f i c a t i o n in t h i s in stru m en t i s t o use a la d d e r network c o n t r o l s t r a t e g y which lo c k s th e dose and time r e g i s t e r s t o g e t h e r . Although th e e a r l i e r , more e l e g a n t , design could compute Leq from a wider range of dose and time in f o rm a tio n , t h i s in c re a s e d range of accuracy only a p p l i e d t o re a d in g s which could not be d isp la y e d due t o th e meter l i m i t a t i o n s .
The intended a p p l i c a t i o n meant t h a t th e instrum ent had t o be r e l i a b l e and easy t o o p e ra te as i t was t o form p a r t of th e home experiment k i t . In a d d i t i o n t o t h i s i t had t o meet a n o n - t r i v i a l design s p e c i f i c a t i o n w ith i n th e r e s t r i c t e d budget of th e Open U n iv e r s it y . As i s e v i d e n t from th e r e s u l t s from t h i s in s tru m e n t, i t s Type 2 c l a s s i f i c a t i o n i s due t o dynamic span r a t h e r than exchange r a t e e r r o r s . Since t h i s i s l i m i t e d by th e use of only two b a t t e r i e s , i t i s probably capable of improvement. This design has been manufactured s u c c e s s f u l l y and i s now used by a number of p u b li c bodies throughout th e world.
Instrum ent number 3 in Table 9.1 i s th e m icroprocessor-ba se d p ro to ty p e using a charge balan cin g a lg o rith m d e s c rib e d in th e e a r l y s e c t i o n s of Chapter Seven. I t was developed t o provide an a l t e r n a t e t o th e instrum ent 2 f o r use by th e Open U n i v e r s i t y . The performance o f t h i s in stru m en t in computing L0q i s not as good as th e previous one, however t h i s instrum ent o f f e r e d th e a d d i t i o n a l f e a t u r e of p r o v id i n g th e s t a t i s t i c a l l e v e l s , a f e a t u r e p a r t i c u l a r l y sought a f t e r by th e couse team. The c o s t p e n a lty of in c lu d in g a m icroprocessor embodied w i t h i n th e in stru m en t was a major f a c t o r a g a i n s t t h i s u n i t . The second h y b r id instrum ent was e v e n t u a ll y adapted f o r i n t e r f a c e t o th e analogue t o
d i g i t a l c o n v e rto rs of th e BBC model B microcomputer and s p e c i a l softw are was prepared t o f a c i l i t a t e g e n e r a tio n of th e s t a t i s t i c a l i n d i c e s .
The performance of t h e f i n a l in stru m en t i s t h a t of instrum ent 4 in Table 9 .1 . The exchange r a t e t o l e r a n c e e r r o r s a re w e ll w ith in th e l i m i t a t i o n s imposed by a Type 0 c l a s s i f i c a t i o n . The u n c e r t a i n t i e s in the dynamic span and peak f a c t o r f i g u r e s a re as a r e s u l t of th e l i m i t s imposed by t h e i n s t r u m e n t a t i o n system, however they a l s o r e p r e s e n t performance which i s c o n s i s t e n t w ith a Type 0 c l a s s i f i c a t i o n .