DESIGN PROCEDURE 



for the components to prevent tags making accidental contacts with others, 

 leading almost certainly to short-circuits or non-operation. In addition, 

 fixing everything together gives a measure of portability, enabling the 

 whole job to be put away if more urgent matters intervene. Various methods 

 are possible : everything can be screwed to a piece of wood, or even card- 

 board, or a large old chassis, already provided with a variety of holes, may 

 be pressed into service. The author solders everything, valveholders, 

 potentiometers and all, to a long tagboard; in this way an experimental 

 stage can be set up in about 5 minutes. 



o 



o 



o 

 o 



o 

 o 



o 

 o 



Fourth 

 stage 



1 



Third 

 stage 



I 



First 

 stage 



Second 

 stage 



(a) 



(b) 



Figure 44.2 



Lash ups tend to take up a lot of bench-space, and it would generally 

 be impractical to fashion the entire electrophysiological unit in this way, 

 before building anything in its final form. A reasonable procedure is to 

 develop as a lash up as much as it is eventually proposed to put on one 

 chassis, then do the necessary mechanical design and build it properly 

 before developing the next part. In our example a possible scheme of 

 division might be : 



Chassis 1 Pre-amplifier 



2 Main amplifier 



3 Cathode ray tube, with its brightness, focus and astigmatism 

 controls 



4 Time base and stimulator 



5 EHT and HT power packs. 



Building everything on one huge chassis is, of course, possible, but it may 

 be difficult to get a sufficient separation between, say, the power packs 

 and the pre-amplifier, and the result is anyway likely to be cumbersome. 

 Methods of mechanical construction have been discussed in Part II. 

 The problem of component layout within the framework of the chosen 

 scheme is one of scale and of pattern. Apparatus laid out on a generous 

 scale is easy to work on and less prone to spurious operation caused by 

 unwanted stray capacitances and inductive couplings, but it is bulky and 

 requires more sheet metalwork. The pattern of layout, also, is bound up 

 with the question of stray couplings. Thus a four-stage differential amplifier 

 should be laid out in plan as shown in Figure 44.2a rather than 44.2b. 



675 



