SUNDRIES 



'overall', i.e., a single sheath enclosing all the cores, or each core may have 

 its own screen. Thus the cable bringing the mains into the equipment might 

 be twin core, screened overall, the object being to contain the electric field 

 due to the large alternating voltage present and minimize the interference 

 that such a field would cause if it were allowed to act on the input circuits 

 of amplifiers. On the other hand if the conductors in a multicore cable are 

 all carrying signals, or pulse waveforms, between which 'cross-talk' — due to 

 mutual capacitance — must be eliminated, then the cores must be individually 

 screened. 



Choice of insulation depends on function. PVC is perfectly satisfactory 

 for cables distributing power supplies except EHT, for which polythene is 

 preferable. For carrying signals screened cable is usual and the limiting 

 factor is then the capacitance between the core and the screen. Consider a 

 typical case of a pre-amplifier coupled to a main amphfier by screened cable, 

 and for simplicity's sake assume the circuitry is single-sided, so that the 

 cable is single-cored . Let an upper turn-over frequency of 1 kc/s be acceptable 

 and let the cable be driven from the anode of a pentode valve having a 100 kQ 

 load. Then oJmax ^^ 6 X 10^ and the maximum permissible cable capacitance 

 (stray capacitances being neglected) is 



C = ^ ^ 160 pF 



Screened PVC-insulated 14/0-0076 has a capacitance of about 85 pF/ft., so 

 the cable must be less than 2 ft. long. A polythene insulated cable of com- 

 parable overall thickness has a capacitance of 16-5 pF/ft., allowing a 9\ foot 

 cable — a great improvement. It is possible to get very special cables having 

 a capacitance of only 4 pF/ft.* 



Some cables are shown in Plate 25.9. No. 1 is seven cores of 14/0-0076, 

 individually screened, whilst No. 2 is similar, but 4 cored. No. 3 is 6 cored, 

 14/0-0076, screened overall. Nos. 4 and 5 are both single-cored screened 

 polythene cables. It is important not to fall into the trap of supposing that the 

 thicker cable has a lower capacitance because the core and screening are 

 further apart. The capacitance depends on the ratio of the diameters of 

 the core and screen, and in fact these are both 16-5 pF/ft. cables. The raison 

 d'etre for the thicker — and more expensive — cable is quite irrelevant to most 

 electrobiological work ; it is that the dielectric loss is lower at high (i.e. tens of 

 megocycles) radio frequencies. No. 6 is a double-screened cable, useful for 

 the input cable to cathodally screened cathode followers; the capacitance is 

 22^ pF/ft. 



* Transradio C44. 



326 



