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SILICON SILIC CHIP www.siliconchip.com.au Publisher & Editor-in-Chief Leo Simpson, B.Bus., FAICD Editor Nicholas Vinen Technical Editor John Clarke, B.E.(Elec.) Technical Staff Ross Tester Jim Rowe, B.A., B.Sc Bao Smith, B.Sc Photography Ross Tester Reader Services Ann Morris Advertising Enquiries Glyn Smith Phone (02) 9939 3295 Mobile 0431 792 293 glyn<at>siliconchip.com.au Regular Contributors Brendan Akhurst Ian Batty David Maddison B.App.Sc. (Hons 1), PhD, Grad.Dip.Entr.Innov. Associate Professor Graham Parslow Dave Thompson SILICON CHIP is published 12 times a year by Silicon Chip Publications Pty Ltd. ACN 003 205 490. ABN 49 003 205 490. All material is copyright ©. No part of this publication may be reproduced without the written consent of the publisher. Printing and Distribution: Derby Street, Silverwater, NSW 2148. Subscription rates: $105.00 per year in Australia. For overseas rates, see our website or the subscriptions page in this issue. Editorial office: Unit 1 (up ramp), 234 Harbord Rd, Brookvale, NSW 2100. Postal address: PO Box 139, Collaroy Beach, NSW 2097. Phone (02) 9939 3295. E-mail: silicon<at>siliconchip.com.au ISSN 1030-2662 Recommended & maximum price only. 2  Silicon Chip Publisher’s Letter SPICE streamlines circuit design This month’s tuitional article on the topic of SPICE simulation (page 38) will have a particular interest for readers who would like an insight into the ways in which we produce the circuits for our projects. These days we seldom bother with building a “bird’s nest” or a protoboard layout on the workbench. There are a number of reasons for this. First, the components are simply too small and usually have too many pins to produce a bird’s nest. Second, even if we did wire up a bird’s nest or a protoboard, the stray wiring capacitance and inductance would be so unpredictable that reliable operation would be unlikely. Or if it did work by some fluke, it might be very hard to reproduce the same performance on a PCB. Second, the key devices in many circuits are now surface-mount types and the only way to wire these into a prototype layout would be to use adaptor boards of some sort. Even then, stray wiring capacitance and inductance would be a problem. Finally, many of our microprocessor circuits, particularly those using the PIC16F88, are variations on past themes and the real “smarts” are in the software. The major part of the design is in writing and debugging the program. So in virtually every project these days, we proceed directly to producing a prototype PCB, designed using the powerful and highly regarded CAD package, Altium Designer (produced in Australia, by the way). Part of the design process for the PCB will involve trying to make provision for any circuit changes which might prove to be necessary, without producing another iteration of the board. If this can be done successfully, we save time and money. But the PCB design does not simply involve using a pencil circuit sketch or a more elegant CAD rendition which becomes the “netlist”. In the case of most analog circuits, we need to run SPICE simulations to ensure that the proposed design will actually work. In fact, SPICE simulation takes the place of the bird’s nest or the more elaborate protoboard layout. As described in this month’s article on SPICE simulation, this process allows as many iterations as we need, to be confident that the circuit will work as intended. Nor is there any need to do any instrument testing, because the SPICE program will simulate that too. So for example, SPICE can show how a filter circuit will respond to an impulse, or model the input surge current into a power supply, or show how deliberate overloads will affect the circuit – all without blowing a single fuse or letting the “smoke out” of any expensive semiconductors. However, despite all that initial simulation, after assembling the prototype PCB we sometimes find that the performance is not what we wanted. This can happen no matter how carefully the PCB has been laid out – and this happened with the Graphic Equaliser in this month’s issue. You can see that there would be no practical way in which that circuit could be prototyped in the traditional way – it is simply too large and complicated. Perhaps inevitably though, the initial performance of the prototype PCB was not up to scratch. Its boost and cut were excessive and the distortion and residual noise were too high. And here I will let you into a secret: we had not done any initial SPICE simulations, because the circuit was a miniaturised variation of a design we presented back in 1989. Sorting out the problems with the prototype could have taken many days of component changes and subsequent testing but we did not have time for that. Instead, we simply did a few SPICE simulations of the key circuit sections and this pointed to the solution. The results can be seen on page 18 of this issue. Leo Simpson siliconchip.com.au