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PUBLISHER'S LETTER Publisher & Editor-In-Chief Leo Simpson, B.Bus. Editor Greg Swain , B.Sc.(Hons.) Technical Staff John Clarke, B.E.(Elec.) Robert Flynn Advertising Manager Paul Buchtmann (02) 979 5644 Mobile: 018 28 5532 Victorian Representative McDonald Woodside & Associates Pty Ltd, 143a Como Parade East, Parkdale, Victoria 3194. Phone (03) 587 5155. Contact: Cameron McDonald . On mechanical & acoustic filters Ever looked at one of those fancy new communications receivers and marvelled at how much performance is crammed into their small cases? They are jewels of miniaturisation with more features than any one person is ever likely to use: scanning, lots of memory for storing wanted stations, comprehensive coverage, multiple modes. You name it, you can have it, provided you are prepared to pay the price. But have you ever wondered just how much better these modern communications receivers are compared with the best that was on offer 30 or 40 years ago? That is one of our stories this month. The conclusions will surprise you. Essentially, while great strides have been made in making communications equipment smaller, more reliable and crammed with operating conveniences, today's manufacturers haven't really managed to make them receive signals any better. To me, the really interesting aspect of those old communications receivers was their use of mechanical filters to give unsurpassed selectivity. Typically, they worked at 455kHz or at 500kHz. Now how could this be? How could anything mechanical work at 500kHz? I'm not going to give away the whole story here - you can read it yourself, beginning on page 82. But while the concept of anything mechanical working at 500kHz may seem staggering, one of our projects this month uses a surface acoustic wave filter which works at over 300MHz. How's that again? An acoustic device working at 300MHz? Impossible! Not so. Surface acoustic wave filters, SAW filters for short, have been widely used in TV and communications equipment for quite a few years now although it is only just recently that they got into the realms of 300MHz. They are a resonant device which works by propagating an acoustic wave across a piezoelectric crystal. Often, when designers are trying to model physical systems, such as loudspeaker systems for example, they use electrical analogs or equivalents. This helps them understand the system better and therefore ultimately leads to better performance. However, the use of mechanical filters and SAW filters shows that it is possible to go the other way and get results which are almost unthinkable. Isn't it true that, the more you read about electronics, the more fascinating it becomes? Now have a look at that article comparing two communications receivers on page 82. Those designers of 40 years ago certainly knew what they were about. Leo Simpson Regular Contributors Jennifer Bonnitcha, B.A. Garry Cratt, VK2YBX John Hill Jim Lawler , MTETIA Bryan Maher, M.E., B.Sc. Jim Yalden, VK2YGY Darren Yates Bob Young Photography Glen Cameron Editorial Advisory Panel Philip Watson , MIREE, VK2ZPW Norman Marks Steve Payor, B.Sc., B.E. SILICON CHIP is published 1 2 times a year by Silicon Chip Publications Pty Ltd. All material copyright (c). No part of the contents of this publication may be reproduced without prior written consent of the publisher. Typesetting: Magazine Printers Pty Ltd , Rozelle, NSW 2039. Printing: Macquarie Print, Dubbo, NSW 2830. Distribution: Network Distribution Company. Subscription rates: $42 per year in Australia. For overseas rates, refer to the subscription page in this issue. Liability: Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. Editorial & advertising offices: Unit 39, 5 Ponderosa Pde, Warriewood , NSW 2102. Postal address: PO Box 139 , Collaroy Beach, NSW 2097. Phone (02) 979 5644. Fax (02) 979 6503 . ISSN 1030-2662 AUGUST 1990 3