Custom designed LSI for instrumentation b y G o r d o n T a y l o r T

During the 1970s the demands of the electronics and communications industries for high precision instrumentation at competitive prices set new problems for the in,irument design engineer. Faced with a challenge to provide accuracy, reliability and versatility in widely different environments and applications an early decision was made to explore the latest technology for digital frequency meters and universal counter-timers. This paper discusses the consequences of this decision.

1. Introduction Analysis of a range of instruments showed that a high proportion of every instrument had many common features including switching logic, time base dividers and gates, basic counting circuits, a display drive and complex input and output gating requirements. One of the most significant advances in the industry is the introduction and development of large scale integration (LSI) - the technology of the future and apparently and ideal solution. If all the common circuits can be provided in a single package the resulting device will be applicable to a complete family of instruments.

An unsuccessful world-wide search for a commercially available LSI package to meet the exacting specifications for a new range of instruments led to the conclusion that the solution lay in a custom designed package where the expertise of the instrument design engineer could be applied to produce a custom built device designed specifically to fit the requirements. A single-chip 60MHz universal counter-timer with many additional features was chosen as the device to be used as the basis for an ever growing family of instruments. In practice the large number of LSI devices used has kept costs down thus enabling the designers to direct their time and energy to the development of 'more technology for the same price'. This policy has been successfully maintained even in the face of high inflation.

After careful evaluation of all available technologies the Ferranti CDI (Collector Diffusion Isolation) process was selected for a number of reasons, including its speed (allowing more high-speed circuitry to be designed on to the chip) and the inherent reliability deriving from its simpler manufacturing process.

~Racal-Dana, Maidenhead, Berks, U.K.

2. Advantages of LSl approach The first and most obvious benefit of LSI is the reduced component count. But that does not mean wasted space inside all LSI counters. In many cases the gain in space has allowed the inclusion as standard of extra features such as time interval averaging, standard data outputs, frequency burst measurement and more sophisticated input circuits.

Less obvious is that the designer has been able to improve isolation of sensitive circuitry and has had room to lay extra tracking for monitor points, data outputs and control lines to be brought out to a PCB edge as standard features for direct connection to the outside world without using flexible or wired connections.

The elimination in a typical counter of up to 50 IC packages minimising connections, the reduction in the number of soldered'joints and the intrinsically high MTB Fcharacteristics of LSI devices themselves have all contributed radically to improving the reliability of modern counters and timers. Additionally servicing costs are reduced since the major part of the circuits can be checked by a single package replacement.

The LSI chip shown in Fig. 1 consumes considerably less power than its discrete IC equivalents, and thus reduces instrument power requirements. Apart from the reduced demand on the internal power supply or extended battery life in portable instruments, this also lowers the internal temperature which, in turn, allows the operating temperature range of the complete instrument to be widened. All the circuitry contained on an LSI device can be fully temperature tracked and compensated by means of an on-chip detector and external regulator to maintain its full specification over a wide ambient temperature range.

In the production and test department a simple

MICROELECTRONICS JOURNAL, Vol. 10 No. 2 © 1979 Mackintosh Publications Ltd., Luton. 37

Custom designed LSI for instrumentation continued from page 37

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Fig. 1 The counter-on-a-chip which is the heart of the 99 series of digital frequency meters and universal timer counters.

construction technique is possible. Therefore, the complete PCB and, in some cases, the entire instrument can be checked on an ATE system far more thoroughly than by hand. The test engineer saves time due to the application of ATE and no longer needs to be trained in or even understand the complex logic and other circuits designed into the chip.

The policy to design one LSI chip with many features has not only enabled the production of a family of frequency meters and counter-timers (there are 10 different instruments at present) but also has generated applications in other equipment. Of particular interest, the same LSI chip is used in synthesised signal generators where it is applied to the monitoring and

Fig. 2

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The Racal-Dana 9082 520MHz synthesised signal generator has a built-in frequency monitor and control system based on the LSI counter-on-a-chip.

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Bringing the power of the computer to the laboratory bench the 900 series microprocessor controlled timer counter is seen here with the calculator keyboard in its operating position.

~g. 3

control of the output stages (Fig. 2). For users such as the military, government

establishments and workshops where large numbers of frequency meters or signal generators are in service this commonality avoids the need to hold large stocks of different ICs and simplifies the task of the service technician, who must familiarise himself with all the instruments in his charge.

The counter-on-a-chip approach has produced a complete family of instruments meeting the requirements of military, government and commercial establishments. Performances once thought to be unobtainable are achieved while costs are kept at a low level. In fact when the effects of inflation and devaluation are taken into account the current range of frequency meters and counter-timer costs less than their predecessors of ten years ago.

3. Microprocessors With a growing demand for programming facilities, which enable individual instruments to form part of larger systems, and particularly in view of the international acceptance of the IEEE/IEC interface, the microprocessor has obvious applications in converting measurement data into the required formats. This enables measurement of phase, rise and fall time etc. to be made, without additional special-to-type circuits, together with processing of the measurement data to produce the additional functions by m/lthematical computation.

The inclusion of microprocessor control has enabled all the features of a good timer counter, reciprocating counter and a calculator together with operating capabilities and systems interface options to be

combined in a single instrument (Fig. 3). A principal feature is the use of a calculator-type keyboard for all controls thus simplifying front panel controls. The keyboard is located in a front panel drawer which is pushed out of sight when not in use. LED displays are used to indicate such details as function selected, measurement time, system control, external reference and all vital input parameters. The main display comprises an eleven digit assembly and there are two separate three digit read out assemblies to display trigger level settings for both input channels.

By incorporating microprocessor control any measurement can be made at the touch of a button which automatically selects the appropriate gate time or multiplier to provide a quick reading. The resolution may then be selected through a single keyboard entry and, for frequency and period measurements, both the voltage range and trigger level can also be set automatically to minimise measurement errors.

4. Conclusion Custom built or standard LSI devices will not be rendered obsolete by the advent of the microprocessor. There are many areas in the instrumentation field where the custom built chip, standard LSI or uncommitted logic array (ULA) is preferred. There is every reason to believe that the trend to 'more specification for the same price' will continue as LSI and VLSI devices and manufacturing processes are further developed. There is no doubt at all that the application of state-of-the-art LSI to current and future developments will bring further improvements to the performance/cost ratio for the customer and at the same time utilise the engineering skills of their designers more effectively.

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