Roger Riordan: Career

In the 1960's telephone lines still used analog techniques. Digital integrated circuits were still primitive, the first analog ICs were just appearing, and amplifiers used large numbers of transistors.

In international telecommunications groups of individual telephone lines were fed into modulators, so that each conversation was shifted into a different frequency band in a sub channel. Then each subchannel, containing perhaps 16 conversations, was passed on to another modulator which shifted the frequency of the whole lot, allowing a large number of subchannels to be packed into a single channel, and fed into an undersea cable.

At the other end filters were used to separate the different channels, which were then fed into demodulators to shift their frequencies back to base level, and then another demodulator separated out the individual conversations, which more filters routed to the appropriate phones.

The modulators and demodulators used large numbers of complex filters, assembled from ferrite cored inductors and capacitors.  The inductors were large and expensive but gave relatively poor performance and the whole system was very expensive and rather unreliable.

In 1966 H.J. Orchard, an English engineer, published a paper describing the use of transistors to produce a simulated inductor, using only amplifiers, resistors and capacitors, to replace the expensive ferrite cored inductors.  The circuit was complicated, and did not give a particularly good performance.  I read it, and thought "there must be a better way of doing this".  The first integrated operational amplifiers had just been released, and after a bit of doodling I devised a circuit which appeared to meet the requirements.

I ordered a couple of op amps (at vast expense), and while I waited for them I designed a circuit using discrete transistors.  I built a prototype, which worked almost immediately, and gave outstanding performance.  I did some measurements and wrote a short paper.  The op amps I had ordered were the Fairchild OP02, which was notoriously fragile.  I built a circuit using them, which lasted just long enough to show that the op amp version could work.

My letter was published in Electronics Letters in 1967, and my idea was quickly adopted by the industry.  But not long afterwards digital techniques were developed, and by about 1980 they had completely supplanted the old analog techniques, so no one was interested in active filters anymore. CSIRO did not patent the idea, and we never made any money from it.

The original paper is reproduced below.

Simulated Inductors Part 1.

Simulated Inductors Part 2.

 © Roger Riordan 2004-2019

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