October 2008 – History

Scanning the Past: A History of Electrical Engineering from the Past
Submitted by Bob Morrison, Editor

Copyright 1995 IEEE. Reprinted with permission from the IEEE publication, “Scanning the Past” which covers a reprint of an article appearing in the Proceedings of the IEEE Vol. 83, No. 7, July 1995.

Donald B. Sinclair
Fifty-five years ago this month, the PROCEEDINGS OF THE INSTITUTE OF RADIO ENGINEERS (IRE) included a paper by Donald B. Sinclair on the use of the twin-T circuit in impedance measuring instruments. At the time he was a design engineer with the General Radio Company in Cambridge, MA, where he worked for nearly four decades.

Sinclair was born in 1910 in Winnipeg, Canada, and operated an amateur radio station in the attic of his home as a teenager. He spent three years as an undergraduate at the University of Manitoba before transferring to the Massachusetts Institute of Technology (MIT). In 1931, he graduated in electrical engineering at MIT, where he subsequently earned an M.S. degree in 1932 and a doctorate in 1935. After a year as a research associate at MIT he joined the General Radio Company in 1936. He coauthored an IRE paper published in 1936 on a method of determining the residual inductance of a variable capacitor at radio frequencies. In another IRE paper published in 1938, he discussed the parallel resonance method for precise measurements of impedance.

In his July 1940 paper, Sinclair explained how the parallel twin-T network had been employed in a commercial impedance bridge which was capable of precision measurement at much higher frequencies than with earlier instruments. The new impedance bridge covered the range from 500 kHz to 30 MHz, which included a portion of the shortwave spectrum as well as the standard radio broadcast band. An important advantage of the twin-T null instrument was that it enabled use of a variable air capacitor. Sinclair also explained how to correct for small errors due to residual parameters. In still another IRE paper published in November 1940, he described a radio-frequency bridge instrument which could be used for measurements to 60 MHz.

During the same period, Sinclair developed a precision radio receiver for use in field strength measurements over a range of frequencies from 100 MHz to 3000 MHz. A version of this receiver was adopted for use in radar countermeasures by the military during World War II. During the war, Sinclair divided his time between General Radio and the Radio Research Laboratory at Harvard University, a center for countermeasures research and development. He flew as an observer on early ferret plane flights in Europe and received a Presidential Certificate of Merit in recognition of his wartime contributions.

Sinclair served as Chief Engineer at General Radio from 1950 to 1960 and as President of the company from 1963 to 1973; he retired in 1974. He was elected a Fellow of the IRE in 1943, served as IRE President in 1952, and was elected to the National Academy of Engineering in 1965. He served as a Visiting Professor at MIT for a time and also served several years on the local school board. He died in 1985 at age 75.
James E. Brittain
School of History , Technology and Society
Georgia Institute of Technology