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Audio Phase Shift Network Design Project
Enhanced Central Electronics 20A

The key to good opposite sideband suppression with single sideband transmitters using the phasing method rests on the audio phase shifter. Simple audio phase shifters as designed by Dome and Norgaard were able to achieve 40db opposite sideband suppression over the range of 275 to 2750Hz. Beyond this audio base band, sideband suppression rapidly deteriorates and “splatter” type interference occurs.

In April 1955, Wes Schum challenged two Stanford University electrical engineering students, Taylor Howard and Alfred Faries, to develop a far superior phase shift network. The prize? A shiny new Central Electronics 20A exciter.

Off to work the two young men went and the fruitful results were presented to Wes in June, 1955. Developing such a network design….one where long-term sideband suppression was required to exceed 50db over the range of 160 to 3300Hz…required tremendous calculation precision. When one considers the primitive nature of mainframe computers in the early 1950s versus the computing power, compact size and easy of use of Today’s laptop machines, it is small wonder that the task was accomplished in better than three months.

The results of this study were never integrated into an enhanced 20B exciter, as was once envisioned, but became the cornerstone of a remarkably new transmitter, the Central Electronics 100V. For anyone interested in the design of complex networks or having a fascination of the goings on in a cutting edge firm as was Central Electronics the following thesis offers an incredible opportunity to look back and appreciate their hunt for perfection.

The complete Howard and Faries paper is here. It is facinating reading. Enjoy!

Here is a final surprise about the 100V’s phase shifter that nobody outside of Joe Batchelor and Wes Schum knew until now:

Stanford University was not the only place Wes Schum and Joe Batchelor nosed around for a superior design for the planned 100V transmitter. Their search brought them to, of all places, the Radio Corporation of America and before one of the world’s premiere passive network designers.

A skilled RCA engineer helped further refine the work completed by Stanford’s electrical engineering students and the result is contained in a note penned by Joe Batchelor. Look closely as the text encircled by the little hand drawn box: “RCA Lab net furnished by Stu Seeley”. Yes, that’s right. The same Stuart William Seeley that invented, along with D. E. Foster, the Foster-Seeley FM Discriminator. Review the PS-2 schematic in your 100V and 200V transmitters and you’ll see Mr. Seeley’s work

Now, you know the whole story!

© 2012 Central Electronics