Thursday, February 12, 2009

Homodyne ≠ Synchrodyne

I thought I'd found a horrible incongruity in history. Here we have an early PLL tuner and one book claims D.G. Tucker invented it, another book claims Francis Morley Colebrook did. It's my worst-case research scenario, both sources are credible, and I have to tease out the truth. Strangely it was a datasheet in a Chinese parts catalog that first reconciled my problem.
Colebrook was writing papers in the 1920s about rectifying detection. This work was derivative of Karl Braun and Jadadis Bose and a paper by E.V. Appleton on the synchronization of oscillators. But he made progress in calculating radiation resistance. Like a good researcher he built on the existing information and made some progress. Colebrook made the original homodyne receiver in 1924. His design mixed the received signal with a locally generated sine wave at the same frequency as the carrier wave to extract the signal from the carrier in a simple detector. Wikipedia completely skips this in their entry. Krzysztof Iniewski book Wireless Technology affirms my assertion. (His English is poor, I correct it in the quote)
"The use of PLL-based frequency synthesis has it's roots in the evolution of coherent communication systems. ...In 1924, a team of British engineers led my F.M. Colebrook the Homodyne (later renamed Synchrodyne) method, which consisted of a local oscillator, a mixer and an audio amplifier."
The Homodyne suffered from the fact that reception was valid only when the oscillators phase and frequency were very close to the incoming signal. Any slight shift in phase would cause the frequency to drift which in turn would cause signal strength to deteriorate. This short coming is what spurred later research in Automatic Frequency Control (AFC.) In 1931
Henri de Bellescize applied to patent an improved homodyne tuning circuit with AFC. That was the first PLL. henri cannot have derived his work from Tucker as he predates it. But before Colebrook was Robinson.

In March 1922, F.N. H. Robinson applied for a patent on a tuning circuit in which the carrier of the incoming signal is filtered out in a path separate from the main signal path. That signal was amplified in a regenerative tuned amplifier before being recombined with the original input signal. The basic idea is to reinforce the carrier signal, in other words a homodyne... J. Evans points out plainly what this is and also that nobody noticed.

So if Colebrook invented the precursor to the PLL, and Henri derived the true PLL, what did D.G. Tucker do? there were a series of small modifications by different invetors.. some plainly redundant. Walton in 1930, Reimann 1932, Jarvis 1933, Urte 1934?, Starnecki 1934, Oltze 1938. These primarily improve selectivity. More here.

In 1932 D.G. Tucker with R.A. Seymour and J. Garlick were co-authoring academic papers on the Synchrodyne. They did not in one step invent the synchrodyne. A series of small improvements led it to them. There were no new engineering principles here. Zero. This page addresses the confusion. Regenerative tuning circuits were called homodynes. That's where the erroneous idea that one homodyne became the synchrodyne arose. The synchrodyne wasn't even a radio, it was designed for measurement!

First of all, the technical name for this device is the Zero-IF Receiver. The name refers to it's zero intermediate frequency (IF). I
n this circuit the oscillator phase, is the reference for the PLL frequency synthesizer. It's controlled by the DC output voltage of the quadrature detector. Received frequencies are detected directly, without the need for super-heterodyne conversion. It's elegant and much less noisy than the homodyne models that led to it.

Colebrook went on to work at The national Physical Laboratory Radio Division in the 1940s. There he worked on the "Pilot ACE" one of the UKs first computers. D.G. Tucker became a member of the Newcomen Society and wrote some very detailed academic texts on the history of electrical engineering.