Message	Date	From	Subject
4673	2010-05-11 11:20:34	Tim	Nonlinearity = important in oscillators
I am reminded this past week, in two very different forums, at how the nonlinearity of the active device in an oscillator is fundamental to its operation. Sometimes we live in a world where gain compression and nonlinearity are things we are fighting against, instead of taking advantage of them. The first is the latest QEX, where N1EKV very usefully shows the transition of a bipolar oscillator from a high-gain class A stage to a lower gain class C stage as it reaches equilibrium. The second is I think more telling: it's a 1960 HP Journal article about the occasional production line HP Wien bridge oscillators that would have a very low level of distortion - but which had serious amplitude instabilities. The author traces through a very nice analysis that shows that a minimal amount gain compression/nonlinearity/distortion is necessary for even the most cleverly simple AGC-stabilized oscillators. On the web at http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1960-04.pdf That HP Journal article reminds me how easy it is to make bad AGC in a radio that overshoots, in an attempt to make all signals the same loudness. I have recently rediscovered that it is in many respects easier to "hear" - that is, distinguish signal from noise - with a radio that has no or minimal AGC. Tim N3QE

I am reminded this past week, in two very different forums, at how the nonlinearity of the active device in an oscillator is fundamental to its operation.

Sometimes we live in a world where gain compression and nonlinearity are things we are fighting against, instead of taking advantage of them.

The first is the latest QEX, where N1EKV very usefully shows the transition of a bipolar oscillator from a high-gain class A stage to a lower gain class C stage as it reaches equilibrium.

The second is I think more telling: it's a 1960 HP Journal article about the occasional production line HP Wien bridge oscillators that would have a very low level of distortion - but which had serious amplitude instabilities. The author traces through a very nice analysis that shows that a minimal amount gain compression/nonlinearity/distortion is necessary for even the most cleverly simple AGC-stabilized oscillators. On the web at http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1960-04.pdf

That HP Journal article reminds me how easy it is to make bad AGC in a radio that overshoots, in an attempt to make all signals the same loudness. I have recently rediscovered that it is in many respects easier to "hear" - that is, distinguish signal from noise - with a radio that has no or minimal AGC.

Tim N3QE