Re: Oscillator Distortion

On 2024-11-18 08:30, chuck wrote:

Like any PLL or ILL or AGC , you only have samples to correction error and never "continuous" correction unless integrated to a DC level. It is a discontinuous error correction with a filter to hold between inputs.
 There are many ways to correct error by limiting the gain > 1 yet ensuring it can reach 1 to oscillate.  S/H, non-linear soft limiters, filters, peak detectors/comparators. It all depends on your specs for lock-in time and THD/IMD specs of the sine wave.
  >  > There's no requirement that the amplitude regulation be continuous.

Any oscillator with a nonlinear or bilinear gain control element that has to respond during a cycle has to deal with the distortion caused by that element.  OTAs, JFET variable resistors, PIN diode attenuators, Vactrols, light bulbs, and so on, all have that problem.  Tail current sources can avoid it, because you can make them as stiff as you like by cascoding, and filter the control voltage as well as you like.  (I often use two- or three-pole capacitance multipliers on the supply rails of discrete circuitry, which is a similar idea.)
Once the gain is close to 1.000, the amplitude growth or decay happens over many cycles, so the update rate can be correspondingly low--just enough to compensate for temperature and power supply variations.
Something like a baby-scale loop can help with that, to allow the amplitude to stabilize rapidly before slowing down the time constant on the control voltage.
If the oscillator needs to be tunable, you get into slightly different tradeoffs, but you still don't have to deal with nonlinear gain control elements.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
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