Re: Oscillator Distortion

chuck <donnyduck@gmail.com> wrote:
On 2024-11-18 3:14 p.m., Phil Hobbs wrote:

 

 
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.)

 

 
Cheers
 
Phil Hobbs
 

 

The FET regulator is a filtered servo with fast cutoff and very slow
build up over thousands of cycles. The same is done with other
non-linear limiters using exponentially orders of magnitude higher
resistance when regulating so the decay rate is slow and low distortion.
 
Phil I think you had different assumptions for your au contraire opinion
on tail currents vs quasi-linear low decay currents when regulated
effectively.  I don't see the difference except a non-linear device
starts up faster.

You’re apparently missing the distinction between a control element that
has to respond within a cycle, such as a JFET variable resistor used as a
feedback element,   and one that just sits at a very slowly varying
operating point, such as a cascoded BJT tail current source with a big
emitter resistor and lots of bypassing on the base.

The first kind gets run through its (inevitably somewhat nonlinear) I-V
curve on every half cycle, regardless of the bandwidth of the control loop.
This contributes an amount of distortion that isn’t improved by narrowing
the loop BW.  The second kind’s distortion can be reduced to any desired
degree by careful design.

The active element’s nonlinearity is of the first kind, of course, but
that’s just amplifier design.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics