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In Reply to: RE: "Low frequency ringing"... posted by Steve O on March 18, 2015 at 11:18:21
Steve, If following the 10:1 rule of thumb, this means that the time constant of the PS to the input and driver stages should be 10 times that of the PS to the output stage, or what? Probably this is obvious to the discussants, but not to me.
However, this is a great thread for learning from. Thanks.
Follow Ups:
The time constant rule of thumb applies to interstage coupling when those stages are contained within an overall feedback loop. The basic idea behind this is to stagger the RC time constants (or, effectively the Fc) of cascaded gain stages to avoid stacking of phase shift at or near one frequency. IMO, the power supply is not directly involved in this although a high impedance supply may incite the ringing of an inherently unstable design due to susceptibility to outside variation at low frequencies.
I'll look for a schmatic I can edit into here where the components of interest can be highlighted for clarity.
Thanks, Steve. Glad I asked the question, because it appears I was on the wrong track to understanding this. Someone else mentioned "motor-boating", which I think can be a consequence of PS filtering. That set me on the wrong path.
See attached image for time constants under discussion. In this example (Heath W-3AM) the time constant ratio is ~ 5:1.
An interesting aside is that when considered at the base units, the unit of the RC product is the second (s).
Thanks for going to all that trouble to illustrate the principle. I get it now.
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