Scott Seidman wrote:


> Most likely the latter. I can remember spending considerable time in class
> figuring out just how must delay a feedback system can tolerate before
> going unstable, but "almost" all of them could tolerate "some", if I recall
> correctly, based upon just when the phase would kick a nice negative
> feedback into the much less pleasant positive variety. Of course, delay
> generally hurts performance, and often simple stability is much less than
> what one needs.

The feedback loop can't provide any correction at the frequencies higher
then 1/(6*Tdelay).
The conservative rule for the stability would be having a unity loop
gain at 1/(8*Tdelay).

(This assumes that the delay is the dominating source of the phase shift
in the loop, the response is monotonic and the bandwidth of the loop is
from 0 to Fmax).

>
> While this example is extremely nonlinear, one of my favorite lab exercises
> in my biomedical engineering course was simply using a pot to zero a chart
> recorder (yeah, messy ink, green paper, clogged pens.... that's the stuff!)
> line, but the pot was wired through a delay line. We changed the delay
> until we started to oscillate. Because our brains tend to "sample" in
> strange ways, and we produce semi-ballistic responses, people did
> considerably better than a run of the mill feedback system would do in the
> same situation. Wonderfully laid out lab, though.
>



Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com