-------- Bob kb8tq writes: > > The "modern" approach to that is to modulate or dither with a > > good long PRNG to whiten the noise, and while good in theory, > > it is not _that_ easy to get right in practice. > > Of course one could simply cave in to the fact that for the sort of > gain required here, a pure DC controller works quite well. > > I know - that takes all the fun out of it .....
It's not like PI(D) controllers were unknown when the 5065 was designed, if anything people were better at "servo technology" back then, than they are today, so I cannot imagine they would not have considered them, and for some reason ruled them out. One problem with PI(D) that if your DUT has impedance (thermal in our case) on the timescale of the main oscillatory external disturbances (air-cons in our case), then at best a PI(D) will introduce a delay in the oscillation, at worst it will amplify it. Today you can buy PID controllers where then processor will attempt to phase-lock the primary disturbance and average it out. That works great until somebody props the door open for ten minutes (=half the air-con period) to fix the hinges. Dont ask me how I know. This is why I suspect the wien-bridge approach may not be a just a homage to the HP200, I think they deliberately wanted to shift the frequencies well north of the 137 Hz. Dithering a PI(D) with a PRNG gets you the same effect, but with wideband noise instead of a single spectral peak. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.