[time-nuts] "The Penultimate HP5065 A15"

[Bob kb8tq]

Hi

> On Aug 4, 2020, at 5:23 PM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> 
> --------
> 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.

In the same era (1960’s) you could still find OCXO’s done with AC bridge
controllers. Their main advantage is getting rid of DC offset voltage issues. 
Back in the day this was pretty tough to deal with. 

The world then discovered op-amps and they progressively got better and 
better. Yes indeed we see chopper based op-amps which are (to a great 
degree) as good as any AC loop. 

AC loop OCXO’s pretty much died out by the mid 70’s. Op-amp based controllers 
have dominated things since the late 70’s. The same is true of a lot of servo 
systems. You simply don’t need the audio drive anymore ….

Bottom line still is: how much effective thermal gain can you reasonably expect
to see on something built like the physics package? The structure is large (not 
a good thing). The two heated zones are coupled (also not a good thing). There
is no reason to expect it to have a better thermal gain than your typical OCXO. 

If your controller would do ok for an OCXO, it should do fine for the physics
package. Compare the HP 105 controller to the 5065 setup to see the similarities ….

Bob


> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at 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.