On 01/08/17 13:49, rickman wrote:
Brian Reay wrote on 8/1/2017 7:58 AM:
On 01/08/17 12:00, Gareth's Downstairs Computer wrote:
Continuing my googling following last night's
BHI lecture, and following up on the Shortt
and Hope-jones clocks, here is a mechanical
phase locked loop, and in Meccano! ...

http://www.meccanotec.com/shortt.html

While the article refers to a 'phase lock loop', it isn't really.. There
doesn't seem to be any measurement of error in the slave which is then
use
use used to 'pull it' to reduce the error- which is how a true phase lock
loop works.

The system seems to operate more as follows, the slave is designed to run
'very nearly right'. It receives precise pulses from the master which it
will naturally sync to. The same will happen if you have two
oscillators on
nearly the same frequency if you 'feed' the output of one to the tuned
circuit of the other. (Including harmonics.) This is used, for
example, by
some amateurs to lock radio oscillators to GPS locked references.

Still, it is an clever system and of interest.

The Shortt clock *does* make a measurement of the phase. It checks to
see if the phase is fast or slow. In one case it invokes a spring that
tweeks the phase of the slave. In the other case it does not invoke the
spring allowing the clock to continue running unadjusted. The default
behavior of the slave clock is to run a bit slow and the adjustments
speed it up (or the other way round, I can't recall exactly).

The measurement may be binary and the adjustment is the same, but that
does not make it anything other than a phase locked loop.


Hmm, I half see your point but I'm not entirely convinced.

I'm just not convinced that the description truly 'maps' to that of a
true PLL.

I don't doubt that it works nor do I suggest it isn't a very clever bit
of design. I'm just not sure about the terms used.