Re: [Ntp] Frequency transfer in NTP

[Miroslav Lichvar <[e-mail]>]

On Sun, Jan 31, 2021 at 11:30:03PM +0100, Magnus Danielson wrote:
> perfectly. The first PLL loop will feed the "worst case" input into the
> next where it will amplify it most, and so it goes. The problem is
> already well analyzed classically, and what you need to do is to
> increase the damping factor, and hence reduce Q, of the PLL and this is
> done by increase the P factor of a PI-loop PLL.

Yes, you can dampen the loops to avoid the overshoot, but that will
have a negative impact on the timekeeping performance. You can
minimize the phase error, or frequency error, but not both at the same
time.

> You also use a statement that you cannot in the protocol convey both
> time and frequency errors. This is just plain incorrect. A phase
> measurement will include both.

The point that I'm trying to make is that transfering time and
frequency separately works better than time transfer alone.

> I have before mentioned that incorrect setting of parameters compared to
> polling rate will create overshoots. This is because P and I parameters
> needs to be scaled correctly with regards to the polling rate, or else
> the balance between them will alter the actual damping factor and thus
> going from over-damped to under-damped.

PI loops are extremely simple, but difficult to tune. I have plenty of
experience with them in PTP. I find it interesting that you would
consider using one in NTP, where the conditions are more variable,
there is more filtering, non-constant update interval, etc. Making one
comparable to the standard NTP loop would be tricky.

> I can provide a range of references, unfortunatly not all in online
> sources. I could do a fairly long derivation of equations for it, and
> you can find posts from me where I do that.

If you wanted to be constructive, it would be best if you showed us an
example of a loop that doesn't overshoot in the step response and
otherwise performs similarly to ntp or chrony.

We can compare it a chain of servers and see if it performs better
than the ntpd and chronyd patched for frequency transfer.

-- 
Miroslav Lichvar