and how you handle these control transitions between a slow loop, and
the next fast loop, and next fast etc etc , whether in software, or in
hardware , example selecting which divider output to use, is non trivial.
In hardware, I wll normally freeze the value, and wait a full cycle
until the hardware change of divider that interrupts the counter has
rippled through the system.
In software, there is a littl emore determinicity as it can be known
which and what edge and counter ststus etc can be jam loaded , and also
progressively slower loop constants can run simultaneously. Also, step
changes to (software) loop constants can be realized (difficult in
hardware)
On 4/03/2022 11:16 am, Bob kb8tq wrote:
Hi
On Mar 3, 2022, at 3:36 PM, Erik Kaashoek <e...@kaashoek.com> wrote:
The GPSDO I'm building started with frequency locking but now I'm adding
phase locking so the time stamping counter can be on GPS time.
A first version works with a PI controller setting the vc-tcxo Vtune DAC
based on the phase difference of the 10 MHz with the PPS phase. Due to
tolerances the tcxo frequency range is big and is set by a 16 bit DAC where
1 bit is about 2e-11 frequency change.
Take a look at just how good the LSB ( or maybe even LSB’s … ) on your
DAC really are. Some are not all that great and others are quite impressive.
Cost is indeed part of that. Summing a pair of lower resolution parts often
turns out to be pretty cost competitive.
With 2^16 steps a LSB of 2x10^-11 (and perfect linearity) you would get
a 1.3 ppm tune range. If it’s symmetric you would get +/- 0.65 ppm. That
probably is not “enough” to compensate for much aging on a low cost
TCXO…..
Once the DAC value is close to the correct frequency the loop catches
nicely but if the setting is far off catching takes a long time.
A possible solution is to use the frequency error to set the DAC close to
the optimal frequency for catching.
Speed of catching is important as the design is intended to only be
switched on when needed.
Does anyone have pointers to info on how to do quick catching in such a
control loop?
The “normal answer” is to use a sequence of control loops. You start off with
a fast / wide band loop and then once it gets “good enough” you go to a
somewhat narrower loop. Your state monitoring tells you when it’s got to an
adequately settled point and you step again. It is not uncommon to have
anywhere from a few to a dozen or so of these steps.
Bob
Erik
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--
Glen English
RF Communications and Electronics Engineer
CORTEX RF
Pacific Media Technologies Pty Ltd trading as Cortex RF
ABN 40 075 532 008
PO Box 5231 Lyneham ACT 2602, Australia.
au mobile : +61 (0)418 975077
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