[time-nuts] Distributed DDS

Javier Serrano javier.serrano.pareja at gmail.com
Thu Jan 27 10:15:14 UTC 2011

Dear nuts,

Triggered by Ulrich's very interesting thread on nasty DDS features, I would
like to submit for comments an idea for an application of DDS technology
which hopefully does not suffer from them. We have a need at CERN to
distribute RF signals (those used for driving particle-accelerating
cavities) over long distances (several km) in a phase-compensated way. We
can already have a phase-compensated fixed 125 MHz clock everywhere thanks
to the White Rabbit (WR) network (see
http://www.ohwr.org/projects/white-rabbit/wiki). The nodes in the WR network
will be made with FMC carrier boards in several formats, e.g. this PCIe card
with on-board DDS and PLL chips:
http://www.ohwr.org/projects/fmc-pci-carrier/wiki. The idea to transmit an
arbitrary RF signal from one node to another is the following:

- Feed the reference RF to one of the nodes (equipped with a suitable
interfacing FMC) and have the DDS take the place of the usual VCO in a PLL
configuration, i.e. have the DDS track the incoming RF signal by suitably
modifying its control word in real time. Of course, the RF must be a
relatively stable signal. In this configuration I expect to have the low
frequency noise of the RF reference at the output of this PLL, not the
artifacts described in the nasty DDS features thread. The DDS is clocked
with the WR 125 MHz clock or a clock derived from it. All WR nodes have easy
access to this clock.

- Time-tag all correction words sent to the DDS in this "PLL node" with a
good WR UTC tag.

- Broadcast these correction words with their associated UTC tags to all
interested nodes, with enough time in advance so that everybody gets them by
some suitable UTC "execution time". Typical delays through a WR network are
in the 10-100 us area and upper-bounded so one can do worst-case design.

- Have all receiving nodes replay these control words at the same UTC times

This Distributed DDS (D3S) scheme should result in good (as good as the
UTC-distribution capabilities of WR) RF signal re-generation everywhere,
with a constant delay with respect to the original signal which is not a
problem for us. It also has the potential of sending more than one RF signal
through the same link quite naturally. As this is a new application for us,
I was wondering if some among you have been confronted with such problems in
the past or have tried to do something similar to this D3S idea.



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