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 everywhere. 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. Thanks! Javier _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
