South Coast Repeater and Beacon Group

GPS Locking the beacons

Overview - by Andy G4JNT

GB3SCX, GB3SCF and GB3SCC need a 10MHz master reference signal to lock their sources- it is intended to adapt the other beacons for locked operation eventually.

The first frequency locking trial, on GB3SCX, made use of a high stability ovenned oscillator that had started out life as part of the Decca Navigator chain.After its well-bedded-in crystal operation after some 20 years of operation, I had kept it running continuously for six months and built into a battery backed up module. When controlling GB3SCX it typically kept the beacon within a few tens of Hz of nominal - a few parts in 10^-9, not bad for a middle-aged OCXO.

After the power supply failure in June 2005, the battery flattened so this unit would then have to be re-bedded in and would be unlikely to achieve the same level of stability for many months - in fact when GB3SCX resumed operation a month later, it proved to be several 100s of Hz out and drifted tens of Hz for a few days.

When the controller was rebuilt as part of the post power supply failure update, we decided to build in a Jupiter-T GPS module.This particular GPS module is unique in that is provides a 10kHz reference output, disciplined by the GPS signal. Previous measurements on this output suggested that it typically maintains a short term stability, measured over a few tens of seconds, of a few parts in 10^-9.The long term frequency stability is determined by the caesium standards on the GPS satellites and is, near enough, exact.

Another unit was added to the beacon hardware that contained a good quality voltage controlled TCXO module supplying 10MHz. This is divided down to 10kHz and phase locked to the GPS derived 10kHz timing signal.By employing a Phase Locked Loop time constant of 30 seconds, the worst of the GPS short term variations should be ironed out. This type of simple GPSDO was written up in 'Scatterpoint' and is also published here The 10MHz signal is buffered and sent to each unit that wants it.

The result now is that all beacons that are locked to this reference, are phase locked together, and maintain a short term frequency stability of a couple of parts in 10^-9.

An additional advantage of having a GPS receiver on site is that an accurately timed one pulse-per-second (to 300ns timing accuracy) signal is available, as well as precise time of day on the data stream from the GPS module. These can be used together to enable coherent and/or time locked signalling methods. Distribution connections are available within the beacon controller module for supplying Motorola Oncore compatible binary code (9600 baud, 5V levels) 1 PPS (5V level) and of course the 10kHz and 10MHz reference signals.

Future experiments on site can now make use of precise frequency and time locking, phase locked to all other transmissions from there.

More details of frequency locking the RF sources can be found at SCC_Locking.htm