Friday, 6 February 2015

Beacons

After the Martlesham 23 and 3cm beacons failed a few weeks ago my thoughts turned to upgrading the 3cm beacon, once again.
The 23cm beacon, GB3MHZ, was quickly returned to service, no fault found (FNF), but the 3cm beacon was taken completely out of service.
Two faults were found.
One was that the driver output was down and that could explain the reports of poor signals levels for the last year or so.
The other fault was more serious. The RDDS had failed. The result was that the beacon was no longer locked but had defaulted to the bottom end of the VCXO range, giving an output around 10368.780MHz
At this point it was decided that the beacon should have a complete rebuild and incorporate MGM ( digital modes) at last. For the last few years the beacon has been running frequency locked but with FSK keying and plain carrier only.
The other big change was that the group decided that we should adopt the Danish Next Generation Beacon (NGB) platform. This is well developed and supported in Europe. In addition to running the Danish developed PI4 digital mode, it is easy to change the mode to one of several others as conditions and  bands dictate.
A DDS board and a 1GHz clock board were quickly ordered and have arrived. The clock board generates the required clean DDS clock and can be shared amongst all the other bands, if required. The clock board is driven by our G3RUH 10MHz GPSDO. NMEA output from the GPS time locks the digital mode sequence.
Each new band will require a new DDS board. We are thinking about 13cm next. 
The photo shows the DDS board in its screened box.








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