GPS Synchronising of LO's
Stability and accuracy of microwave local oscillators (LO's) is of prime importance especially for weak signals where antenna heading may also be a variable. If one further variable can be eliminated then it all helps to gain a contact. With the advent of the Luis Cupido system using Altera CPLD devices synchronising (or disciplining) LO's to an ultra stable Global Postioning System (GPS) becomes a reality.
One way to accomplish this is to discipline a 5 or 10Mhz ovened xtal oscillator to the GPS system. The latter generates an extremely accurate 1 pulse per second output, or an equally accurate 10khz waveform.
The device shown left uses a Jupiter GPS module to receive the GPS signal and output the 10khz waveform to a Luis Cupido type module. A 10Mhz reference signal is produced from an ovened 10Mhz VCXO** using a derivative of the now familiar G8ACE oscillator,whose output is divided down in the CPLD and a comparison made at 10khz. The error signal is filtered and fed back to the VCXO. A secondary output from the 10Mhz oscillator is provided to drive the reference input on a microwave LO at 10, 24, 47, 76 or 134Ghz. These latter units tend to use the same type of Cupido module as described above, with the reference input of 10MHz and a VCXO frequency in the region of 90-100MHz.
An RS232 interface is provided to allow setting of the GPS receiver parameters via a PC, as well as providing the usual GPS positional information. The GPS receiver has a back-up battery to retain satellite almanacs and configuration settings.
Currently this synchronising is applied to my 10, 24 and 47Ghz systems. This makes finding the local 24 & 47Ghz beacons (also frequency standard controlled) very easy to even if its a very low level signal as one can merely dial up the frequency on the FT290 tunable IF and be confident that its correct and then devote all ones efforts to getting the antenna pointing correct.
**The 10Mhz reference oscillator kits are available from John G8ACE
A recent addition has been to add a display, shown here on the left to the GPS box, which reads the NEMA data and displays Lat & Long , Locator, Time, number of satellites being tracked, Lock status and DOP information.
To some extent the above has now been superseded by obtaining off eBay an ex -equipment Rubidium Standard which can be used at home or portable if required. It has been converted to run off 12V by providing a 12 to 20V converter.