Signal superimposition

1. A method of transmitting two signals over a single transmission path for synchronizing generators within an aircraft electrical system, the method comprising the steps of transmitting a first of the two signals and modulating the first signal with a second of the two signals, the second signal having a higher frequency content than the first signal and in which the second signal comprises a digital word, and the digital word is sent in a period contained within a first or a second period of the first signal.

2. A method as claimed in claim 1 , wherein the first signal comprises first periods of a first signal level separated by second periods of a second signal level.

3. A method as claimed in claim 2 , wherein the second signal comprises a plurality of pulses each pulse having a duration less than the first period of the first signal, and wherein the second signal is transmitted within the first period of the first pulse.

4. A method as claimed in claim 2 , in which the commencement of the first period is marked by a signal transition, and a guard period in which the second signal is not transmitted is provided immediately in advance of the signal transition.

5. A method as claimed in claim 2 , in which the commencement of the first period is marked by a signal transition, and a guard period in which the second signal is not transmitted is provided immediately following the signal transition.

6. A method as claimed in claim 2 , in which digital data is transmitted during the second period.

7. A method as claimed in claim 1 , in which the first signal is a synchronisation signal.

8. A method as claimed in claim 1 , wherein the second signal has a shorter inter-pulse period than the first signal.

9. A method of synchronising generators within an avionics system having a plurality of generator controllers so as to perform no-break power transfer, the method comprising providing a reference signal for frequency and/or phase synchronisation, the reference signal being transmitted along a transmission path, and transmitting messages the between the generator controllers along the transmission path such that the generator controllers can co-operate to perform the no-break power transfer, the messages being sent during a mark or space period of the reference signal.

10. A generator control system comprising a plurality of generator controllers responsive to a signal path, wherein a first signal is transmitted on the signal path, and a second signal is transmitted on the signal path, the second signal modulating the first signal and having a higher frequency content than the first signal, where the first signal is a reference signal for synchronising a generator controller of the plurality and the second signal conveys commands or status data to the generator controller of the plurality.

11. A generator control system as claimed in claim 10 , in which the first signal comprises a plurality of first periods of a first signal level separated by second periods of a second signal level.

12. A generator control system as claimed in claim 11 , in which the second signal comprises a digital word, and the digital word is sent in a period contained within the first period of the first signal.

13. A generator control system as claimed in claim 12 , wherein the commencement of the first period is marked by a signal transition of the first signal and a guard period is provided before and after the transition in which the second signal is not transmitted.

14. A generator control system as claimed in claim 13 , in which the controllers synchronise with the first signal by identifying the signal transition surrounded by the guard period.

15. A generator control system as claimed in claim 11 , in which data is transmitted as a digital word within the second period.

16. A control system as claimed in claim 10 , wherein the second signal has a shorter inter-pulse period than the first signal.

17. A device comprising a controller for a generator, wherein the controller has a signal connection, wherein the controller is configured to obtain generator synchronization timing data from a timing reference signal on the signal connection, and wherein the controller is further configured to exchange with other generator controllers command signals modulated onto the timing reference signal and having a shorter inter-pulse period than the timing reference signal.

18. A method of transmitting two signals over a single transmission path, for synchronizing generators within an aircraft electrical system, the method comprising: providing a first signal comprising a series of first pulses, each of the first pulses having a first pulse duration, and transmitting said first signal over the transmission path; and providing a second signal comprising a series of second pulses, each of the second pulses having a duration, each of the second pulse durations being less than the first pulse duration, and transmitting said second signal over the transmission path, whereby said second signal is transmitted within said first pulse duration.