Apparatus for and method of synchronising oscillators within a data communication system

• US 6,701,133 B1
• Filed: 10/06/2000
• Issued: 03/02/2004
• Est. Priority Date: 11/03/1997
• Status: Expired due to Term

• Current Assignee: Wireless Systems International Ltd. (CommScope Holding Company, Inc.), Magseis FF, LLC
• Original Assignee: Fairfield Industries Incorporated
• Inventors: Bateman, Andrew, Bennett, Richard Michael
• Primary Examiner(s): Urban, Edward F.
• Assistant Examiner(s): Nguyen, Simon
• Application Number:
  US09/530,625
• Time in Patent Office: 1,243 Days
• Field of Search: 455/70, 455/255, 455/71, 455/265, 455/47, 455/267, 455/48, 455/142, 455/104, 455/203, 455/500, 455/208, 455/502, 455/260, 455/507, 455/517, 455/522, 455/702, 455/703, 375/260, 375/279, 375/284, 375/356, 375/362, 375/365, 375/366-368, 370/319, 370/324, 370/344, 370/350, 370/441, 370/480, 370/491, 370/500, 370/503, 370/518, 370/526
• US Class Current: 455/71
• CPC Class Codes: G01V 1/223 Radioseismic systems
• CPC Class Codes: H03J 7/047 Automatic frequency control using an auxiliary signal, e.g. low frequency scanning of the locking range or superimposing a special signal on the input signal

Abstract

A method and apparatus is provided for locking a local oscillator to a reference oscillator. The reference oscillator is arranged to transmit a signal containing two reference pilot tones in a signal sideband. The apparatus samples the received signal and makes a frequency shifted copy where the shift is equal to the expected separation between the reference tones. The non-shifted and shifted signals are then combined in the frequency domain to constructively reinforce one of the reference pilot tones which is then searched for. Once the tone has been identified, a correction to the local oscillator frequency is calculated. The apparatus may be of particular use in a radio telemetry system in which it is important that the frequency of the local oscillators in the transmitters and receivers are very accurately matched to each other and where, in order to reduce interference, all the clocks used in the system are locked to a master clock.

Claims

1. A data communication system comprising a master unit containing a master clock and at least one remote unit, the or each remote unit having a local oscillator, in which the master unit transmits a signal containing at least first and second pilot signals at predetermined frequencies, and in which the or each remote unit is arranged to receive the pilot signals, to compare the frequencies of the received pilot signals with a local record of their frequencies, and to adjust the frequency of the local oscillator to reduce the difference between the received and expected pilot signal frequencies to below a threshold, wherein the received pilot signals comprise a pilot tone recovered from both the upper and lower sidebands of a carrier signal comprising the received signals.

2. A data communication system comprising:

a master unit having a master clock and a transmitter;

a remote unit having a local oscillator;

in which the master unit transmits a carrier signal with upper and lower sidebands;

the sidebands have at least first and second pilot signals of predetermined frequencies;

the remote unit receives the transmitted signal;

the remote unit compares the frequencies of the received pilot signals with a local record of pilot signal frequencies;

the remote unit adjusts the frequency of the local oscillator to reduce the difference between the received pilot signal frequencies and the local record of pilot signal frequencies below a threshold;

the received pilot signals comprise two pilot tones recovered from both the upper and lower sidebands of a carrier signal; and

the received pilot signal comparison comprises locating the received pilot signals by looking for two tones in the received signal that are separated by the frequency difference between the pilot tones.

3. A data communication system comprising:

a master unit having a master clock and a transmitter;

a remote unit having a local oscillator;

in which

the master unit transmits a carrier signal with upper and lower sidebands;

the sidebands have at least first and second pilot signals of predetermined frequencies;

the remote unit receives the pilot signals;

the remote unit compares the frequencies of the received pilot signals with a local record of pilot signal frequencies;

the remote unit adjusts the frequency of the local oscillator to reduce the difference between the received pilot signal frequencies and the local record of pilot signal frequencies below a threshold; and

the received pilot signal comparison comprises using a frequency shifted copy of the received pilot signals to locate the received pilot signals.

4. The data communication system of claim 3 in which:

the received pilot signals comprise two pilot tones; and

the frequency shifted copy of the received pilot signals is displaced from the received pilot signals by a frequency shift substantially equal to the separation of the two pilot tones.

5. The data communication system of claim 4 in which:

the pilot tones are inserted into a baseband signal transmitted by the master unit; and

the transmitted signal is modulated to include information.

6. The data communication system of claim 5 in which the remote unit further comprises:

a local transmitter and a local receiver in which the power of the received pilot signals is used to estimate the transmission loss between the master unit and the remote unit; and

that estimate is used to control the transmission power of the local transmitter.

7. The data communication system of claim 6 in which:

the remote unit demodulates the received signal to recover the baseband signal and pilot tones.

8. The data communication system of claim 7 in which:

the remote unit modifies the received signal by mixing it with the signal of the local oscillator; and

the local receiver has a phase detector such that both the in-phase and out-of-phase components of the received signal are output from the detector.

9. The data communication system of claim 8 in which:

the remote unit has a signal processor to locate the pilot tones within the demodulated signal; and

the pilot tones are used to adjust the frequency of the local oscillator.

10. The data communication system of claim 9 in which:

the signal processor locates the pilot signals by looking for a known frequency difference between the pilot tones.

11. The data communication system of claim 10 in which:

the pilot tones are cross-correlated to improve identification of the pilot tones.

12. The data communication system of claim 11 in which:

the signal processor copies the demodulated signal; and

frequency shifts the copy by a frequency nominally equal to the frequency difference between the pilot tones.

13. The data communication system of claim 12 in which:

the pilot tones are identified by performing a frequency domain analysis on the received and processed signals.

14. The data communication system of claim 13 in which:

the results of the frequency domain analysis on the received and processed signals are properly combined to enhance the pilot tones.

15. The data communication system of claim 14 in which:

a spread of frequencies is correlated to look for Doppler-shifted versions of the pilot signals.

16. The data communication system of claim 15 in which:

the signal processor searches to find the largest peak of the correlated signals; and

the signal processor estimates a measure of confidence that the pilot tones are correctly identified by measuring the amount by which the largest correlation peak exceeds the remaining signals.

17. The data communication system of claim 16 in which:

the signal processor further compares the magnitude of the largest correlation peak with the magnitude of the second largest correlation peak to derive a ratio to indicate the degree of confidence that the pilot tones are correctly identified.

18. The data communication system of claim 17 in which:

once the signal processor has found the pilot tones with sufficient confidence, a new control voltage for the oscillator is calculated and generated based upon the error between the expected pilot tone frequencies and the received pilot tone frequencies.

19. A method of synchronizing a local oscillator with a master oscillator comprising:

providing a local receiver unit with a local record of expected pilot tone frequencies stored thereon;

transmitting first and second pilot tones in a modulated signal from a master unit;

receiving the modulated signal with said local receiver unit;

demodulating the received signal;

analyzing the demodulated signal to identify the first and second pilot tones by searching for at least one of the frequencies of the pilot tones or the frequency difference between the pilot tones;

comparing the frequencies of the demodulated pilot tones with the expected frequencies of the pilot tones stored in the local record to measure the frequency error; and

modifying the frequency of the local oscillator to reduce the frequency error.

20. A method of synchronizing a local oscillator with a master oscillator comprising:

providing a master unit having a master clock and a transmitter;

providing a remote unit having a local oscillator;

transmitting a carrier signal with upper and lower sidebands from said master unit, wherein the sidebands have at least first and second pilot signals of predetermined frequencies;

receiving the transmitted signal at the remote unit;

recovering two pilot tones from both the upper and lower sidebands of the carrier signal;

comparing the frequencies of the received pilot signals with a remote unit local record of pilot signal frequencies; and

adjusting the frequency of the local oscillator to reduce the difference between the received pilot signal frequencies and the local record of pilot signal frequencies below a threshold,

wherein the step of comparing comprises locating the received pilot signals by looking for two tones in the received signal that are separated by the frequency difference between the pilot tones.