Telecommunications – Transmitter and receiver at separate stations – With control signal
Reexamination Certificate
2000-10-06
2004-03-02
Urban, Edward F. (Department: 2683)
Telecommunications
Transmitter and receiver at separate stations
With control signal
C455S047000, C455S502000, C370S503000, C370S526000, C375S362000
Reexamination Certificate
active
06701133
ABSTRACT:
The present invention relates to an apparatus for and a method of synchronising oscillators within a communication system. An example of such a system is a radio telemetry system where incoming data at a baseband frequency is frequency converted for transmission at radio frequencies.
In a radio based communication system, input data, for example speech or signals from a sensor generally occupy a relatively low frequency range, referred to in the art as the baseband. The baseband signals are frequency up converted for transmission over the radio link. Frequency up conversion can be performed by mixing (ie multiplying) the baseband signals with a frequency derived from a local oscillator.
Assuming, for the purposes of simplicity, that the baseband signal comprises a continuous tone having a frequency &ohgr;
B
, and that the local oscillator has a frequency &ohgr;
L
, then it is well known in the art that the result of the mixing will produce signals at frequencies &ohgr;
L
−&ohgr;
B
and &ohgr;
L
+&ohgr;
B
this mixing process gives rise to the two side bands which are centred on the local oscillator frequency &ohgr;
L
. One or both of these signals may then be transmitted in accordance with known transmission schemes. Assuming for simplicity that only the upper side band is transmitted, then the receiver is arranged to recover the baseband by mixing the incoming signal (&ohgr;
L
+&ohgr;
B
) with a signal &ohgr;
L
′ derived from a local oscillator.
This will produce signals at the sum and difference frequencies &ohgr;
L
′+&ohgr;
L
+&ohgr;
B
and &ohgr;
L
−&ohgr;
L
′+&ohgr;
B
. If &ohgr;
L
and &ohgr;
L
′ are identical then the recovered signal would be at the frequency &ohgr;
B
(the term &ohgr;
L
′+&ohgr;
L
+&ohgr;
B
being ignored as it occurs outside a system passband). However if the local oscillators are not accurately matched then the term &ohgr;
L
−&ohgr;
L
′) will be non zero and a frequency error would be introduced into the recovered signal. The above discussion has ignored the contribution of the phase shift, but this is also significant and in order to accurately recover the baseband signal the outputs &ohgr;
L
and &ohgr;
L
′ from each of the oscillators must be accurately matched both in frequency and phase.
A seismic data transmission system must recover amplitude, frequency and phase information from each of the geophones (or hydrophones) if the subsequent data processing for normal move out correction, dip move out correction and the like is to be accurately performed. It is important that the frequency of the local oscillators in the transmitters and receivers in such a radio telemetry system are very accurately matched. One approach to achieve this is to use a highly stable and accurate frequency reference in each receiver and transmitter unit. However, if frequency errors of approximately one part per million or less are required over a wide temperature range, then high accuracy temperature controlled references are required, these tend to be expensive, bulky and power hungry and are therefore not suitable for use in portable battery powered radio units.
U.S. Pat. No. 4,188,579 describes a system wherein a local oscillator in a receiver is tuned to a transmitter oscillator by referring to two pilot signals in the received signals.
According to one aspect, the invention provides 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.
According to another aspect, the invention provides 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 pilot signals comprise two pilot tones and the pilot signal comparison comprises locating the received pilot signals by looking for two tones in the received signals which are separated by the frequency difference between the pilot tones.
According to another aspect, the invention provides 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 pilot signal comparison comprises using a frequency shifted copy of the received signals to locate the received pilot signals in the received signals.
The invention also relates to corresponding methods of synchronising oscillators.
It is thus possible to provide a communication system in which the remote units can be provided with relatively inexpensive local oscillators which are controlled so as to lock on to a master oscillator.
Preferably the pilot signals have a narrow frequency spread and may be regarded as pilot tones.
Preferably the pilot tones are inserted into a baseband signal which is modulated by the transmitter. The pilot tones may both occur in the or each sideband of the transmitted signal. They may be modulated to include information for identification purposes or information transfer purposes.
Advantageously the power of the received pilot tones may be used to estimate the transmission loss occurring between the base station and a remote unit. This estimate may then be used to control the transmit power of the remote unit, in order to ensure that it's signal is received with sufficient, but not excessive, power at the base station.
Preferably the or each remote unit is arranged at least intermittently, to monitor the signal from the master unit and to demodulate the signal therefrom to recover the baseband signals and hence the pilot tones. The or each remote unit down converts the incoming radio frequency signal by mixing it with a local oscillator. The or each remote unit includes a quadrature detector within its radio receiver such that both the in-phase and quadrature components of the baseband signal are output from the detector.
A signal processor is arranged to locate the pilots tones within the demodulated baseband signal and to use these to adjust the frequency of the local oscillator. Such signal processing may be performed in the analogue or digital domain and in hardware or software. Digital signal processors are now available at reasonable costs and are especially suited to this processing task. Preferably the signal processor is arranged to identify the pilot tones by looking for the known frequency difference between the pilot tones.
The use of two pilot tones for frequency locking of the local oscillator signific
Bateman Andrew
Bennett Richard Michael
Fairfield Industries, Inc.
Jackson Walker L.L.P.
Nguyen Simon
Tidwell, Esq. Mark A.
Urban Edward F.
LandOfFree
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