Oscillators – Automatic frequency stabilization using a phase or frequency... – With intermittent comparison controls
Patent
1992-02-21
1992-12-15
Grimm, Siegfried H.
Oscillators
Automatic frequency stabilization using a phase or frequency...
With intermittent comparison controls
331 18, 331176, H03L 102, H03L 706
Patent
active
051720752
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a self-calibrating temperature-compensated frequency source, and, particularly but not exclusively, to a method and apparatus of maintaining an operating frequency of a frequency source, which is normally locked to a received external reference frequency, when said external reference frequency is not being received.
In, for example, a radio communication system, the carrier frequency is one of a number of parameters used to specify the system. Accurate knowledge of the carrier frequency of a received signal can enhance the performance of a system to a degree depending on the signal processing techniques employed. In the case of mobile communication systems, for example, the remote unit (e.g. car phone) is desirably inexpensive, small and lightweight, has low power consumption and transmits a relatively low power signal. Also, the base station frequency has to process weak signals in a noisy radio environment.
Such mobile communication systems could benefit significantly in performance if the remote unit contained an accurate frequency source from which the carrier frequency could be derived. However, many of the characteristics of highly accurate frequency sources make them unsuitable for mobile applications; notably these sources possess one or more of the following attributes:
(a) appreciable size and weight
(b) high power consumption
(c) high cost.
Further, due to the plethora of systems and services competing for access to a limited range of radio frequencies available, there is an increasing need to improve the efficiency of use of the electro-magnetic spectrum, by allowing an increased number of users to access a given band of frequencies. For example, in the mobile radio area, there is a trend towards the use of closer channel spacing with reduced guard bands between adjacent channels, implying the need for accurate frequency control if unacceptably high levels of mutual interference are to be avoided. The satisfactory operation of new services such as cellular telephone systems is dependent on accurate frequency control, and the future trend is likely to be towards a requirement for increasing levels of frequency accuracy for the signals transmitted by the remote units in such systems. Also, commercial pressures are tending to require equipment to be low-cost, small, lightweight and with low power consumption.
It is known to maintain accuracy of a frequency signal of a remote unit by locking the frequency signal to an external reference frequency signal which is received by the remote unit from a base transmitter station. As long as the external reference frequency signal is being received, reasonable frequency accuracy of the remote unit frequency source can be maintained. The remote unit is therefore able to transmit, when necessary, within the correct band of frequencies for the particular specification.
However, there are times when it will not be possible for the remote unit to receive the external reference signal, e.g. when the remote unit is out of range of the base signal transmitter or when there is an obstruction between the base transmitter and the remote unit which prevents transmission reaching the remote unit. If the remote unit is then required to transmit, it will be capable of maintaining the required level of frequency accuracy only for as long as its local frequency source does not drift from the locked setting by more than allowable limits (Note that in generally known systems the stimulation which causes the remote unit to transmit will have been generated locally, as the control signalling which would normally be used to stimulate transmission is not being received).
For a remote unit which, due to requirements of cost effectiveness, uses an inexpensive uncompensated crystal oscillator as its frequency source, a substantial amount of frequency drift can occur due to changes in the temperature of the crystal and associated components. With the highly specific frequency requirements discussed above, temperature changes may cause the frequency so
REFERENCES:
patent: 4921467 (1990-05-01), Lax
Sizer Geoffrey D.
Yerbury Michael J.
Advanced Systems Research Pty. Limited
Grimm Siegfried H.
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