Oscillators – Frequency stabilization – Temperature or current responsive means in circuit
Reexamination Certificate
2000-09-18
2003-02-18
Kinkead, Arnold (Department: 2817)
Oscillators
Frequency stabilization
Temperature or current responsive means in circuit
C331S158000, C331S1160FE, C331S066000, C331S074000
Reexamination Certificate
active
06522212
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a device and a method for temperature compensation by a determination of the cut-angle of crystals used in oscillators. The invention relates also to device and a method for determining the identity (or difference) of the cut-angle of crystals used in oscillators.
In particular, the invention addresses to problems how an effective temperature compensation can be achieved in a clock oscillator without the need of performing an estimation procedure which requires a temperature cycling during a production process. The temperature compensation technique of the present invention is particularly useful for all applications where two clock oscillators each having a crystal are used, e.g. in multistandard applications—one example being a GSM-AMPS dual mode operation). Since the temperature compensation using two “matched crystals” requires two crystals, it is most suitable for applications that need to have two reference frequencies for operation. For example, two reference frequencies in a dual mode subscriber station operating in the GSM and GPS systems, require two different frequencies of 13 MHz and a selected reference frequency (the company SiRF e.g. has selected 24.5525 MHz). However, the invention is not restricted to use in reference oscillators and it may be used for any type of oscillator for any particular application purpose.
Generally, in such crystal oscillators the oscillation frequency temperature characteristic is mainly determined by the resonant frequency temperature characteristic of the respective crystals and the invention is particularly directed to compensate this resonant frequency temperature characteristic of crystal oscillators. However, any other circuit element in the two crystal oscillators can be matched according to the invention in order to cancel out temperature effects of such other circuit components. Therefore, although the invention will hereinafter be explained with reference to the crystal matching of crystals used in oscillators, the inventive concept is generally applicable to matching of other circuit components which determine the resonance frequency temperature characteristic of the oscillators.
BACKGROUND OF THE INVENTION
Crystals (crystal oscillators) are fundamental to radio communication equipment and to many other electronic circuits which require a very stable resonant frequency. The very high quality factor (Q=10000 . . . 5000000) and a small temperature coefficient make them most attractive as the frequency determining elements in frequency oscillator circuits. A particular useful crystal is a AT-cut crystal which covers a fundamental frequency range from 0.5 MHz to 30 MHz. Most of the considerations below are made for the AT-cut crystals, however, it should be noted that the invention is not restricted to this special type of crystal. Other crystal types may be used as well.
When using crystal oscillators in radio communication equipment, the long term stability due to temperature dependent frequency drift is important. Firstly, it is desired that the oscillator keeps a predetermined frequency stable over a long period of time independent from temperature changes. Secondly, if there is a frequency drift due to ambient temperature variations, it is desired that a well defined control is carried out to tune the frequency back to the desired frequency. For doing so, it is necessary to have an exact knowledge of the AT-cut crystal temperature characteristic. Of course, the resonant frequency temperature characteristic of the crystal oscillator is not only determined by the resonant frequency temperature characteristic of the crystal itself but also from additional active circuitry which may be used in the oscillator in addition to the crystal. Hereinafter, it is, however, assumed that the resonant frequency temperature characteristic of the crystal oscillator is mainly dependent on the resonant frequency temperature characteristic of the crystal itself.
DESCRIPTION OF THE PRIOR ART
As already indicated above, in the prior art several solutions are available in order to obtain a well defined temperature characteristic of a crystal. The temperature characteristic must be known also to perform the temperature compensation in case the temperature changes during operation.
FIG. 1
shows an overview of the relation between the available temperature range, initial frequency deviation, frequency drift over temperature and cost for a standard 13 MHz crystal. As shown in
FIG. 1
, a small initial frequency deviation and a small frequency drift over temperature may only be obtained at very high cost. For reasonable costs (see the first line for 100% cost) a quite large initial frequency deviation of ±30 ppm and a large drift over temperature of ±50 ppm must be expected. Whilst the selection of a crystal with a low frequency drift over temperature may be a straight forward solution to build crystal oscillators with low frequency drift over temperature, this approach is certainly a very highly cost intensive one.
As already explained above, instead of just selecting a crystal at very high cost, an alternative approach is to select a moderately priced crystal and to provide the oscillator with a frequency control input (e.g. voltage controlled) and use this frequency control input as a temperature dependent control signal to apply a control voltage tuning the frequency of the oscillator back to the desired frequency dependent on the temperature changes. For example, a voltage controlled variable capacitor can be used in the oscillator circuit in order to pull the resonance frequency (frequencies) of the crystal and thus of the oscillator back to the desired frequency. Using voltage controlled crystal oscillators, different temperature compensation principles can be distinguished.
A first approach uses a pre-detuned control signal voltage, where pre-detuning is a function of the temperature and where the pre-detuning is selected to counteract the frequency drift of the crystal. Such temperature dependent control signals can for example be generated by a NTC/PTC resistor network.
For a digitally compensated crystal, the temperature response of the crystal (if known) is digitized and stored in a look-up table. The control voltage used for tuning the resonant frequency of the oscillator is adjusted according to the current ambient temperature which is sensed by a temperature sensor and the values stored in the look-up table. Both types of crystal oscillators are widely used and are known as VCTCXO and DCTCXO.
Two further solutions for achieving a temperature compensations of crystals (or crystal oscillators) are to place the crystals (or the crystal oscillators) into a temperature controlled oven. Since the temperature is usually kept constant well above the maximum ambient temperature, the crystal itself is operating at a constant temperature and hence independent of the ambient temperature. This achieves a lowest temperature dependent frequency drift.
Another approach is to use a feedback loop to compensate temperature effects. In this case, the crystal oscillator is synchronized to a common frequency standard (master clock) like the DCF77 or is synchronized to a base station like in the GSM system. That is, crystal oscillators used in such feedback loops (e.g. in a GSM network) may use the network reference for temperature compensation purposes assuming that the network reference is available and stable whenever the temperature compensation is required.
Whilst usually the temperature dependent resonance frequency drift is unwanted and a temperature compensation is applied to retune the resonant frequency, an advantageous use of the temperature dependent frequency drift is for temperature sensing purposes where a strong temperature dependence of the resonant frequency is desirable. However, obviously for performing a precise tuning of the resonant frequency or for an accurate temperature measurement, a precise knowledge about the frequency drift versus temperature is necessary. T
Kinkead Arnold
Nixon & Vanderhye PC
Telefonaktiebolaget LM Ericsson (publ)
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