Oscillators – Frequency stabilization – Temperature or current responsive means in circuit
Reexamination Certificate
2002-12-17
2004-12-14
Kinkead, Arnold (Department: 2817)
Oscillators
Frequency stabilization
Temperature or current responsive means in circuit
C331S002000, C331S046000, C331S176000, C331S158000, C331S1160FE, C327S105000
Reexamination Certificate
active
06831525
ABSTRACT:
This invention relates to oscillator arrangements, and is particularly concerned with providing an oscillator output signal with improved frequency stability.
BACKGROUND OF THE INVENTION
It is well known to provide an oscillator with a resonator in order to determine an oscillation frequency. The resonator may take any of various forms such as crystal, ceramic, dielectric, cavity, and coaxial cable resonators; for brevity herein only crystal oscillators are discussed below but the term “resonator” is used herein to include any of such devices or structures. A crystal resonator may use a rubidium or caesium crystal, or may use any of a variety of piezoelectric materials, such as quartz, employing any of a variety of types of acoustic waves, such as bulk, shallow bulk, and surface acoustic waves, in determining its resonant frequency. The term “crystal resonator” is used herein to include any such resonators, and in particular to include both bulk wave oscillators and surface acoustic wave (SAW) oscillators. The term “crystal oscillator” is used herein to mean an oscillator including a crystal resonator.
Crystal oscillators are frequently used to provide an oscillator output signal with a stable frequency. However, it is known that the output signal of a crystal oscillator has a frequency which is dependent upon various parameters such as temperature, acceleration, microphonics, and ageing. It is also known to compensate to some extent for changes of temperature by using a temperature-compensated crystal oscillator, or to reduce temperature changes by placing the crystal resonator and associated circuitry in a temperature-controlled oven.
In some applications a crystal oscillator is required to provide an extremely stable timing or frequency reference. For example, base stations of cellular wireless communications systems need a very accurate timing reference which they derive from GPS (global positioning system) satellites in normal operation, but in the absence of a GPS reference signal they are required to maintain the timing reference to an accuracy of about 7 &mgr;s in a 24-hour period. The frequency stability of the oscillator providing this reference must therefore be about 0.08 ppb (parts per billion).
For such frequency stability, sensitivity to acceleration is not significant because the crystal oscillator is stationary, microphonics can be avoided with good mechanical design, and ageing over the 24-hour period is very small, for example less than 0.02 ppb, so that temperature sensitivity is a dominant factor. A temperature compensated crystal oscillator may provide a frequency stability of only about 1 ppm (part per million), an ovenized crystal oscillator may provide a greater but still inadequate frequency stability, and a doubly ovenized crystal oscillator may still only provide a frequency stability of about 0.4 ppb; the steps of temperature compensation, ovenizing, and double ovenizing add considerable and successively increasing costs.
It is known from Onoe U.S. Pat. No. 3,978,432 issued Aug. 31, 1976 and entitled “Oscillator Having Plural Piezoelectric Vibrators Parallel Connected For Temperature Compensation” to provide an oscillator with two or more resonators connected in parallel, at least one of the resonators having a second-order frequency-temperature characteristic and the others each having a third-order frequency-temperature characteristic with resonant frequencies selected to provide a wider compensated temperature range for the oscillator. However, such an arrangement would be very difficult to provide in practice and does not provide frequency stability over an extended temperature range.
It is also known from Hartemann U.S. Pat. No. 4,338,575 issued Jul. 6, 1982 and entitled “Process For Compensating Temperature Variations In Surface Wave Devices And Pressure Transducer Utilizing This Process” to provide two SAW delay line oscillators with closely similar frequencies which are mixed to produce an output frequency that is a function of a parameter, such as force, pressure, or acceleration, to be measured, and to include in one of the oscillator loops an extra temperature-dependent delay for temperature compensation. In this arrangement the delay-temperature characteristic of the extra delay must precisely compensate for drift between the SAW delay lines with changing temperature; the reference assumes this to be linear, an assumption that is not necessarily valid. Further, the reference is concerned primarily with difference measurements for which an increase in output frequency is generally unimportant, and otherwise suggests additional measures. Such additional measures are not practicable to achieve the frequency stability as discussed above.
Accordingly, an improved oscillator arrangement is required to provide the desired frequency stability. Even where such extremely high frequency stability is not required, it is desirable to provide an oscillator arrangement with improved frequency stability, for example to reduce requirements for temperature compensation or ovenizing at any desired level of frequency stability.
SUMMARY OF THE INVENTION
According to one aspect, this invention provides an oscillator arrangement comprising: a first oscillator, comprising a resonator for determining a frequency of the oscillator, for producing a first signal at a frequency f
1
having a dependence upon a predetermined parameter P which includes a term c
1
P
n
where c
1
is a coefficient and n is a non-zero integer; a second oscillator, comprising a resonator for determining a frequency of the oscillator, for producing a second signal at a frequency f
2
having a dependence upon the predetermined parameter P which includes a term c
2
P
n
where c
2
is a coefficient not equal to c
1
; and a mixer for combining the first and second signals to produce an output signal of the oscillator arrangement at a frequency f
1
−f
2
; wherein f
2
=(c
1
/c
2
)f
l
whereby the output signal frequency has substantially zero dependence on P
n
.
For example, the oscillators can comprise crystal oscillators the resonators of which comprise surface or bulk acoustic wave devices.
The first and/or second oscillator can include a frequency divider for producing the first and/or second signal, respectively, from an oscillation frequency of the oscillator determined by the resonator.
In particular, the predetermined parameter P can comprise temperature. For example, the resonators of the oscillators can be selected to have a predominantly linear dependence of frequency upon temperature, with n=1, or they can be selected to have a substantially zero first-order dependence of frequency upon temperature, with n=2, or they can be selected to have a predominantly third-order dependence of frequency upon temperature, with n=3.
A cascade of such oscillator arrangements can be provided in order to reduce temperature dependence for different powers of temperature, i.e. for a plurality of values of n. Thus the invention also provides an oscillator arrangement comprising a first oscillator arrangement as recited above for which the first and second oscillators comprise second and third, respectively, oscillator arrangements each as recited above, n having a first value for the first oscillator arrangement and a second value, different from the first value, for both of the second and third oscillator arrangements.
Such an oscillator arrangement can include one or more frequency dividers each for producing a signal frequency supplied to a respective mixer from a respective oscillator. The mixers and frequency dividers are conveniently constituted by a programmable digital circuit.
Another aspect of the invention provides an oscillator arrangement comprising two oscillators for producing respective signals at two different frequencies each of which has a respective dependence upon a parameter in accordance with a polynomial with respective coefficients which are different for the two oscillators, a product of one of the frequencies and a selected one of the coeffic
Beaudin Steve A.
Xu Hongwei
Kinkead Arnold
Nortel Networks Limited
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