Oscillators – Frequency stabilization – Temperature or current responsive means in circuit
Reexamination Certificate
2002-09-23
2004-09-07
Kinkead, Arnold (Department: 2817)
Oscillators
Frequency stabilization
Temperature or current responsive means in circuit
C331S158000, C331S1160FE, C331S179000, C331S066000, C331S17700V, C331S03600C
Reexamination Certificate
active
06788159
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a temperature compensated oscillator, an adjusting method thereof, and an integrated circuit for temperature compensated oscillator. The invention more particularly relates to a temperature compensated oscillator that corrects frequency fluctuations caused by changes in ambient temperatures by digital control, an adjusting method thereof, and an integrated circuit for temperature compensated oscillator.
2. Description of the Prior Art
A conventional system having a timer function such as a clock and an RTC (Real Time Clock) normally uses a crystal oscillator with a tuning fork crystal resonator operating at 32.768 KHz or an AT cut crystal resonator operating at 4.194304 MHz.
The tuning fork crystal resonator operating at 32.768 KHz can oscillate with low current consumption but suffers from disadvantageous frequency fluctuations caused by temperature changes. Meanwhile, heat from the user's body can change the temperature of the outer part of a watch, but the surrounding part of the resonator is kept at a relatively fixed temperature, and therefore the low current consumption characteristic of the tuning fork resonator can be advantageous in this application.
Conversely, the AT cut crystal resonator operating at 4.194304 MHz suffers less from frequency fluctuations caused by temperature changes than the tuning fork type, while it requires large current for oscillation.
Conventionally, the advantages and disadvantages are balanced against each other, and either the tuning fork type or the AT cut type has been used. In recent years, however, with increasing demands for lower power consumption systems, the AT cut type resonator has been less frequently used. As a result, the disadvantage of the tuning fork type resonator in association with the temperature characteristic has been more often pointed out.
The frequency-temperature characteristic of the tuning fork type crystal resonator can generally be approximated by the following quadric:
&Dgr;
f/f=A
2
(
T−T
0
)
2
+A
0
where T
0
represents the reference temperature that as well as the coefficient of the quadric varies among crystals.
FIG. 16
shows an example of a frequency-temperature characteristic.
Meanwhile, the oscillation frequency of a crystal oscillation circuit is as follows:
f
0
=f
s
(1+1/(2
C
0
/C
1
(1+
C
L
/C
0
))
where f
s
, C
0
, and C
1
represent the resonance frequency, equivalent parallel capacitance, and equivalent series capacitance of the crystal resonator, respectively. C
L
represents the load capacitance of the oscillation circuit. From the equation, assuming that load capacitance C
L
is variable with temperature T, the frequency can be adjusted, so that temperature compensation can be carried out. An example of the variable characteristic of the frequency depending upon load capacitance C
L
is shown in FIG.
17
.
As can be understood from
FIG. 16
, in the temperature range from −35° C. to 85° C., a frequency deviation in the range from −20 ppm to 200 ppm at most must be compensated. Meanwhile, from
FIG. 17
, around the standard value of load capacitance C
L
, 6 pF, load capacitance C
L
must be controlled to be in the range from 1.9 pF to 7.2 pF to achieve the above compensation. In practice, the parasitic capacitance of the input/output portion of the IC is about 1 pF (at the pad or the protection circuit), and therefore as only for the variable capacitance element, the capacitance must be controlled between 0.9 pF to 6.2 pF. At present, it is substantially impossible to control the capacitance ratio of 6.2 pF/0.9 pF=6.89 using a variable capacitor stored in the IC, and with low power supply voltage.
What is generally practiced at present is as follows. The load capacitance C
L
of an oscillation circuit
181
shown in
FIG. 18
is made of a capacitor array
182
as shown in FIG.
19
. Switching elements SW
0
to SW
n−1
are turned on/off to selectively connect capacitive elements C
0
to C
n-1
to the oscillation circuit
181
and control load capacitance C
L
. An analog signal output from a temperature detector
183
is converted into a digital signal by an A/D converter
184
, compensation data is read out from a memory
185
using the digital signal as an address, and the switching elements SW
0
to SW
n-1
are turned on/off based on the compensation data.
Compensation is carried out for each predetermined temperature step, and therefore errors on both limits of the temperature range increase when the temperature range expands. In order to reduce the errors, the temperature step width must be reduced, which increases the bit number of the memory
185
. This is illustrated in
FIGS. 20 and 21
. The minimum control unit of the capacitor array
182
in
FIG. 19
must also be reduced, which also increases the bit number of the memory
185
.
According to the conventional method, the load capacitance (capacitors) C
L
of the oscillation circuit is arranged in an array form and controlled by turning on/off the switching elements. According to this method, in order to improve the adjusting precision, the area size of the capacitor array and the bit number of memory are inevitably increased. This makes it difficult to reduce the cost.
SUMMARY OF THE INVENTION
It therefore an object of the present invention to provide a temperature compensated oscillator keeping the area of the capacitor array and the bit number of the memory from increasing and allowing for high precision and an adjusting method thereof.
Preferably, a temperature compensated oscillator according to the present invention includes a temperature detector that outputs an analog signal depending on a temperature, an A/D converter for converting the analog signal from the temperature detector into a digital signal, a memory from which compensation data is read out using the digital signal from the A/D converter as an address, a capacitor array for selectively connecting a plurality of capacitive elements to an oscillation circuit based on the compensation data, the oscillation circuit causing a resonator such as a crystal resonator to oscillate thereby generating an oscillation output signal, and using the capacitor array as a frequency adjusting element for the oscillation output signal, a frequency comparison circuit for comparing the frequencies of an externally input reference frequency signal and the oscillation output signal, a register in which the value of each bit is sequentially determined based on the frequency comparison result from the frequency comparison circuit, a switching circuit for selectively supplying the compensation data read out from the memory and a digital signal output from the register to the capacitor array, a voltage variable capacitive element connected to the capacitor array, and a control voltage generation circuit for generating control voltage to control the capacitance of the voltage variable capacitive element in response to the analog signal from the temperature detector. The digital signal output from the register is supplied to the capacitor array through the switching circuit and the plurality of capacitive elements are connected to the oscillation circuit in response to the digital signal, so that the oscillation circuit carries out oscillation operation. The value of each bit in the register is sequentially determined based on the comparison result for each comparison operation by the frequency comparison circuit to change the frequency of the oscillation output signal. The digital signal output from the register when the frequency of the oscillation output signal from the oscillation circuit is matched with a particular frequency is written in the memory as the compensation data corresponding to the detection temperature that can be addressed using the digital signal output from the A/D converter corresponding to the temperature detected by the temperature detector at the time, so that the writing operation is carried out for each temperature st
Matsumoto Toru
Takahashi Masayuki
Jordan and Hamburg LLP
Kinkead Arnold
Nippon Precision Circuits Inc.
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