Oscillators – Frequency stabilization – Temperature or current responsive means in circuit
Reexamination Certificate
1999-12-23
2001-08-07
Grimm, Siegfried H. (Department: 2817)
Oscillators
Frequency stabilization
Temperature or current responsive means in circuit
C331S1160FE, C331S158000
Reexamination Certificate
active
06271736
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a digital temperature compensating crystal oscillator. More specifically, the present invention relates to a digital temperature compensating crystal oscillator, and a method for stabilizing the frequency thereof, in which not only the vibration phenomenon caused by the conversion of analog signals to digital signals in spite of a constant abient temperature is decreased, but also the output frequency vibrations caused by the noises of analog devices are also decreased, thereby improving the stability and the reliability.
2. Description of the Prior Art
Generally, the temperature compensating crystal oscillator is constituted such that the highly stable quartz oscillator is provided with a temperature compensating means, so that the variations of the frequency due to the abient temperature can be compensated. This crystal oscillator was embodied in the analog form at first.
However, this analog type temperature compensating crystal oscillator showed a limit to meeting the miniaturization-light weight trend. At present, therefore, there is provided a digital method consisting of a couple of individual devices such as an integrated circuit and a crystal oscillator. An example of this digital temperature compensating crystal oscillator is illustrated in FIG.
1
.
As shown in
FIG. 1
, the digital temperature compensating oscillator includes: a temperature sensing section
11
for sensing the abient temperature to output voltage signals; an analog/digital converting section
12
for converting the temperature voltages of the temperature sensing section
11
to digital data; a memory section
13
for storing oscillation compensating data for different temperatures, and for outputting oscillation compensating data, with the output data of the analog/digital converting section
12
serving as the address; a capacitor array section
14
with a plurality of capacitors connected through switching devices respectively, and with the capacitors circuitally connected in accordance with the output oscillation compensating data of the memory section
13
so as to form the required capacitance; and an oscillating section
15
including crystal oscillating elements to form a crystal oscillating circuit so as to a frequency fo, the crystal oscillating elements being connected to the capacitor array section
14
.
As shown in
FIG. 8
a
, the temperature sensing section
11
includes: a start-up circuit
311
for activating the following circuits upon supplying a power; a constant current generating section
312
for being activated by the activating circuit
311
to generate a constant current; a temperature sensing circuit
313
with its current being always constant owing to the constant current generating section
312
, and with its voltage being varied by the ambient temperature; and an output amplifying circuit
314
for amplifying the voltage V
1
of the temperature sensing circuit
313
to a certain level.
Here, the output temperature sensing signals of the temperature sensing section
11
are voltage signals of a certain range (e.g., 0-3 V).
As shown in
FIG. 8
b
, the analog/digital converting section
12
includes: a reference voltage generating section
321
for outputting 2
n
reference voltages (where n is the number of the bits of the converted digital data) after distinguishing the ranges of the output voltages of the temperature sensing section
11
; a switching section
322
consisting of a plurality of switch circuits with their one ends being connected to a respective output terminal of the reference voltage generating section
321
, and with their other ends being commonly connected together, for selecting one of the output voltages; a comparing section
323
for outputting a difference between the output voltage of the switching section
322
and the output voltage B
temp
of the temperature sensing section
11
; and an SAR section
324
for carrying out a switching to supply the reference voltages sequentially to the comparing section
323
upon supplying the power, so as to output the relevant digital data in the form of a converted data D
ADC
when the output value of the comparing section
323
becomes 0.
The memory section
13
may consist of an EPROM.
As shown in
FIG. 8
c
, the capacitor array section
14
includes a plurality of capacitors C
11
-C
1m
connected in parallel. The respective capacitors are grounded through switching devices Q
1
-Qm which are operated by the output data D
0
-DM of the memory section
13
.
As shown in
FIG. 8
d
, the oscillating section
15
includes: a crystal element X-ta1, a plurality of capacitor devices and an MOS transistor. Here, the common contact points A of the capacitor array section as shown in
FIG. 8
c
are connected respectively to, both terminal P
1
and P
2
of the crystal oscillating element X-tal of the oscillating section
15
.
Here, as another embodiment, the digital temperature compensating oscillator includes a digital/analog converter and a varactor diode instead of the capacitor array
14
. The memory section
13
is stored with control voltages which carry out controls in relation with the ambient temperature, the control voltages being supplied to the varactor diode. Thus the voltage which is supplied to the varactor diode is varied in accordance with the sensing temperature of the temperature sensor, so that the oscillation frequency can be controlled.
At any case, however, the temperature sensing section and the analog/digital converter are necessarily provided. These two devices have the analog characteristics, and therefore, there is generated an error of ±2 LSB in the output data of the analog/digital converter due to the undesired noise from the designing stage.
Further, when the temperature voltages V
temp
which are analog signals are converted into digital signals, the temperature voltages V
temp
which are analog signals corresponding to the ambient temperatures are sampled at certain units to convert them into digital data D
ADC
as shown in
FIG. 2
a
. Therefore, as shown in
FIG. 2
b
, a boundary voltage Va exists which corresponds to both of the two temperature data C
n−1
and C
n
. At this boundary temperature voltage, two digital data can be produced. Accordingly, if the output voltage of the temperature sensing section
11
is the boundary voltage Va during the digital conversion, a digital data D
n−1
or a digital data D
n
can be outputted from the analog/digital converting section
12
. Therefore, the output oscillation frequency of the oscillating section
15
can be either f1 or f2.
Therefore, when the digital temperature compensating oscillator converts the analog temperature voltages to digital data, if the ambient temperature lies at the boundary of the resolution of the analog/digital converter, the sampled temperature data can be continuously varied, and therefore, the output frequency is seriously influenced.
In order to solve the problem of the boundary value, Motorola Pendulum-LV and Chronos-LV User's Guide proposes the use of an anti-dither circuit. In this vibration eliminating circuit, only if the temperature data which have been continuously obtained by a certain number of times through the analog/digital converter are all different from the previously stored temperature data, then a new temperature data is outputted. In this method, therefore, only if the temperature sampled data which have been continuously obtained more than a certain number of times are different from the previous temperature, then the frequency compensation with respect to the temperature variation is realized.
However, in this oscillator of the Motorola company in which the above described method is applied, there is adopted a temperature compensating code of 4 bits. Therefore, no influence is received from the variations of the adjacent temperature codes during the sampling. However, if the data hits are increased to more than 10 bits to improve the precision in accordance with the request of user
Grimm Siegfried H.
Renner , Otto, Boisselle & Sklar, LLP
Samsung Electro-Mechanics Co. Ltd.
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