Oscillators – Automatic frequency stabilization using a phase or frequency... – With reference oscillator or source
Reexamination Certificate
2001-05-21
2003-02-04
Pascal, Robert (Department: 2817)
Oscillators
Automatic frequency stabilization using a phase or frequency...
With reference oscillator or source
C331S044000, C331S066000, C331S176000
Reexamination Certificate
active
06515548
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a temperature compensated oscillator, its manufacturing method, and an integrated circuit for a temperature compensated oscillator, and particularly relates to a temperature compensated oscillator for correcting a change in frequency, due to a circumferential temperature of the oscillator, by digital control, its manufacturing method, and an integrated circuit for temperature compensated oscillator.
2. Description of the Related Art
In various kinds of communication devices such as a portable telephone, etc., a system clock having higher frequency accuracy is desired at present as communication speed is increased. Therefore, a temperature compensated crystal oscillator is used.
FIG. 4
shows such a temperature compensated crystal oscillator. This temperature compensated crystal oscillator is constructed by externally attaching a crystal resonator XL to an integrated circuit for a temperature compensated crystal oscillator. The integrated circuit for the temperature compensated crystal oscillator is constructed by a varicap diode CV for adjusting an oscillating frequency of an oscillator circuit
41
, a temperature detector
42
, an A/D converter
43
for A/D-converting an output of the temperature detector
42
, a non-volatile memory
44
for storing compensating data for temperatures experienced by the temperature compensated crystal oscillator, a correcting value signal generating circuit
45
, and a D/A converter
46
. The correcting value signal generating circuit
45
generates a correcting value signal on the basis of the compensating data address-designated by an output of the A/D converter
43
and read from the non-volatile memory
44
. The D/A converter
46
D/A-converts the correcting value signal, and generates a control voltage of the varicap diode CV.
Temperature characteristics of the frequency accuracy of a crystal resonator (AT cut) generally used in a temperature compensated oscillator circuit can be approximated by a cubic function represented as follows.
&Dgr;
f/f=A
3
(
T−T
0
)
3
+A
1
(
T−T
0
)+
A
0
T
0
shows a reference temperature, and is different in accordance with an individual crystal together with a coefficient of the cubic function. For example, there are temperature characteristics of the frequency accuracy as shown in FIG.
3
.
The oscillating frequency of the crystal oscillator circuit is provided as follows.
f
0
=f
s
(1+1/(2
C
0
/C
1
(1
+C
L
/C
0
))
Here, f
s
, C
0
and C
1
respectively show a resonant frequency of the crystal, an equivalent parallel capacity and an equivalent series capacity, and C
L
shows a load capacity of the oscillator circuit. It should be understood from this formula that temperature compensation can be carried out by adjusting the frequency if C
L
can be changed in accordance with temperature T. The varicap diode is used as the equivalent series capacitor C
L
.
The compensating data are set as follows in such a temperature compensated crystal oscillator. The oscillator is really constructed by connecting a crystal oscillator to the integrated circuit for temperature compensated crystal oscillation, and is arranged within a constant temperature bath. Then, a voltage is applied from the exterior to the varicap diode CV every setting of a predetermined temperature. A switch
47
of
FIG. 4
is connected to the side of a terminal A in advance. An oscillating output signal of the oscillator is monitored, and a control voltage V
c
of the varicap diode for obtaining a predetermined frequency is specified, and characteristics of the D/A converter
46
at that temperature are measured. Each coefficient (A
3
, A
1
, A
0
) of the cubic function and data for adjustment of the D/A converter
46
are calculated on the basis of these measuring data, and are stored to the non-volatile memory
44
as the compensating data corresponding to each temperature.
At the actual operating time, the compensating data are read in a state in which data obtained by A/D-converting analog temperature information detected by the temperature detector
42
are set to an address of the non-volatile memory
44
. These compensating data are read and outputted to the correcting value signal generating circuit
45
so that a correcting value signal is generated. This correcting value signal is D/A-converted by the D/A converter
46
so that the control voltage V
c
of the varicap diode CV is generated. The switch
47
of
FIG. 4
is connected to the side of a terminal B, and the control voltage V
c
is applied to the varicap diode CV.
In the conventional method, the compensating data are calculated by so-called off-line processing in which the measuring data at each temperature are once stored to an external device and are separately processed. Thereafter, the compensating data are written to the non-volatile memory. Therefore, it takes much cost and labor.
Further, temperature characteristics of the temperature detector
42
, the A/D converter
43
, the D/A converter
46
and the varicap diode CV, and frequency-temperature characteristics of the crystal resonator XL are individually different from each other. These different temperature characteristics are corrected by approximate values so that it is difficult to improve combination accuracy.
It is also very difficult to reduce cost and improve compensation accuracy since absolute exactness of temperature setting is required to extract the compensating data.
SUMMARY OF THE INVENTION
Therefore, an object of the invention is to provide a temperature compensated oscillator for which it is unnecessary to accurately set and detect temperature at a manufacturing time, and reduce manufacture cost of the temperature compensated oscillator.
In the invention, a temperature compensated oscillator has a frequency comparing circuit for comparing the frequency of an oscillating output signal of it and the frequency of an external reference frequency signal externally inputted, and also has a register for determining each bit on the basis of results of this comparison. A digital signal from the register is supplied to a D/A converter for generating a control voltage of a variable capacity element for adjusting the frequency of the oscillating output signal. A self compensating operation for sequentially determining each bit of the register on the basis of the comparing results of every comparison, and for conforming the frequency of the oscillating output signal to the frequency of the external reference frequency signal is provided. When the frequency of the oscillating output signal is conformed to the frequency of the external reference frequency signal, the digital signal from the register is set as compensating data corresponding to a detected temperature of a temperature detector at that time. The compensating data are determined by performing the self compensating operation every predetermined temperature change detected by the temperature detector. Thus, off-line processing is removed, and it is possible to reduce cost and labor for controlling an absolute temperature at a manufacturing time.
Further, combination accuracy can be also improved by correcting the characteristics of all elements in total.
Further, proper compensating data can be extracted by self-detecting a temperature change at a setting time of the compensating data so that no strictness of temperature setting is required. Therefore, it is expected that total cost performance of an adjustment is greatly improved.
A temperature compensated oscillator of the invention comprising a temperature detector for outputting an analog signal according to temperature, an A/D converter for converting the analog signal from the temperature detector to a digital signal, a memory from which compensating data are read using the digital signal from the A/D converter as an address, a D/A converter for converting the compensating data from the digital signal to the analog signal, a variable capacity element set by the analog signa
Matsumoto Toru
Takahashi Masayuki
Glenn Kimberly E
Jordan and Hamburg LLP
Nippon Precision Circuits Inc.
Pascal Robert
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