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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a temperature detector using crystal
resonators and a temperature compensated oscillator having the temperature
detector.
2. Description of the Prior Art
A temperature compensated oscillator has a voltage controlled oscillator
(VCO) which is a main oscillator, a temperature detector for detecting the
temperature around the main oscillator, and a control voltage generating
circuit for generating a control voltage from the detected temperature,
which maintains the oscillation frequency of the VCO constant by
compensating for the changes in ambient temperature and applying such a
control voltage to the VCO. The temperature detector has an oscillator
with a piezoelectric resonator whose oscillation frequency varies with its
temperature. Temperature is detected in terms of the oscillation frequency
of the oscillator.
One example of such a temperature detector using the combination of an AT
cut and a Y cut crystal resonators is described in a paper entitled "LOW
PROFILE HIGH STABILITY DIGITAL TCXO; ULTRA LOW POWER CONSUMPTION TCXO" by
V. Candelier et al., Proceedings of the 43rd Annual Symposium on Frequency
Control, 1989, pp. 51-54. An AT cut crystal resonator has a small
temperature coefficient of natural oscillation frequency while a Y cut
crystal resonator has a large temperature coefficient of natural
oscillation frequency. In such a case, the temperature detector counts the
output pulses of the oscillator implemented by the AT cut crystal
resonator while the output of the oscillator implemented by the Y cut
crystal resonator is in a high level. The temperature detector resets the
count to zero every predetermined period while delivering the count at the
time just before resetting as a detected temperature signal.
Another example of the temperature detector using a single crystal
resonator is disclosed in a paper entitled "FACTORS INFLUENCING STABILITY
IN THE MICROCOMPUTER-COMPENSATED CRYSTAL OSCILLATOR" by A. Benjaminson,
Proceedings of the 44th Annual Symposium on Frequency Control, 1990, pp.
597-614. This detector has a single SC cut crystal resonator for
temperature detection and excites the resonator by two oscillation
circuits assigned to, respectively, a fundamental harmonic and a higher
harmonic. As a result, the detector generates oscillator outputs, one is a
fundamental harmonic and the other is a tertiary higher harmonic. A signal
representative of a frequency difference between the two oscillation
outputs is used as a detected temperature signal.
The problem with the above-described dual resonator scheme is that the
temperature detection error increases transiently when the temperature is
sharply changed due to the turn-on of a power source or a sharp change in
ambient temperature. Specifically, to enhance accurate temperature
detection, a large ratio is usually selected between the natural frequency
(fA) of the AT cut resonator and the natural frequency (fY) of the Y cut
resonator (e.g. fA=1 MHz and fY=10 kHz). As a result, the Y cut resonator
has a far larger volume and a far larger heat capacity than those of the
AT cut resonator. Therefore, the two resonators are noticeably different
in thermal time constant. When the temperature is sharply changed, a
temperature detection error ascribable to the temperature difference
between the two resonators is not avoidable during the transient period to
a steady temperature state.
On the other hand, the single SC cut crystal resonator scheme is free from
the temperature detection error ascribable to the difference in thermal
time constant. However, this conventional scheme has a drawback that the
oscillation circuit for generating the tertiary higher harmonic increases
power consumption. Specifically, the power consumption by the
semiconductor devices constituting the oscillation circuits and the entire
circuitry including them increases substantially in proportion to
oscillation frequency. Hence, the oscillation circuit assigned to the
tertiary higher harmonic consumes almost three times greater power than
the oscillation circuit assigned to the fundamental harmonic. It follows
that the total power consumption is almost four times greater than that of
the oscillation circuit assigned to the fundamental harmonic.
BRIEF SUMMARY OF THE INVENTION
1. Object of the Invention
It is, therefore, an object of the present invention to provide a
temperature compensated oscillator and a temperature detector which are
free from the above-discussed temperature detection errors ascribable to a
difference in thermal time constant and consumes a minimum of power.
2. Summary of the Invention
In accordance with the present invention, a temperature compensated
oscillator has a pair of AT cut crystal resonator pieces having
substantially the same natural oscillation frequency and different cut
angles from each other, a case member holding the resonator pieces in an
oscillatable state, electrode films formed on opposite surface of each
resonator piece, and leads connecting the electrode films to terminals
disposed located outside the case member. The resonator pieces are each
connected to respective one of oscillation circuits to be excited by the
latter, thereby producing two different oscillation outputs. A difference
in frequency between the two oscillation outputs is representative of a
detected temperature. The two resonator pieces have substantially the same
natural oscillation frequency and, therefore, substantially the same
dimensions and time constant. Since the resonator pieces are accommodated
in a single case, their temperature remain the same at all times to
eliminate detection errors.
BRIEF DESCRIPTION OF THE DRAWINGS
The above-mentioned and other objects, features and advantages of the
present invention will become more apparent by reference to the following
detailed description of the invention taken in conjunction with the
accompanying drawings, wherein:
FIG. 1 is a block diagram showing a temperature detector embodying the
present invention;
FIG. 2 is a graph representative of natural oscillation frequency to
temperature characteristics of crystal resonators included in the
embodiment;
FIG. 3 is a block diagram schematically showing a temperature compensated
oscillator having a temperature detector embodying the present invention;
FIG. 4A is a section showing a specific configuration of the crystal
resonators included in the embodiment;
FIGS. 4B and 4C are perspective views each showing the construction of a
particular element included in the configuration of FIG. 4A; and
FIG. 5 is a view similar to FIG. 4A, showing another specific configuration
of the crystal resonators.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, a temperature detector of the present invention
comprises a first and a second AT cut crystal resonator Q1 and Q2, a first
and a second oscillator 51 and 52, an up-down counter 53, and a ROM 60.
The first and second resonators Q1 and Q2 have almost the same natural
frequencies f1 and f2, respectively, and have the different cut angles
from each other. For example, the cut angles of the first and the second
AT cut crystal resonators Q1 and Q2 are 35.degree.25' and 35.degree.17',
respectively. The first and second oscillators 51 and 52 include the first
and second crystal resonators Q1 and Q2 as a part of their resonating
circuit, respectively. Thus, the first oscillator 51 oscillates the first
resonator Q1 at the frequency f1 and the second oscillator 51 oscillates
the second resonator Q2 at the frequency f2. The first oscillator 51
outputs rectangle wave signal having the frequency f1 as a first
oscillated signal. In the same way, the second oscillator 52 output a
rectangle wave signal having the frequency f2 as a second oscillated
signal. The up-down counter 53 receives the first and second oscillated
signals on the count-up and count-down terminals, respectively. The
counter 53 is reset at every predetermined period, which is sufficiently
short compared with changing period of the temperature, and outputting its
counting value every predetermined period as a phase difference data
signal. The phase difference data signal fluctuates responsive to the
temperatures of the first and second crystal resonators. The reset
operation is achieved by using a reset signal which is frequency-divided
signal of one of the first and second oscillated signal.
FIG. 2 shows specific resonance frequency to temperature characteristics of
the crystal resonators Q1 and Q2. In the figure, the abscissa and the
ordinate indicate respectively temperature and relative frequency which is
the variation rate (.DELTA.f/f) of the resonance frequency. Specifically,
curves AT1 and AT2 represent respectively the changes in the resonance
frequencies of the crystal resonators Q1 (AT cut with cut angle of
35.degree.25') and Q2 (AT cut with cut angle of 35.degree.17') with
respect to temperature. Therefore, the phase difference data signal
monotonously increases (or monotonously decreases) with the elevation of
temperature. The ROM 60 stores a table showing correspondence between the
phase difference data signal and the temperature, and outputs a
temperature signal which represents the detected temperature, in response
to the received phase difference data signal.
FIG. 3 shows an embodiment of a temperature compensated oscillating circuit
of the invention. Referring to FIG. 3, the oscillating circuit comprises a
first and a second crystal resonator Q1 and Q2, a first and a second
oscillator 51 and 52, an up-down counter 53, a ROM 54, a digital-to-analog
conventing circuit (DAC) 55 and a voltage controlled oscillator (VCO) 56.
In FIG. 3, elements having the same reference numeral are the same as the
elements shown in FIG. 1. Thus, further descriptions are eliminated. The
up-down counter 53 outputs a frequency difference data signal. The ROM 54
receives the frequency difference data signal as an address signal, and
outputs a digital control voltage data signal. The DAC 55 converts the
digital control voltage data signal into an analog control voltage signal
to maintain a frequency of an output signal of the VCO 56 at a frequency
fo. The ROM 54 stores a table which is designed to offset a frequency
drift of the VCO 56.
Referring to FIGS. 4A-4C, a specific arrangement of the crystal resonators
Q1 and Q2 will be described. There are shown flat resonator pieces 1 and 2
corresponding respectively to the AT cut crystal resonators Q1 and Q2
which have substantially the same natural oscillation frequency and
different cut angles, as stated earlier. The resonator pieces 1 and 2 are
retained by support means in a vertical position and in parallel to each
other. The support means so retaining the resonator pieces 1 and 2 is
accommodated in a metallic case 8. Electrodes 3 and 4 for electrical
connection are formed on opposite sides of the resonator piece 1 by the
evaporation of gold. Likewise, electrodes 5 and 6 are provided on the
resonator piece 2 by the evaporation of gold. The resonator pieces 1 and 2
are firmly retained by the upper end of a generally T-shaped conductive
support 7 at the lower ends thereof. The conductive support 7 is disposed
in and affixed to the bottom of the case 8 at the lower end thereof. The
electrode 4 of the resonator piece 1 and the electrode 5 of the resonator
piece 2 are electrically connected to the upper end of the conductive
support 7. Elongate conductive terminals 9 and 10 extend throughout the
bottom wall of the metallic case 8 and are affixed to the latter with the
intermediary of insulating members 11 and 12, respectively. The electrodes
3 and 6 of the resonators 1 and 2, respectively, are retained by and
electrically connected to the upper ends of the terminals 9 and 10,
respectively.
To connect the assembly shown in FIGS. 4A-4C to the oscillator adapted for
temperature detection, the bottom of the metal case 8 is placed on a
circuit board, not shown, and then the terminals 9 and 10 are connected in
a predetermined manner. Since the resonator pieces 1 and 2 are located
face-to-face and each in a vertical position, the area of the bottom of
the metal case 8, i.e., the area which the case 8 occupies on a circuit
board is reduced.
FIG. 5 shows another specific configuration of the crystal resonators Q1
and Q2. In the figures, the same or similar parts and elements are
designated by like reference numerals, and redundant description will be
avoided for simplicity. As shown, the AT cut flat resonator pieces 1 and 2
are each retained in a horizontal position by support means which will be
described. The resonator pieces 1 and 2 lie in the same plane. The support
means with the resonator pieces 1 and 2 is accommodated in a metallic case
18. The resonator pieces 1 and 2 are supported by and electrically
connected to the upper end of a rod-like conductive support 17 at the ends
thereof which adjoin each other. Rod-like conductive terminals 19 and 20
extend throughout opposite side walls of the case 18 and are affixed to
the latter with the intermediary of insulating members 21 and 22,
respectively. The ends of the resonator pieces 1 and 2 which are remove
from the above-mentioned adjoining ends are retained by and electrically
connected to the inner ends of the conductive terminals 19 and 20,
respectively.
To mount the resonators 1 and 2 on a printed circuit board, the bottom of
the case 18 is placed on the circuit board, and then the outer ends of the
terminals 19 and 20 are bend downward. Since the configuration shown in
FIG. 4 has the resonator pieces 1 and 2 positioned horizontally in the
same plane, the height thereof above the circuit board is reduced. Such a
configuration is feasible for so-called planar mounting.
In the above-described specific configurations of the quartz vibrators
Q.sub.1 and Q.sub.2, the resonator pieces 1 and 2 have substantially the
same natural oscillation frequency and, therefore, substantially the same
dimensions, thus achieving substantially the same time constant. Moreover,
since the resonator pieces 1 and 2 adjoin each other within a single case,
their temperatures remain the same at all times during operation. This
eliminates temperature detection errors particular to the conventional
oscillator using an AT cut and a Y cut crystal resonator.
In addition, the oscillators 51 and 52 associated with the resonator pieces
1 and 2, respectively, operate with the fundamental harmonics having
substantially the same frequency and, therefore, consume the same amount
of power. Such oscillators 51 and 52, therefore, do not need an increase
in power due to a higher harmonic with which the conventional oscillator
using an SC cut crystal resonator deals.
While the resonator pieces 1 and 2 shown in FIGS. 4A and 5 have their
electrodes 4 and 5 provided on one side thereof commonly connected, the
electrodes 4 and 5 may be separated from each other and led to the outside
independently of each other. As shown in FIG. 1, when the crystal
resonators Q.sub.1 and Q.sub.2 and the oscillators 51 and 52 are commonly
connected to ground, wirings on a printed circuit board will be simplified
if they are commonly connected within a metallic case, as in any one of
the specific configurations described above.
Although the invention has been described with reference to specific
embodiments, this description is not meant to be construed in a limiting
sense. Various modifications of the disclosed embodiments, as well as
other embodiments of the invention, will become apparent to persons
skilled in the art upon reference to the description of the invention. It
is therefore contemplated that the appended claims will cover any
modifications or embodiments as fall within the true scope of the
invention.
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