Oscillator that uses thermostatic oven

Details Oscillator that uses thermostatic oven Oscillator that uses thermostatic oven

2001-03-01

2002-08-13

Walberg, Teresa (Department: 3742)

Electric heating

Heating devices

Combined with diverse-type art device

C330S010000

Reexamination Certificate

active

06433309

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field to Which the Invention Pertains
This invention relates to an oscillator wherein a thermostatic oven is used to stabilize the oscillation frequency, and more particularly to a quartz-crystal oscillator wherein an operational amplifier is used to control a heat source of a thermostatic oven.
2. Background Art
From among oscillators which employ a quartz-crystal element, an oven-controlled oscillator wherein a vibrator such as a quartz-crystal element is held in a thermostatic oven has a stabilized oscillation frequency against a variation of the ambient temperature because an oscillation circuit operates with the quartz-crystal element or the like kept at a constant temperature. The oven-controlled oscillator is used particularly for applications for which a high frequency stability is required such as a comparatively high grade communication apparatus used in a base station of a mobile communication system, for example.
The oven-controlled oscillator usually includes a quartz-crystal element, a thermostatic oven which includes an electric heater and in which the quartz-crystal element is accommodated, and a heat source control circuit for controlling the heater of the thermostatic oven.
FIG. 1
shows a general circuit configuration of a conventional oven-controlled oscillator.
The oven-controlled oscillator includes crystal oscillation circuit
10
, thermostatic oven
1
in which quartz-crystal element
3
is accommodated, heater
6
for heating the inside of thermostatic oven
1
, and heat source control circuit
2
for controlling heater
6
. Heat source control circuit
2
includes thermistor RT
1
thermally coupled to thermostatic oven
1
as hereinafter described, and the remaining part of heat source control circuit
2
except thermistor RT
1
is referred to as control circuit unit
11
. Crystal oscillation circuit
10
is an oscillation circuit that includes quartz-crystal element
3
as a circuit element. Crystal oscillation circuit
10
is a circuit of the Colpitts type, for example, wherein a resonance circuit is formed from quartz-crystal element
3
serving as an inductor component and series capacitors and part of an output of the resonance circuit is amplified and fed back to the resonance circuit by an amplifier (transistor) so that the circuit may oscillate.
As shown in
FIG. 2
, quartz-crystal element
3
is formed from a quartz blank of, for example, an AT cut enclosed in and held by metal vessel
5
from which a pair of leads
4
are led out.
FIG. 3
illustrates a frequency-temperature characteristic of a quartz-crystal element which uses an AT cut quartz blank. As can be seen from
FIG. 3
, the AT cut quartz blank has such a frequency-temperature characteristic of a cubic curve that a point of inflection appears in the proximity of 25° C. of the room temperature and a minimal value of the frequency appears in the proximity of +70° C.
Thermostatic oven
1
is formed, as shown in
FIG. 2
described above, from heater wire
6
serving as a heat source and wound around an outer wall of metal vessel
5
of quartz-crystal element
3
. With thermostatic oven
1
having the configuration just described, metal vessel
5
is heated entirely by heater wire
6
and functions as a thermostatic oven for the quartz blank. According to circumstances, metal vessel
5
and heater wire
6
are insulated from the external air by means of a heat insulator or an adiabatic material (not shown). The internal temperature of thermostatic oven
1
is maintained constant by heat source control circuit
2
. Heat source control circuit
2
detects the internal temperature of thermostatic oven
1
by means of a temperature sensitive element such as thermistor RT
1
thermally coupled to thermostatic oven
1
and controls current to be supplied to heater
6
in response to a result of the detection to try to keep the internal temperature of thermostatic oven
1
, that is, the temperature of quartz-crystal element
3
, at a constant value. Thermistor RT
1
is disposed in the proximity of heater wire
6
, for example. The temperature of thermostatic oven
1
(quartz-crystal element
3
) when such temperature control is performed is set to a temperature that indicates a frequency minimal value on the higher temperature side of the frequency-temperature characteristic of quartz-crystal element
3
. This temperature is referred to as preset temperature.
FIG. 4
shows an example of particular circuit configuration of heat source control circuit
2
. Heat source control circuit
2
includes operational amplifier
7
operating as an inverted differential amplifier. A reference voltage produced by dividing a voltage of power supply Vcc by dividing bias resistors R
1
, R
2
is inputted to a non-inverted input terminal (+) of operational amplifier
7
. A comparison voltage obtained from dividing bias resistors R
3
, R
4
is inputted to an inverted input terminal (−) of operational amplifier
7
through resistor Ra. Here, thermistor RT
1
serving as a temperature sensitive element is used as bias resistor R
3
on power supply Vcc side. This thermistor RT
1
has such a temperature-resistance characteristic that the resistance value decreases as the temperature rises as seen in FIG.
5
. The output terminal and the inverted input terminal of operational amplifier
7
are connected to each other through feedback resistor Rb. In the present circuit, the amplification factor A of operational amplifier
7
is represented by Rb/Ra. The output of operational amplifier
7
is connected through resistor R
5
to the base of transistor
8
whose emitter is grounded. Heater wire
6
of thermostatic oven
1
is connected between the collector of transistor
8
and power supply Vcc.
In the present circuit, various circuit constants are set so that the resistance value of thermistor RT
1
at the room temperature of 25° C. is higher than that of resistor R
4
and a great difference voltage may appear between the reference voltage which depends upon resistors R
1
, R
2
and the comparison voltage which depends upon resistor R
4
and thermistor RT
1
. Consequently, when power supply is made available, the great difference voltage is amplified in accordance with the amplification factor A of operational amplifier
7
and inputted to the base of transistor
8
. Accordingly, high collector current flows to transistor
8
and a great amount of heat is generated from heater
6
. Therefore, when power supply is made available, the internal temperature of thermostatic oven
1
, i.e., the temperature of quartz-crystal element
3
, rises suddenly.
As the internal temperature of thermostatic oven
1
rises, the resistance value of thermistor RT
1
drops, and consequently, the comparison voltage decreases until it indicates a value near to the reference voltage. At this time, the internal temperature of thermostatic oven
1
reaches +70° C. corresponding to the minimal value of the frequency-temperature characteristic of the quartz-crystal element. Accordingly, the oscillation frequency varies from a frequency at the room temperature to another frequency at the minimal point based on the frequency-temperature characteristic and thereafter remains stably at the frequency at the minimal point. Usually, in order to cause the oscillation frequency to be stabilized rapidly after power supply is made available, the amplification factor A of operational amplifier
7
is set to a high value such as approximately 50 to 100, for example, so that the internal temperature of thermostatic oven
1
may rise rapidly.
In recent years, miniaturization of communication apparatus has been and is proceeding, and with the miniaturization, a quartz-crystal element of a reduced size is used popularly. Accordingly, also the heat capacity of the quartz-crystal element as thermostatic oven
1
decreases and the speed of the response of the internal temperature of the thermostatic oven to the power applied to heater
6
increases. Thus, when the internal temperature of the thermostatic ov

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