Oscillators – Electromechanical resonator – Crystal
Reexamination Certificate
2002-07-26
2004-05-18
Nguyen, Linh M. (Department: 2816)
Oscillators
Electromechanical resonator
Crystal
C331S176000
Reexamination Certificate
active
06737928
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a temperature-compensated crystal oscillator (TCXO), and more particularly to a temperature-compensated crystal oscillator with reduced phase noise.
2. Description of the Related Art
Temperature-compensated crystal oscillators are used as a reference frequency source in mobile communication devices such as cellular phone terminals or the like, for example, because they are capable of compensating for the frequency vs. temperature characteristics thereof due to the crystal unit for increased frequency stability. In recent years, there has been a demand for a temperature-compensated crystal oscillator with reduced phase noise for the purpose of maintaining desired communication quality in digital communications.
FIG. 1
shows a circuit arrangement of a conventional temperature-compensated crystal oscillator. As shown in
FIG. 1
, the conventional temperature-compensated crystal oscillator generally comprises a crystal oscillator and a temperature compensating mechanism, which are integrated in an IC (integrated circuit) chip. The crystal oscillator has crystal unit
1
as an inductor and a pair of voltage-variable capacitive elements
2
a
,
2
b
connected respectively to the opposite ends of crystal unit
1
, the voltage-variable capacitive elements
2
a
,
2
b
doubling as oscillating capacitors. Crystal unit
1
comprises, for example, an AT-cut quartz-crystal blank whose frequency vs. temperature characteristics is represented by a cubic curve nearly at the room temperature. Each of voltage-variable capacitive elements
2
a
,
2
b
typically comprises a variable-capacitance diode. Voltage-variable capacitive elements
2
a
,
2
b
and crystal unit
1
jointly make up a resonance circuit. Voltage-variable capacitive elements
2
a
,
2
b
have respective anodes connected to a ground potential as a reference potential and respective cathodes to which there is applied a temperature compensating voltage V
c
via respective resistors
6
a
,
6
b
which serve to cut off high frequency components.
Inverting amplifier
4
with feedback resistor
3
connected thereacross is connected across crystal unit
1
for amplifying the resonance frequency component of the resonance circuit. Inverting amplifier
4
should preferably comprise a CMOS (Complementary Metal Oxide Semiconductor) inverter. DC-blocking capacitors
5
a
,
5
b
are provided respectively to input and output terminals of inverting amplifier
4
. The temperature-compensated crystal oscillator produces an output voltage V
o
from the junction between capacitor
5
b
and crystal unit
1
. The temperature-compensated crystal oscillator may be summarized as a crystal oscillator with voltage-variable capacitive elements
2
a
,
2
b
inserted in its closed oscillation loop. The frequency vs. temperature characteristics of the crystal oscillator is represented by a cubic curve because of the characteristics of the crystal unit.
The temperature compensating mechanism generates a low-level detected-temperature signal in response to the ambient temperature based on, for example, the temperature vs. resistance characteristics of a resistor in the IC chip, and generates the temperature compensating voltage V
c
from a constant voltage source based on or amplifying the detected-temperature signal. As shown in
FIG. 2
, the temperature compensating voltage V
c
has temperature vs. voltage characteristics having a reference voltage V
co
at 25° C., which corresponds to the frequency vs. temperature characteristics of the crystal oscillator, and is represented by a cubic curve superposed on the reference voltage V
co
. Various circuits for generating the temperature compensating voltage V
c
are known to those skilled in the art.
When the temperature compensating voltage V
c
from the temperature compensating mechanism is applied to the cathodes of voltage-variable capacitive elements
2
a
,
2
b
, the capacitances across these voltage-variable capacitive elements change. Since the equivalent series capacitance as viewed from crystal unit
1
also changes, a change in the frequency vs. temperature characteristics of the crystal oscillator can be compensated for and the frequency vs. temperature characteristics can be made flat by thus applying the temperature compensating voltage V
c
which corresponds to the frequency vs. temperature characteristics of the crystal oscillator.
The temperature compensating voltage V
c
is of a positive potential to apply a reverse voltage to the cathodes of voltage-variable capacitive elements
2
a
,
2
b
, so that the capacitances across voltage-variable capacitive elements
2
a
,
2
b
will be reduced in inverse proportion to the applied voltage. That is, the capacitances across voltage-variable capacitive elements
2
a
,
2
b
are changed by the reverse voltage applied thereto, with no current flowing between the anodes and cathodes of voltage-variable capacitive elements
2
a
,
2
b.
However, the above crystal oscillator suffers the following problems by applying the temperature compensating voltage V
c
to the cathodes of voltage-variable capacitive elements
2
a
,
2
b
. As shown in
FIG. 3
, the output signal V
o
of the temperature-compensated crystal oscillator has a waveform represented by a high-frequency voltage superposed on the temperature compensating voltage V
c
. The waveform of the output signal V
o
has an upper limit V
max
and a lower limit V
min
. In order to keep the cathode of the voltage-variable capacitive element positive with respect to the anode thereof, the lower limit V
min
of the output signal V
o
has to be greater than the potential (0 V in
FIG. 3
) of the anode. Therefore, the temperature compensating voltage V
c
which is typified by the reference voltage V
co
needs to be of a value depending on the amplitude level of the output signal, i.e., a value with the half value (V
max
+V
min
)/2 of the amplitude level being added as an offset voltage thereto. However, as described above, the temperature compensating voltage V
c
is generated from the constant voltage source based on or amplifying the low-level detected-temperature signal according to the temperature vs. resistance characteristics of the resistor. The voltage signal generated by the constant voltage source generally contains more noise as the voltage value thereof is higher. Consequently, since the conventional temperature-compensated crystal oscillator needs to set the temperature compensating voltage V
c
(or the reference voltage V
co
) to a higher value depending on the output signal thereof, the output signal contains large noise, deteriorating the phase noise characteristics of the temperature-compensated crystal oscillator.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a temperature-compensated crystal oscillator with good phase noise characteristics.
According to the present invention, the above object can be achieved by a temperature-compensated crystal oscillator comprising a crystal unit having frequency vs. temperature characteristics, a voltage-variable capacitive element inserted in a closed oscillation loop including the crystal unit, an amplifier for keeping oscillation in the closed oscillating loop, means for applying a temperature compensating voltage to an anode of the voltage-variable capacitive element, and means for applying a voltage to prevent a current from flowing through the voltage-variable capacitive element to a cathode of the voltage-variable capacitive element, whereby the frequency vs. temperature characteristics can be compensated for by the temperature compensating voltage applied to the anode of the voltage-variable capacitive element.
Because the temperature compensating voltage is applied to the anode of the voltage-variable capacitive element, the voltage applied to the cathode of the voltage-variable capacitive element is relatively increased to apply a reverse voltage to the voltage-variable capacitive element, thus preventing a direct cur
Asamura Fumio
Kubo Kuichi
Katten Muchin Zavis & Rosenman
Nguyen Linh M.
Nihon Dempa Kogyo Co. Ltd.
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