Rubidium atom oscillator generating a stable frequency

Details Rubidium atom oscillator generating a stable frequency Rubidium atom oscillator generating a stable frequency

2001-09-27

2003-05-27

Pascal, Robert (Department: 2817)

Oscillators

Automatic frequency stabilization using a phase or frequency...

Molecular resonance stabilization

C331S094100

Reexamination Certificate

active

06570455

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to rubidium atom oscillators and, more particularly, to a rubidium atom oscillator used for a reference frequency source for broadcasting, a clock source established in a subordinate office of a lower part of a main office which constitutes a digital synchronous network or a clock source provided in a base station of mobile communications.
In recent years, in the market of a source of reference frequency, there is a demand for a small, low cost, high performance rubidium atom oscillator. In order to realize a rubidium atom oscillator that satisfies such requirements, simplification of circuits and selection of low cost parts of a rubidium atom oscillator are advanced. Consequently, as for a voltage controlled crystal oscillator (VCXO) used as a slave oscillator, a small general-purpose rubidium atom oscillator having a large manufacturing deviation and a large degradation of an output frequency has been used. Accordingly, it is desired to make an improvement with respect to stability in frequency or influence of circumference noise.
2. Description of the Related Art
FIG. 1
shows a composition of a conventional rubidium atom oscillator. The rubidium atom oscillator shown in
FIG. 1
comprises: a voltage controlled crystal oscillator (VCXO)
80
which oscillates a frequency of about 10 MHz; a frequency synthesis part
81
; a low-frequency oscillator
82
; an atomic resonator
83
which uses rubidium atoms; a preamplifier
84
; a synchronous wave detector
85
; an alarm (ALM) circuit
86
which generates an alarm when a resonance signal output from the preamplifier
84
is not detected; a sweep circuit
87
which generates a sweep waveform to VCXO
80
; a switching circuit
88
which is controlled by an output of the alarm circuit
86
so as to select a synchronized signal from the synchronous wave detector
85
when an alarm output is detected and to select a sweep signal from the sweep circuit
87
when the alarm output is not detected; and an integrator
89
which integrates the output of the switching circuit
88
so as to generate a control voltage for VCXO
80
.
A description will now be given of an operation of the above-mentioned rubidium atom oscillator. An output of VCXO
80
is supplied to outside as an output (Rb-OSC) of the rubidium atom oscillator, and also supplied to the frequency synthesis part
81
. The frequency synthesis part
81
synthesizes and multiplies the output frequency of VCXO
80
so as to generate the atomic resonance frequency (6.8346282 . . . GHz). Moreover, the frequency synthesis part
81
performs phase modulation by an output signal of the low-frequency oscillator
82
. The low-frequency oscillator
82
oscillates a frequency of 100-200 Hz. The output of the frequency synthesis part
81
is supplied to the atomic resonator
83
.
FIG. 2
shows the internal composition of the atomic resonator
83
. As shown in
FIG. 2
, the atomic resonator
83
includes: a magnetic shield case
83
′ accommodating the atomic resonator; a lamp house
830
accommodating a rubidium lamp
832
; a high-frequency source
831
; a cavity
833
which constitutes a cavity resonator; a resonance cell
834
in which rubidium atoms (gas) are filled; a photodiode
835
which detects a rubidium light; and a microwave excitation antenna
836
.
An atomic resonator is accommodated in the magnetic shield case
83
′. The lamp house
830
and the cavity
833
are temperature-controlled at 90 degrees and 70 degrees, respectively. The rubidium lamp
832
provided inside the lamp house
830
emits a light by electrodeless discharge caused by high-frequency excitation of rubidium atoms (gas) being carried out by the high-frequency source
831
. The cavity
833
is tuned to the atomic resonance frequency (=6.8346 . . . GHz), and the microwave output from the frequency synthesis part
81
(refer to
FIG. 1
) is emitted from the microwave excitation antenna
836
. The microwave is irradiated to the rubidium atom enclosed in the resonance cell
834
. The photodiode
835
detects the light of the rubidium lamp
832
which passed through the resonance cell
834
. If the frequency of the microwave irradiated to the rubidium atoms matches the resonance frequency of rubidium atom, an amount of light received by the photodiode decreases due to a light-microwave double resonance, thereby, generating a resonance signal (a reduction in the amount of light is regarded as a detection of a resonance signal).
Returning to
FIG. 1
, the preamplifier
84
amplifies the output of the photodiode
835
. The amplified output is supplied to the synchronous wave detector
85
as an atomic resonance output, and also supplied to the alarm circuit
86
. Based on existence of the resonance signal in the output of the atomic resonator
83
, the alarm circuit
86
distinguishes the states of frequency lock and unlock, and outputs an alarm signal to outside. The switching circuit
88
switches the signal to be supplied to the integrator
89
according to the alarm signal. That is, the switching circuit
88
selects the output of the synchronous wave detector
85
in a non-alarm state in which the resonance signal is detected. On the other hand, the switching circuit
88
selects an output of the sweep circuit
87
which generates a voltage which carries out the sweep of the output frequency of VCXO
80
in a state in which the resonance signal has not been detected. The output of the switching circuit
88
is supplied to the integrator
89
. The integrator
89
integrates the input signal, and changes the input signal into a control signal.
The synchronous wave detector
85
carries out synchronous detection of the resonance signal generated by the atomic resonator
83
by the output frequency of the low-frequency oscillator
82
, i.e., the same frequency as the phase modulation in the frequency synthesis part
81
. The integrator
89
smoothes the output of the switching circuit
88
into a direct-current signal, and outputs the directcurrent signal as an error signal. By applying the error signal output from the integrator
89
to VCXO
80
as a frequency control voltage, the output frequency of VCXO
80
is kept equal to the resonance frequency of rubidium atoms with respect to stability of frequency (a frequency lock is carried out).
As mentioned above, VCXO is used for the conventional rubidium atom oscillator. Since VCXO enables a frequency variable by an external control voltage, a change in the frequency of VCXO, which is caused by a change in an outside environment, such as temperature, a power supply, and noise, or aging, is large as compared with the crystal oscillator (XO) of a fixed frequency output. Such a characteristic change is especially large in a general-purpose small VCXO that has come to be used in recent years.
FIG. 3
is a graph showing changes in the characteristics of VCXO and XO with passage of time. In
FIG. 3
, a horizontal axis expresses lapsed days (day), and a vertical axis expresses a rate of change in frequency (&Dgr;f/f
0
). Af is a change in frequency and f
0
is a basic frequency of crystal oscillators. It can be appreciated from the graph of
FIG. 3
that the change in the characteristic of VCXO with the passage of time is larger than that of XO.
In order to correct such a frequency change and aging of VCXO, generally, a more steep frequency variable characteristic is given to VCXO. For this reason, the frequency stability of VCXO tends to be influenced by a circumference noise, etc. Therefore, when a rubidium atom oscillator is constituted using VCXO, there is a problem in that VCXO becomes a major cause of a characteristic degradation such as degradation in the short-term stability (a rate of stabilization within a short time) of a rubidium atom oscillator or phase noise degradation (instability due to phase change).
Furthermore, since an amount of change with the passage of time is large, the frequency of VCXO is swept during a period (about 10-30 minutes) until a

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