Oscillators – Molecular or particle resonant type
Reexamination Certificate
2000-03-28
2001-10-09
Mis, David (Department: 2817)
Oscillators
Molecular or particle resonant type
C331S003000, C331S074000
Reexamination Certificate
active
06300841
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an atomic oscillator suitable for use as a standard source in broadcasting, a clock source in a digital synchronous network, etc.
2. Description of the Related Art
FIG. 6
of the accompanying drawings is a block diagram schematically showing a conventional rubidium atomic oscillator (hereinafter simply called “atomic oscillator”). The atomic oscillator
101
is divided into chiefly three blocks: a high frequency (HF) block
102
as a first block, an optical microwave unit (OMU)
103
as a second block, and a low frequency (LF) block
104
as a third block.
The HF block
102
not only generates an output frequency signal to be supplied to outside as the output of the atomic oscillator
101
, but also produces from the output frequency signal a signal from which a microwave {rubidium resonant (transitional) frequency signal of approximately 6.8346 GHz} to be produced in the OMU
103
originates. For this purpose, in an example, the HF block
102
essentially includes a voltage-controlled crystal oscillator (VCXO)
120
as a standard oscillator, a first LC tank circuit
121
, a phase modulator circuit
122
, a second LC tank (resonator) circuit
123
, an amplitude modulator circuit
124
, a matching circuit
125
, and a frequency synthesizer
126
.
The VCXO
120
generates an output frequency signal (e.g., 10 MHz) to be supplied to outside as the output of the atomic oscillator. In an example, each of the first and second LC tank (resonator) circuits (hereinafter simply called “tank circuit”)
121
,
123
includes a non-illustrated transistor amplifier, and a tank filter composed of a coil (L) and a capacitor (C) in parallel; the individual tank circuit
121
,
123
varies a bias of the transistor amplifier to distort an input frequency signal and then samples from the resultant signal a multiplied-frequency component, thus multiplying the input frequency signal by a predetermined natural number (3 for example). Firstly in the first tank circuit
121
, the output frequency signal (10 MHz for example) of the VCXO
120
is multiplied by a natural number (3, for example); this is, 10 MHz×3=30 MHz. Then in the second tank circuit
123
, the resultant output frequency signal of 30 MHz is multiplied by the same natural number; this is, 30 MHz×3=90 MHz.
In the meantime the phase modulator circuit
122
modulates the output (30 MHz) of the first tank circuit
121
in phase (or in frequency) using the output (phase modulation signal fm of 155 Hz, for example) of a below-described low frequency oscillator
141
of the LF block
104
. The amplitude modulator circuit
124
modulates the output (90 MHz) of the second tank circuit
123
by the output (approximately 5.3101 HMz) of the frequency synthesizer
126
.
The frequency synthesizer
126
, as is well known in the art, is a kind of phase-locked loop (PLL) circuit equipped with a non-illustrated voltage-controlled oscillator (VCO) for generating a frequency signal for the above-mentioned amplitude modulation (amplitude modulation signal of approximately 5.3101 MHz). The output of the VCO is compared in phase with the output of the VCXO
120
, and the output frequency (amplitude modulation frequency) of the VCO is controlled in such a manner that the output of the VCO is synchronized in phase with that of the VCXO
120
. The PLL-type frequency synthesizer
126
is also equipped with a variable frequency divider (programmable frequency divider) so that fine adjustments of the above-mentioned amplitude modulation frequency can be made in accordance with frequency setting information given from an external source.
Further, the matching circuit
125
takes an impedance matching between the HF block
102
and the OMU
103
so that the signal (90 MHz) amplitude-modulated in the amplitude modulator circuit
124
is input to the OMU
103
via the matching circuit
125
as a high frequency signal from which the above-mentioned resonant frequency signal originates.
Then the OMU
103
, which is a box-shaped atomic resonator, detects and outputs a signal (atomic resonance signal) when a rubidium atom in the resonator box is resonated (transited) as a microwave to be a resonant (transitional) signal of a rubidium atom occurs in the resonator box. For this purpose, as shown in
FIG. 6
, the OMU
103
includes a rubidium lamp
131
, a resonance cell
133
in which a rubidium atom is charged (loaded), a cavity resonator
132
equipped with a varactor diode
134
and a photo diode (PD), and a pre-amplifier (PA)
136
. The resonator box of the OMU
103
is treated with magnetic shielding
130
so that resonance of rubidium atom is prevented from being influenced by a possible magnetic field due to a peripheral circuit.
The above-mentioned rubidium lamp
131
is a lamp for emitting light (high frequency discharge) by exciting a coil
1312
with the output of an exciter
1311
, thereby irradiating the light to a resonance cell
133
in the cavity resonator
132
.
The varactor diode (high-natural-number multiplier)
134
produces in the cavity resonator
132
a microwave (resonant frequency signal) having an amplitude-modulated component ±5.3101 MHz at either side about 90×76=6.840 GHz by multiplying the output (phase-and amplitude-modulated high frequency signal of 90 MHz) by a high natural number (76 for example) of the matching circuit
125
; the lower part of this microwave frequency (6.840 GHz−5.3101 MHz=6.8346 GHz) is a value approximating to the resonant frequency of rubidium atom. The cavity resonator
132
is tuned to approximately 6.8 GHz (designed in such a manner that resonance will occur under a microwave of approximately 6.8 GHz).
Since the requested output frequency of the VCXO
120
is oddlessly 10 MHz, simply multiplying such output frequency 10 MHz using the first and second tank circuits
121
,
123
and the varactor diode
134
does not suffice to produce the resonant frequency of rubidium atom (approximately 6.8346 GHz). Consequently in the conventional rubidium atomic oscillator
101
, the input signal to the OMU
103
is modulated in amplitude whereupon the amplitude-modulated component of the resultant input signal is multiplied.
The photo diode
135
receives the light emitted from the rubidium lamp
131
and traveling through the resonance cell
133
and outputs an electrical signal in accordance with the quantity of the received light as an atomic resonance signal. Assuming that a microwave frequency produced by the varactor diode
134
coincides with the resonant frequency (approximately 6.8346 GHz) of rubidium atom, an atomic resonance will occur so that the light from the rubidium lamp
131
will be absorbed in part by the rubidium atom in the resonance cell
133
, resulting in reduced quantity of the received light.
Accordingly an electrical signal (atomic resonance signal) to be output from the photo diode
135
has information about a difference (error frequency) between the microwave frequency, which is produced in the cavity resonator
132
, and the resonant frequency of rubidium atom (approximately 6.8346 GHz). The pre-amplifier
136
serves to amplify the output of the photo diode
135
to an appropriate level in advance.
The LF block
104
serves as a servo circuit which detects the above-mentioned frequency-error information (error signal) from the atomic resonance signal output from the OMU
103
and which then controls the output frequency of the VCXO
120
in such a manner that the detected error signal is minimal (ideally 0). In an example, the LF block
104
is composed of a low-frequency voltage control oscillator (VCO)
141
, an amplifier
142
, a synchronous detector
143
, and an integrator
144
.
The low-frequency VCO
141
generates a phase modulation signal fm for the phase modulator circuit
122
and, in the meantime, the amplifier
142
amplifies an atomic resonance signal from the OMU
103
up to an appropriate level. And the synchronous detector
143
ta
Atsumi Ken
Koyama Yoshito
Matsuura Hideyuki
Nakajima Yoshifumi
Nakamuta Koji
Fujitsu Limited
Helfgott & Karas P C
Mis David
LandOfFree
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