Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
2000-07-19
2001-10-16
Mis, David (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S096000, C331S17700V
Reexamination Certificate
active
06304153
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to satellite communication systems, and more particularly, to a resonator tuning system for satellite communication systems.
BACKGROUND ART
Satellites and other spacecraft are in widespread use for various purposes including scientific research and communications. These scientific and communications missions, however, cannot be accurately fulfilled without wireless communication between a ground station and the spacecraft. In many applications, the satellite relies upon a wireless communication to send and receive electronic data to perform attitude and position corrections, diagnostic status checks, communication calculations and other functions. Without accurate wireless communication, proper satellite function is hindered and at times adversely effected.
Many modern spacecraft use resonator tuning systems for changing communication frequencies. The prior art systems for changing communication frequencies are predicated upon coupling a voltage controlled capacitor (varactor) to a resonator in order to change its resonance frequency. There are two general cases for resonator tuning systems, the lumped case and the distributed case.
In the lumped case, the resonance frequency ƒ
0
—
lumped
of a resonator with capacitance C and inductance L is given by:
f
0
_
lumped
=
1
1
L
·
C
(
1
)
Tuning is effected by connecting the varactor having capacitance C
var
(V
control
), either in series or in shunt with the resonator. When connected in series, the resonant frequency becomes:
f
0
_
lumped
Series
=
1
1
L
·
C
·
C
var
(
V
control
)
C
+
C
var
(
V
control
)
(
2
)
When, on the other hand, the varactor is connected in shunt with the resonator, the resonance frequency is given by:
f
0
_
lumped
Shunt
=
1
1
L
·
(
C
+
C
var
(
V
control
)
)
(
3
)
In the distributed case, the varactor is used to terminate a transmission line of length equal to one-quarter wavelength at the frequency of interest. Then, by virtue of the impedance inverter effect, the input impedance of the transmission line acts inductive, with an inductance value that is a function of the terminating capacitor value. Thus the effective inductance L, of the resonator being loaded by the transmission line, is changed and consequently its resonance frequency is changed.
The fundamental disadvantage of these approaches stems from the fact that the semiconductor varactor is not a pure capacitor, but contains an intrinsic parasitic resistance that introduces losses in the resonator, thus lowering its unloaded Q. The consequence of a reduction in the unloaded Q may be appreciated by examining the carrier to noise (C/N) ratio in a Voltage Controlled Oscillator (VCO), where (C/N) is given by:
C
N
=
(
2
×
Q
L
×
Δ
f
)
2
×
P
o
(
Loss
×
f
o
)
2
×
(
2
×
k
×
T
×
B
×
NF
)
(
4
)
Where Q
L
is the loaded Q of the resonator, Loss is the loss factor in the resonator, f
0
is the frequency of oscillation, &Dgr;f is the offset frequency from f
0
, P
0
is the output power of the oscillator, k is Boltzrnann's constant, T is the absolute temperature, B is the measurement bandwidth, and NF is the noise figure of the amplifier. Examination of equation (4) reveals that in order to obtain high C/N ratio, the loaded Q must be high. The loaded Q is highest when it experiences minimum external loading.
The disadvantages associated with these conventional resonator tuning techniques have made it apparent that a new technique for resonator tuning is needed. The new technique should maintain the high, unloaded Q properties of resonators, while significantly reducing phase noise. Additionally, the new technique should allow superior frequency tuning capability. The present invention is directed to these ends.
SUMMARY OF THE INVENTION
It is, therefore, an object of the invention to provide an improved and reliable resonator tuning system. Another object of the invention is to maintain the high, unloaded Q properties of resonators, while significantly reducing phase noise.
In accordance with the objects of this invention, a resonator tuning system is provided. In one embodiment of the invention, a resonator tuning system includes an apparatus coupled to a resonator for changing the resonator's resonance frequency. Tuning of the resonator is accomplished by varying the coupling to a capacitor or varactor, rather than by varying the capacitor. An interferometer, such as a Mach-Zender interferometer, is coupled to the resonator and changes the resonance of the resonator by applying an actuation voltage to vary the coupling to the resonator. In this way, a resonator may be tuned and maintain high unloaded Q properties while being coupled to a varactor or other load.
The present invention thus achieves an improved resonator tuning system. The present invention is advantageous in that it allows superior frequency tuning capability over the prior art.
Additional advantages and features of the present invention will become apparent from the description that follows, and may be realized by means of the instrumentalities and combinations particularly pointed out in the appended claims, taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 4733182 (1988-03-01), Clarke et al.
patent: 5210763 (1993-05-01), Lewis et al.
Gudmestad Terje
Hughes Electronics Corporation
Mis David
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