Oscillators – Solid state active element oscillator – Significant distributed parameter resonator
Reexamination Certificate
2000-01-13
2001-05-29
Lam, Tuan T. (Department: 2816)
Oscillators
Solid state active element oscillator
Significant distributed parameter resonator
C331S1170FE
Reexamination Certificate
active
06239663
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high-frequency oscillator. More particularly, the invention relates to a high-frequency oscillator using Field-Effect Transistors (FETs) and electromagnetically-coupled transmission lines, which is applicable to the microwave or millimeter wave ranges.
2. Description of the Prior Art
In a high-frequency oscillator using an FET where the output of the FET is positively fedback to its input, generally, some phase error tends to occur between the input and output of the FET due to the 1/f noise and the white noise generated in the FET. The resonant or oscillation frequency of the oscillator shifts automatically so as to eliminate or decrease the phase error according to the Kirchhoff's law. Thus, the oscillation frequency tends to fluctuate, resulting in a wide-based spectrum B in FIG.
1
.
Ideally, the oscillation frequency of the oscillator is kept at a single value and therefore, it has a linear spectrum A shown in
FIG. 1
, i.e., it is expressed by the well-known &dgr; function.
The word “phase noise” is defined as the ratio (P
F
/P
0
) of the power level P
F
at the frequency apart from the central oscillation frequency by an offset (i.e., off-carrier) frequency &Dgr;f with respect to the power level P
0
at the central oscillation frequency, which is expressed by the unit “dBc/Hz”. It is preferred that the value of the phase noise is as small as possible. In other words, as the value of the phase noise becomes smaller, the capacity or performance of the oscillator becomes higher.
The fluctuation of the oscillation frequency, i.e., phase noise, varies dependent upon the 1/f noise and the load QL of the resonator. Thus, to reduce the phase noise, it is required to decrease the 1/f noise and to increase the load Q
L
.
The main cause of the 1/f noise is the time constant distribution of the recombination centers existing at the surfaces of the semiconductor of the FET and the interfaces therein. Therefore, not only the 1/f noise is difficult to be controlled but also it tends to affect the lateral-type semiconductor device such as the FET. To suppress the effect of the 1/f noise, a vertical-type semiconductor device such as a heterojunction bipolar transistor is often used instead of the FET. Alternately, a suitable measure to increase the load Q
L
is often taken.
Various high-frequency oscillators formed by the FET that operates in the microwave or millimeter-wave range have been developed and reported, an example of which is shown in FIG.
2
. The prior-art oscillator of
FIG. 2
is disclosed in the Japanese Non-Examined Patent Publication No. 9-260945 published in October 1997.
As shown in
FIG. 2
, this prior-art oscillator is equipped with a dielectric resonator
109
to decrease the phase noise. A bypass capacitor
100
, an oscillation FET
101
, a feedback stub
102
, a varactor element
103
, an oscillation circuit
104
, an output matching circuit
105
, an output circuit
106
, capacitors
107
and
111
, and a coupling line
108
are formed on a Gallium Arsenide (GaAs) substrate
112
.
In the prior-art oscillator shown in
FIG. 2
, the load Q
L
is increased to reduce the phase noise by using the dielectric resonator
109
with a high Q value. For example, if the value of the load Q
L
is increased to ten times its original value, the phase noise is decreased to one-hundredth ({fraction (1/100)}) (i.e., −20 dB). Thus, this oscillator has an advantage that the phase noise can be effectively decreased. However, it has a problem that the mass productivity is not high and that the oscillator size or scale is large.
Another example of the prior-art high-frequency oscillators of this sort is shown in
FIG. 3
, which is of the series feedback type and which has an advantage that both the mass productivity and the reproducibility are excellent.
As shown in
FIG. 3
, this prior-art oscillator comprises an FET
222
. A transmission line
217
, which serves to generate a negative resistance, is connected to the source of the FET
222
. A transmission line
221
is connected to the drain of the FET
222
. The line
221
is further connected to an output terminal
213
through a dc-blocking capacitor
223
b
. A transmission line
220
is connected to the gate of the FET
222
through a dc-blocking capacitor
223
a
. The line
220
is further connected to a transmission line
218
through a capacitor
219
. The line
218
serves as an inductor determining the oscillation frequency. The capacitor
219
serves to determine the oscillation frequency along with the line
218
.
A gate bias circuit or line
224
is connected to the gate of the FET
222
. The circuit
224
comprises an inductor
224
a
and a voltage source
224
b
providing a bias voltage V
g
. A drain bias circuit or line
225
is connected to the connection point of the transmission line
221
and the capacitor
223
b
. The circuit
225
comprises an inductor
225
a
and a voltage source
225
b
providing a bias voltage V
d
.
To examine the performance of the prior-art oscillator shown in
FIG. 3
, the inventor carried out simulation using a known circuit simulator under the following condition, in which the 1/f noise was not considered.
On the assumption that the prior-art oscillator of FIG.
3
is formed on a GaAs substrate with a thickness of 40 &mgr;m, the relative dielectric constant &egr;r was set as 12.6. The transmission line
217
was supposed to be a microstrip line with a width of 10 &mgr;m and a length of 100 &mgr;m. The transmission line
218
was supposed to be a microstrip line with a width of 10 &mgr;m and a length of 900 &mgr;m. The capacitance of the capacitor
219
determining the oscillation frequency was set as 100 fF. The transmission line
220
was supposed to be a microstrip line with a width of 300 &mgr;m and a length of 270 &mgr;m. The transmission line
221
was supposed to be a microstrip line with a width of 300 &mgr;m and a length of 276 &mgr;m. The capacitance of the dc-blocking capacitors
223
was set as 1 pF. The gate bias voltage V
g
was set as −0.5V. The drain bias voltage V
d
was set as 4.5V.
With respect to the FET
222
, the FET
222
was supposed to be an FET having the AlGaAs/InGaAs heterojunction and the gate width Wg of 200 &mgr;m. Then, a nonlinear FET model was obtained by using the “Curtice Cubic” model. Using the nonlinear FET model thus obtained, the output power of the oscillator was analyzed by the Harmonic Balance method while the nonlinear FET parameters as shown in
FIG. 4
were used. In
FIG. 4
, Lg, Ls, Ld are the gate, source, and drain inductances, respectively, and Cpg and Cpd are the gate and drain parasitic capacitances, respectively. The analysis was carried out using the oscillator testing bench prepared for the microwave simulator produced by the HP EEsof Inc. and named “Series-IV Libra”.
The simulation result thus obtained is shown in
FIGS. 5 and 6
.
FIG. 5
shows the output power characteristic of the prior-art oscillator of
FIG. 3
, in which the output power is 14.2 dBm at the frequency of 99.3 GHz. In this case, the phase noise is given as shown in
FIG. 6
, in which the phase noise is −103.0 dBc/Hz at the offset frequency of 100 kHz.
The measured value of the phase noise of the prior-art oscillator of
FIG. 3
was approximately −60 to −40 dBc/Hz at the highest at the offset frequency of 100 kHz from the oscillation frequency of 100 GHz, which is higher than that of the above-described simulation result. This is due to the fact that the 1/f noise was not considered in the simulation.
As a result, it is found that from the view point of usefulness, the phase noise of the prior-art oscillator of
FIG. 3
is not satisfactorily low for the use in the high frequency range of 60 GHz or higher.
As explained above, the prior-art oscillator shown in
FIG. 2
has the problem that the mass productivity is not high and the oscillator size or scale is large. The prior-art oscillator shown in
FIG. 3
has the problem that the phase noise
Foley & Lardner
Lam Tuan T.
NEC Corporation
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