Microwave oscillation apparatus capable of suppressing spurious oscillation

In a microwave oscillation apparatus including a negative resistance element, a microstrip line having a first end connected to the negative resistance element and a second end connected to a terminating resistor, and a dielectric resonator magnetically coupled to the microstrip line, a capacitive stub is provided on the microstrip line at a distance (1/4) .lambda..sub.s (2N-1) from the first end thereof, where .lambda..sub.s is a wavelength of a spurious oscillation frequency component and N is a positive integer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a stable microwave oscillation apparatus. 
2. Description of the Related Art 
A prior art microwave oscillation apparatus is comprised of a negative 
resistance element, a microstrip line having an end connected to the 
negative resistance element and another end connected to a terminating 
resistor, and a dielectric resonator magnetically connected to the 
microstrip line, to thereby generate a microwave output having a resonance 
frequency f.sub.o determined by the microstrip line and the dielectric 
resonator. In this prior art microwave oscillation device, in order to 
stabilize the oscillation, the terminating resistor is connected to a 
quarter-wave open end for the resonance frequency f.sub.o or a spurious 
oscillation frequency f.sub.s, and as a result, the ground impedance of 
the terminating resistor is made zero at the resonance oscillation 
frequency f.sub.o or the spurious oscillation frequency f.sub.s. This will 
be explained later in detail. 
In the above-mentioned prior art, however, the ground impedance of the 
terminating resistor is not zero even at a frequency slightly different 
from the resonance oscillation frequency f.sub.o or the spurious 
oscillation frequency f.sub.s. Therefore, it is impossible to completely 
stop spurious oscillations. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a microwave 
oscillation apparatus which can suppress spurious oscillations. 
According to the present invention, in a microwave oscillation apparatus 
including a negative resistance element, a microstrip line having a first 
end connected to the negative resistance element and a second end 
connected to a terminating resistor, and a dielectric resonator 
magnetically coupled to the microstrip line, a capacitive stub is provided 
on the microstrip line at a distance (1/4) .lambda..sub.s (2N-1) from the 
first end thereof, where .lambda..sub.s is a wavelength of a spurious 
oscillation frequency component and N is a positive integer. That is, the 
capacitive stub serves to change the frequency characteristic of the 
negative resistance element viewed from the terminating resistor. As a 
result, the impedance of the terminating resistor viewed from the negative 
resistance element can be large enough to suppress the oscillation at 
spurious frequencies. In other words, the capacitive stub reduces the 
negative resistance value of the negative resistance element at the 
spurious frequencies, thus stabilizing the operation of the microwave 
oscillation apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before the description of the preferred embodiments, prior art microwave 
oscillation apparatuses will be explained with reference to FIGS. 1 
through 4. 
In FIG. 1, which illustrates a first prior art microwave oscillation 
apparatus, reference 1 designates a negative resistance element which is 
connected to a microstrip line 2 whose characteristic impedance Z.sub.o 
is, in this case, about 50.OMEGA.. A dielectric resonator 3 is located in 
proximity to the microstrip line 2, so that the dielectric resonator 3 is 
magnetically coupled to the microstrip line 2. Therefore, a resonance 
frequency f.sub.o is determined by the microstrip 2 and the dielectric 
resonator 3. In order to realize a non-reflection end at frequencies other 
than the resonance frequency f.sub.o, a terminating resistor 4, whose 
resistance value is about 50.OMEGA., is connected between the microstrip 2 
and ground. Even in this case, since oscillations at spurious frequencies 
occur in accordance with a relative relationship between a reflection 
coefficient toward the negative resistance element 1, and a reflection 
coefficient toward the terminating resistor 4, oscillations at spurious 
frequencies may occur in accordance with the reflection coefficient toward 
the negative element 1. 
Also, it is impossible to connect the terminating resistor 4 to ground in 
an ideal state, since the terminating resistor 4 is actually connected to 
ground via a printed board or the like, so that the terminating resistor 4 
has a parasitic capacity. 
In FIG. 2, which illustrates a second prior art microwave oscillation 
apparatus, an open end stub 5 having a length .lambda./4 where .lambda. is 
the wavelength of the resonance frequency f.sub.o is connected to the 
terminating resistor 4 of FIG. 1. As a result, the input impedance of the 
open end stub 5 is zero at frequencies f.sub.o /(2N-1) where N equals 1, 
2, . . . ; however, the non-reflection condition is not satisfied at the 
end of the microstrip line 2 on the terminating resistor 4 at other 
frequencies. Therefore, when the reflection coefficient toward the 
terminating resistor 4 viewed from the negative resistance element 1, and 
the reflection coefficient toward the negative resistance element 1, 
satisfy an oscillation condition, a spurious oscillation or a mode jump 
may occur, to thereby destabilize the oscillation of microwaves. 
In FIG. 3, which illustrates a third prior art microwave oscillation 
apparatus, an open end stub 5' having a length .lambda..sub.s /4 where 
.lambda..sub.s is the wavelength of a spurious frequency f.sub.s, is 
connected to the terminating resistor 4 of FIG. 1 (see: JP-A-64-16106). As 
a result, the input impedance of the open end stub 5' is zero at 
frequencies f.sub.s /(2N-1) where N equals 1, 2, . . . ; however, the 
non-reflection condition is not satisfied at the end of the microstrip 
line 2 on the terminating resistor 4 at other frequencies. Therefore, when 
the reflection coefficient toward the terminating resistor 4 viewed from 
the negative resistance element 1, and the reflection coefficient toward 
the negative resistance element 1, satisfy an oscillation condition, 
another spurious oscillation or a mode jump may occur, to thereby 
destabilize the oscillation of microwaves. 
In FIG. 4, which illustrates a fourth prior art microwave oscillation 
apparatus, both of the open end stubs 5 and 5' are connected to the 
terminating resistor 4 of FIG. 1 (see: JP-A-3-140003). Even in FIG. 4, the 
non-reflection condition is not satisfied at the end of the microstrip 
line 2 on the terminating resistor 4 at frequencies other than the 
resonance frequency f.sub.o and the spurious frequency f.sub.s. Therefore, 
when the reflection coefficient toward the terminating resistor 4 viewed 
from the negative resistance element 1, and the reflection coefficient 
toward the negative resistance element 1, satisfy an oscillation 
condition, another spurious oscillation or a mode jump may occur, to 
thereby destabilize the oscillation of microwaves. 
In FIG. 5, which illustrates a first embodiment of the present invention, a 
capacitive stub 6 is provided on the microstrip line 2 at a distance 
.lambda..sub.s /4 from the end A thereof on the side of the negative 
resistance element 1 of FIG. 1. Note that the distance .lambda..sub.s /4 
can be replaced by (1/4) .lambda..sub.s (2N-1) where N equals 2, 3, . . . 
. That is, the capacitive stub 6 serves to change the frequency 
characteristic of the negative resistance element 1 viewed from the 
terminating resistor 4. As a result, the impedance of the terminating 
resistor 4 viewed from the negative resistance element 1 can be large 
enough to suppress tile oscillation at spurious frequencies other than the 
resonance frequency f.sub.o. In other words, the capacitive stub 6 reduces 
the negative resistance value of the negative resistance element 1 at the 
spurious frequencies, thus stabilizing the operation of the microwave 
oscillation apparatus. 
As shown in FIG. 6A which shows a reflection coefficient to frequency 
characteristic toward the negative resistance element 1 viewed from the 
terminating resistor 4 in the apparatus of FIG. 1, oscillations occur in 
the proximity of the spurious frequency f.sub.s. Contrary to this, in the 
first embodiment, as shown in FIG. 6B, the negative resistance of the 
negative resistance element 1 is made small in the proximity of the 
spurious frequency f.sub.s. As a result, no oscillation occurs in the 
proximity of the spurious frequency f.sub.s. 
Thus, the prior art microwave oscillation apparatuses as illustrated in 
FIGS. 2, 3 and 4 are intended to realize a non-reflection end without 
changing the impedance on the side of the negative resistance element 1 
viewed from the terminating resistor 4. On the other hand, tile first 
embodiment is intended to shift the impedance of the negative resistance 
element 1 viewed from the terminating resistor 4, to suppress the spurious 
oscillation. 
Also, as described above, the object of the open end stubs 5 and 5' is 
different from that of the capacitive stub 6. Therefore, the open end stub 
5 and/or the open end stub 5' are added to the elements of FIG. 5, as 
illustrated in FIGS. 7, 8 and 9 which illustrate second, third and fourth 
embodiments, respectively, of the present invention, thus further 
suppressing the oscillation at spurious frequencies. 
In the above-described embodiments, the negative resistance element 1 is 
comprised of a GaAs FET, however, the negative resistance element 1 can be 
formed by using a bipolar transistor, a Gunn diode, or the like. 
As explained hereinbefore, according to the present invention, since 
oscillations at spurious frequencies can be suppressed, a stable microwave 
oscillation apparatus can be obtained.