Transistorized microwave oscillator of oscillation frequency multiplying type

A transistorized microwave oscillator comprising a resonator 2 coupled to a transistor 4, 5 and a feedback capacitor 7' around the transistor which produces a fundamental oscillation frequency and harmonics thereof. The feedback capacitor is located within a cut-off waveguide 21 and couples oscillations into the waveguide which blocks the lower undesired frequencies.

BACKGROUND OF THE INVENTION 
The present invention relates to a microwave oscillator, and more 
particularly to a transistorized microwave oscillator of the oscillation 
frequency multiplying type. 
A typical transistorized microwave oscillator of the prior art is composed 
of a fundamental wave oscillator circuit involving a resonator and a 
frequency multiplier circuit for generating a required harmonic by 
multiplying the fundamental frequency of this oscillator circuit. Such an 
oscillator uses a frequency multiplier having a varactor diode for 
generating the required harmonic, requires a holding mechanism, a biasing 
circuit and the like for the diode, and accordingly has the disadvantages 
of complex structure and higher cost. Moreover, since the oscillator 
circuit and the multiplier are directly connected to each other, the load 
impedance of the former is the input impedance of the latter. Because of 
the consequent absence of buffering between them with respect to impedance 
fluctuations, any impedance fluctuation of the multiplier would become a 
load fluctuation of the oscillator circuit, resulting in the disadvantages 
that the start of oscillation is unreliable, and sometimes the oscillation 
even fails to start at all, furthermore frequency and output level 
fluctuations due to temperature variations are unstable, and abnormal 
oscillation is more likely to occur. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide a microwave 
transistor oscillator which has a simple structure and, moreover, whose 
electrical characteristics are stable. 
According to the invention, there is provided a transistorized microwave 
oscillator comprising: oscillating means, including at least a transistor 
for oscillation and feedback oscillating, for generating a fundamental 
oscillating frequency and its harmonic components; and a cutoff waveguide 
connected to said oscillating means for cutting off said fundamental 
frequency and undesired lower-order harmonics to thereby provide a desired 
higher-order harmonic, wherein a part of said feedback comprises means for 
coupling said oscillating means to said cutoff waveguide.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates the structure of such a conventional transistorized 
microwave oscillator. FIG. 2 shows an example circuit of the oscillator 
structure shown in FIG. 1. In FIGS. 1 and 2, the transistorized microwave 
oscillator comprises an oscillator circuit 100 and a frequency multiplier/ 
filter circuit 10. 
The oscillator circuit 100 built in a metal chassis 1 comprises an 
oscillator transistor 4 (for example, a GaAs FET), a high Q resonator 2 
(for example, a dielectric resonator), a microstrip line 3 and a feedback 
capacitor 7. The microstrip line 3 comprises a substrate made of a 
ceramic, and metal patterns formed thereon, as will be apparent from FIG. 
3B. The line 3 couples the resonator 2 to the gate 5 of the transistor 4. 
The capacitor 7, which comprises a dielectric substrate and metal patterns 
formed thereon, is connected between the drain and the source of the 
transistor 4. DC bias voltages are supplied to terminals Bl and B2 and 
then to the gate and source of the transistor 4 through feedthrough 
capacitors 8 and 9, which block high frequencies. The drain of the 
transistor 4 is grounded. The output of the oscillator circuit 100 is 
coupled to the frequency multiplier/filter circuit 10 through a coupling 
antenna 11. 
The frequency multiplier/filter circuit 10 comprises a frequency 
multiplying diode 12 and a waveguide filter including resonating rods 13. 
The diode 12 multiplies the fundamental oscillation frequency of the 
oscillator circuit to provide harmonics thereof. The waveguide filter 
filters the harmonics to supply a desired Nth (N=2, for example) harmonic 
to a terminal OUT. 
The microstrip line 3 and feedback capacitor 7 are mounted on a metal 
subcarrier, as will be obvious from FIG. 3B, which is soldered to the 
metal chassis 1. 
The transistor 4 may be a silicon bipolar transistor. Also, the resonator 2 
may comprise a metal cavity resonator. 
With the conventional structure, the frequency multiplying diode is used 
for extracting the needed Nth harmonic. Also, the use of the diode 
necessitates a mechanism to hold the diode, a biasing circuit and the 
like. As a result, the structure has the disadvantages of complexity and a 
higher cost. It further has the following electrical disadvantage that the 
oscillator circuit 100 and the diode 12 are directly connected 
electromagnetically to each other, the load impedance of the former is the 
input impedance of the latter. In other words, because of the consequent 
absence of bufferings effect between them with respect to impedance 
fluctuation, any impedance fluctuation of the diode 12 would become a load 
fluctuation of the oscillator circuit 100, resulting in unreliable 
starting of oscillation, which may sometimes even fail to start at all. 
Moreover, it leads to the instability of frequency and output level 
fluctuations due to temperature variation, and an abnormal oscillation. 
FIGS. 3A and 3B illustrate the structure of one preferred embodiment of the 
invention, which is free of these disadvantages. FIG. 4 shows an example 
of circuitry corresponding to the microwave oscillator structure shown in 
FIGS. 3A and 3B. In FIGS. 3A, 3B and 4, the transistorized microwave 
oscillator comprises an oscillator circuit 100' and a cutoff waveguide 21. 
Constituent elements performing respectively the same functions as the 
corresponding ones in FIG. 1 are assigned the same reference numerals. 
The oscillator of the present invention is featured by including the cutoff 
waveguide 21. The waveguide 21 has an input section which is squeezed in 
shape to cut off the fundamental oscillation frequency and unneeded 
lower-order harmonics and thereby provide a needed higher-order harmonic. 
A length l shown in FIG. 3A constitutes a cutoff size against the 
fundamental oscillation frequency and unneeded lower-order harmonics. 
Another feature of the present oscillator is that coupling of the required 
Nth harmonic between the oscillator circuit 100' and the waveguide 21 is 
achieved by the use of a metal pattern 17 of the feedback capacitor 7', 
which is provided within the waveguide 21. The feedback capacitor 7' being 
located within the waveguide 21 also acts as an antenna for coupling the 
oscillator circuit 100' to the waveguide 21. 
The feedback capacitor 7' comprises a substrate 18 and metal patterns 17 
and 19 formed thereon. The metal pattern 19 is grounded to a metal 
subcarrier 20. The microstrip line 3 comprises a dielectric substrate 15 
and metal patterns 14 and 16. The pattern 14 is connected to the gate of 
the transistor 4. The metal pattern 16 is grounded to the subcarrier 20, 
which is soldered to the chassis 1. 
In the oscillator illustrated in FIGS. 3A and 3B, the output circuit 
consists of the cutoff waveguide 21, which electromagnetically separates 
the oscillator circuit 100' from other circuits. Since this oscillator 
enables the output circuit to be coupled to the Nth harmonic component of 
the oscillator circuit 100', between whose fundamental oscillation wave 
level and the Nth harmonic level is a conversion loss of L dB (L&gt;0), there 
is in effect an attenuator of L dB between the oscillator circuit 100' and 
other circuits. That is, the output circuit is in effect provided with an 
attenuator, resulting in a substantial improvement of the effect of 
impedance variation in the output circuit of the oscillator circuit 100'. 
FIG. 5 shows the comparison of the source drain voltage V.sub.SD vs. 
oscillation level characteristic of the present oscillator of FIG. 3A and 
3B and that of the conventional oscillator of FIG. 1. In FIG. 5, solid 
lines a and b illustrate the characteristics of the oscillator of FIGS. 3A 
and 3B, and broken lines c and d, illustrate examples of abnormal 
oscillation to which the conventional oscillator of FIG. 1 is susceptible. 
The solid line a and the broken line c refer to characteristics at the 
fundamental oscillation frequency of 10 GHz, and the solid line b and the 
broken line d, to characteristics at the 2nd harmonic frequency of 20 GHz. 
Since, as is evident from FIG. 5, there is a level difference (i.e. a 
conversion loss) of 4 to 6 dB between the fundamental oscillation wave 
level and the 2nd harmonic level at the source-drain voltage V.sub.SD of 
about 3.5 V, the present invention provides bufferings equivalent to a 
return loss of 8 to 12 dB. This buffer effect enables the start of 
oscillation to be reliably achieved, stabilizes the frequency and output 
level fluctuations due to temperature variation, and fully prevents, as 
the solid lines a and b show, the abnormal oscillation represented by the 
broken lines c and d. While these instances of abnormal oscillation are 
shown at the voltage V.sub.SD of about 3 V in the diagram, they are mere 
examples. 
As described above, according to the present invention, the feedback 
capacitor constituting a part of the oscillator circuit is utilized as the 
coupling circuit for extracting the needed harmonic from the oscillator 
circuit. Also, the feedback capacitor is concurrently used as an antenna. 
The Nth harmonic component of the fundamental oscillation wave radiated 
from the antenna is extracted into another circuit through the cutoff 
waveguide. Since the waveguide and the oscillator circuit are connected to 
each other by a mere waveguide having the width of cutoff size, there are 
needed no complexly structured coupling antenna of a frequency multiplier 
circuit of the conventional oscillator. Also, the oscillator circuit can 
be readily incorporated with the waveguide by a solid piece of metal. 
Therefore, the invention contributes to cost reduction through the 
simplified structure and the manufacturing ease it makes possible.