An oscillator for very high microwave or millimeterwave frequencies employs a pair of negative-resistance semiconductive devices, each mounted in its own resonant cavity. The two cavities are coupled together by an iris in their common wall. An output waveguide is coupled symmetrically to both sides of the common wall to load both cavities equally. An adjustable mode-control element projects into the cavity to the near vicinity of one of the devices to induce the oscillator to start in the desired mode. Additional dielectric or metallic tuners in the cavities provide a wide variation of frequency.

FIELD OF THE INVENTION 
The invention pertains to very high frequency oscillators, particularly at 
millimeter wave frequencies, using negative-resistance semiconductive 
devices as their driving elements. At these frequencies, the amount of 
power available from any one active device is very limited, so it is 
desirable to combine the power capabilities of several active devices. 
PRIOR ART 
Previous attempts to increase the power output of semiconductor oscillators 
have involved methods of coupling two or more semiconductive devices into 
a single resonant cavity in order to synchronize their oscillations. U.S. 
Pat. No. 3,810,045 issued May 7, 1974 to Thomas G. Ruttan, and assigned to 
the assignee of the present invention describes a push-pull oscillator 
using two Gunn devices in a single waveguide-type cavity approximately one 
full wavelength long. The Gunn devices were positioned near the opposite 
ends of the cavity, so that when a full-wavelength mode was excited, the 
fields at the devices were 180.degree. out of phase, in other words, the 
devices were in push-pull operation. While Ruttan's oscillator did combine 
the outputs of the two devices, there were problems with oscillation in 
some of the many modes that could exist in this extended cavity in 
addition to the desired single-wavelength mode. 
Other attempts to obtain push-pull operation involved a conductive septum 
in the H-plane of the cavity connecting to back-to-back 
negative-resistance diodes. U.S. Pat. No. 3,617,935 issued Nov. 2, 1971 to 
Katuhiro Kimura et al. describes such oscillators. However, the additional 
structural complexity caused by the presence of the septum also introduced 
the possibility of new unwanted modes. 
U.S. Pat. No. 3,562,665 issued Feb. 9, 1971 to R. D. Larrabee describes an 
oscillator in which an insulated septum divides the resonant cavity into 
two mutually insulated half-cavities coupled by irises in the septum. This 
structure can eliminate some of the modes associated with a rf-floating 
septum but has the problem that rf bypassing all of the cavity circulating 
current is quite difficult and a bypass choke of this size has its own 
undesired resonant modes. 
SUMMARY OF THE INVENTION 
The principal objective of the present invention is to provide a 
high-frequency oscillator in which the outputs of a plurality of 
negative-resistance semiconductive devices can be combined to produce 
increased power. 
Another objective is to provide a means of combining the outputs of a 
plurality of devices in which oscillation is restricted to the desired 
mode. 
A further objective is to provide a multi-device oscillator which is easily 
tunable over an extended frequency range. 
These objectives are realized by dividing the resonant circuit of the 
oscillator into two adjacent, closely coupled cavities. An equal number of 
negative-resistance devices is coupled into each cavity, preferably by 
locating the devices and their coupling means within the cavities 
themselves. Coupling between the two cavities is provided by an iris in 
their common wall, and coupling to the useful load is preferably provided 
by an output located adjacent the common wall to couple equally from both 
cavities.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1, 2 and 3 are respectively a side section, a horizontal section, and 
a top view of an embodiment of the present invention. The resonant cavity 
structure 20 is made as a hollowed-out, one-piece metal block 22 covered 
by a generally flat metallic lid 24. Two resonant cavities 26 are cut into 
block 22 leaving a thin, common dividing wall 28 separating them. In wall 
28 a coupling iris 30 is cut to mutually couple the electromagnetic fields 
of cavities 26. An iris 32 in cover plate 24 is located symmetrically 
centered above common wall 28, coupling fields from both cavities 26 into 
a rectangular output waveguide 34 extending out vertically through 
cover-plate 24. In each cavity 26, preferably near an outer wall thereof, 
is a semiconductive negative-resistance device 36, such as a Gunn diode. 
The upper terminals of diodes 36 are conductively connected to the 
cover-plate 24 of cavity structure 20. The lower terminals of diodes 36 
are conductively connected to bias terminals 38 for supplying dc bias to 
diodes 36. The connection comprises a post 40 extending across the height 
of cavity 26 to couple diodes 36 with the electromagnetic fields of 
cavities 26. The bias connection exits through the wall of cavity block 22 
via an rf choke 42 comprising an insulating, thin sleeve 44 lining a hole 
in block 22. In series with lead 38 are sections of low-impedance coaxial 
line 46 extending radially to contact insulator 44, alternating with high 
impedance sections 48 having smaller diameter center conductors. 
It has been found that for the oscillator to start in its proper cavity 
mode, it is useful to have a mode-suppressor rod 50 in close proximity to 
device-coupling rod 40. Suppressor 50 may be of high dielectric constant, 
such as sapphire, or alternatively may be of an rf lossy material such as 
a dielectric loaded with carbon. Rod 50 enters cavity 26 on the midplane 
and is mounted on a screw 52 such that its separation from coupling rod 40 
is adjustable. A mode supprssor 50 may be incorporated near each active 
device 36 as shown, or alternatively in some cases a single suppressor 50 
may be adequate. 
Also projecting into cavity 26 is a tuning rod 54, also located on the 
midplane of cavity 26. A tuning rod 54 may be used in each cavity 26 as 
shown, or alternatively a single rod 54 in one cavity 26 may produce 
adequate tuning. Tuning rod 54 is mounted on a screw 56 to adjust its 
penetration into cavity 26 to tune the frequency of the oscillator. Tuner 
54 may be dielectric, such as sapphire; in which case it is located to 
penetrate a region of cavity 26 where the rf electric field is high to 
achieve maximum tuning. Alternatively, tuner 54 may be a metallic rod; in 
this case the tuner is preferably located in a region of cavity 26 where 
the rf magnetic field is high, whereby tuner 54 acts as an inductive tuner 
to raise the cavity frequency by displacing magnetic field. In operation a 
dc bias current is supplied to each active device 36 through its bias lead 
38 from a bias current supply (not shown). If the devices 36 are not 
perfectly similar, it may be preferable to have a separate, adjustable 
bias supply for each device. Oscillation is set up in cavities 26 in a 
push-pull mode with magnetic field pattern as shown by dotted lines 58. In 
this mode, each of cavities 26 contributes equally to the coupling of 
power out into waveguide 34 through load coupling iris 32. Cavity coupling 
iris 30 synchronizes the oscillations in the two cavities 26. 
FIG. 4 shows a modification of the mode suppressor. Here mode suppressor 
rod 50' enters through the end 58 of cavity 26 which is close to coupling 
rod 40. 
FIG. 5 illustrates a somewhat different embodiment in which coupling rod 
40' is located at a sufficient distance from end wall 58 such that tuner 
rod 54' may be located between coupling rod 40' and end wall 58. In this 
case, as mentioned above, tuner 54' is in a region of high rf magnetic 
field and may be a metallic, inductive-tuning rod. Another embodiment is 
to move coupling rode 40' towards iris/coupling wall 28 such that tuner 54 
is dielectric and located between coupling rod 40 and outside wall 58. 
FIGS. 6, 7 and 8 illustrate alternative embodiments of couplng iris 30. 
FIG. 6 is an end section of the oscillator of FIGS. 1, 2 and 3 in which 
iris 30 is a shallow cut-out in common wall 28 located directly below 
output coupling iris 32. FIG. 7 shows an alternative capacitive iris 30' 
extending entirely across the width of cavities 26. The capacitive iris 
produces a different frequency relationship of the various cavity modes 
and may be desirable for some applications. FIG. 8 illustrates a 
full-height inductive iris 30" which produces stronger coupling between 
cavities 26 than the partial-height iris 30. 
FIG. 9 illustrates a different embodiment in which four active devices 36' 
are located in each cavity 26 to further increase the generated power. The 
electromagnetic fields associated with all the devices in a single cavity 
26 are synchronous. It is to be understood that the number of devices in 
each cavity may be chosen to suit the application. To maintain the 
balanced push-pull operation, it is necessary that the number of devices 
in each cavity 26 be equal. 
FIG. 10 illustrates an alternative embodiment of the rf bypass 42'. Here 
the coupling rod 40 is connected to the bias supply terminal 38 through a 
simple coaxial bypass capacitor formed by a metallic cylinder 60 passing 
through a hole in cavity block 22 and insulated therefrom by a thin walled 
dielectric cylinder 44. The isolation of the bypass of FIG. 10 may not be 
quite as good as the choke structure illustrated in FIG. 1 but it is 
mechanically simple and rugged. 
The embodiments described above are intended to be illustrative and not 
limiting. Many other embodiments will be obvious to those skilled in the 
art. The invention is intended to be limited only by the following claims 
and their legal equivalents.