Oscillators – With distributed parameter resonator
Reexamination Certificate
1999-10-08
2003-04-08
Kinkead, Arnold (Department: 2817)
Oscillators
With distributed parameter resonator
C331S1070DP, C331S1070DP, C331S1070DP
Reexamination Certificate
active
06545553
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an oscillator and in particular, although not exclusively, to a Gunn diode oscillator susceptible to automated mass production. The present invention relates also to a method of assembling oscillators and to a method of tuning oscillators.
DESCRIPTION OF THE PRIOR ART
Gunn diode oscillators are important components of radar systems. They are formed from gallium arsenide substrates which are processed in a particular manner to form devices known as Gunn diodes. A Gunn diode is incorporated into an oscillator package to form an oscillator which can generate microwave frequency electromagnetic radiation. Application of a dc voltage across a Gunn diode causes high frequency electron pulses across junctions within it, which pulses cause an oscillating electric field to be set up in the vicinity of the Gunn diode. The Gunn diode is located in, and supported by, a metallic oscillator body which defines a waveguide, supports other components of the oscillator, and uses the electric field to generate microwave frequency electromagnetic radiation. A prior art bias-tuned, second harmonic, radial mode oscillator is shown in section in FIG.
1
.
In
FIG. 1
, a Gunn diode oscillator comprises generally a body
10
in which is defined an elongate waveguide
11
of rectangular cross-section, a bore
12
containing an RF choke
13
, and a composite bore
14
on the opposite side of the waveguide to the bore. A Gunn diode chip
15
is ultrasonically scrubbed onto the top surface of a gold-plated, first heat sink
16
. The Gunn diode
15
is enclosed by an annular, electrically-insulating tube
17
, which is made from alumina, and an electrically-conductive lid
18
. A gold-plated radial disk
19
is held in contact with the lid
18
by application of a constant spring force on an electrically conducting-connector
20
, which passes along the central axis of the RF choke
13
. The surface of the Gunn diode
15
closest to the RF choke
13
is electrically connected to the connector
20
by way of a wire bond or Maltese cross connection
21
made to the junction of the alumina tube
17
and the lid
18
.
The RF choke
13
comprises alternate high impedance and low impedance sections. In this example, the low impedance section is a brass disk
22
, and the high impedance section is of air
23
. The thickness of each of the sections
22
and
23
is equal to three quarters of the wavelength of the electromagnetic radiation to be generated. Further low and high impedance sections
22
and
23
may be incorporated in the choke
13
. The connector
20
, and thus the radial disk
19
, is electrically insulated from the oscillator body
10
by a layer of plastics material (not shown) which is wrapped around the substantially cylindrical form of the choke
13
. The surface of the Gunn diode
15
furthest from the RF choke
13
is electrically and thermally connected to the oscillator body
10
by the first heat sink
16
and a second heat sink
24
. The Gunn diode oscillator can be activated to generate microwave frequency electromagnetic radiation by application of a dc voltage across the oscillator body
10
and the electrical connector
20
.
A short circuit
25
, in the form of an elongate rectangular cross section metallic element, is movable axially along the waveguide
11
. Movement of the short circuit
25
relative to the position of the radial disk
19
and Gunn diode
15
causes variation of the output power of the oscillator, as will be appreciated by those skilled in the art. Microwave frequency electromagnetic energy is radiated in the direction of the arrow.
The first heat sink
16
is connected to the second heat sink
24
by the mating of a threaded axial bore
26
of the second heat sink
24
and a screw thread
27
formed on the primary surface of the first heat sink
16
. This connection is made by rotation of the first heat sink
16
relative to the second heat sink
24
, achieved by rotation of a screwdriver or the like having a blade inserted in a groove
28
formed in the end of the heat sink
16
furthest from the RF choke
13
, to screw the heat sinks
16
and
24
together. Rotation of these components is made until the surface of a flange
29
of the first heat sink
16
which is furthest from the RF choke
13
is in firm contact with a surface of the second heat sink
24
which is closest to the choke
13
. A reasonable degree of thermal and electrical contact is thus made between the first and second heat sinks
16
and
24
.
The second heat sink
24
, and thus the first heat sink
16
, is supported in the oscillator body
10
by the clamping action of an annular screw
30
. The screw
30
has a thread
31
on its outer surface which mates with a threaded bore
32
of the composite bore
14
. The second heat sink
24
is fixed relative to the body
10
by virtue of frictional forces at the washer shaped contact areas
35
and
36
. A flange
29
of the first heat sink
16
and a part of the second heat sink
24
which is adjacent the flange
29
extend through a bore
33
linking the bore
32
with the waveguide
11
so that the Gunn diode
15
is supported in and located in the waveguide
11
.
Because it is difficult to predict what effect the mating of the threads
26
and
27
will have on the concentricity of the outer cylindrical surface of the second heat sink
24
and the central axis of the first heat sink
16
, it is usual to provide the bore
33
with a diameter 3 or 4% larger than the outer diameter of the flange
29
. It has been found that the unscrewing of the annular screw
30
and subsequent re-screwing can cause the operating characteristics of the Gunn diode oscillator to change. In addition to a significant change in the level of the output power, a frequency change of up to 2% can occur as a result of such a re-assembly. This obviously introduces some undesirable variables into the characteristics that can be expected on oscillator assembly, even when tight component tolerances can be achieved. This frequency pulling and the changing of the output power has been thought to be caused by different alignment of the bond wires or Maltese cross connections
21
with respect to the axis of the waveguide
11
. However, the inventor has found that the frequency pulling and output power change caused by the re-assembly is the result largely of the outer surface of the flange
29
, and thus the outer surface of the part of the second heat sink
24
which is closest to the choke
13
, being either more or less concentric with the bore
33
than previously. The concentricity has a bearing on the shape of the air gap between the outer surface of the heat sink
24
and the bore
33
and particularly on the minimum distance between the bore
33
and the outer surface of the heat sink
24
. The inventor has performed experiments which show that the amount of frequency pulling is dependent on the amount of offset from concentric of the bore
33
and the outer surface of the flange
29
of the heat sink
16
.
SUMMARY OF THE INVENTION
In accordance with a first aspect of the present invention, there is provided an oscillator for generating microwave frequency radiation comprising:
an oscillator body having formed therein a waveguide and a substantially cylindrical bore intersecting the waveguide; and
a package comprising a heat sink having a substantially cylindrical portion and an oscillatory semiconductor device supported at one end of the heat sink;
in which the diameter of the cylindrical portion of the heat sink in relation to the diameter of the bore is such that the heat sink is supported in the bore by one of a) an interference fit, and b) a tight sliding fit, with the semiconductor device being located in the waveguide.
This oscillator is not susceptible to variations in frequency and output power which can occur due to variations in concentricity of the heat sink with the bore in which it is supported. This oscillator is also likely to be cheaper to produce than the prior art oscillators because it is not nec
Bird John
Flatters Gary Stephen
Whitworth Bernard
Kilpatrick & Stockton LLP
Kinkead Arnold
Mitel Semiconductor Limited
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