Coaxial DC block

Details Coaxial DC block Coaxial DC block

2003-01-07

2004-09-28

Pascal, Robert (Department: 2817)

Wave transmission lines and networks

Coupling networks

Nonreciprocal gyromagnetic type

C333S012000

Reexamination Certificate

active

06798310

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
REFERENCE TO MICROFICHE APPENDIX
Not applicable.
FIELD OF THE INVENTION
The present invention relates generally to electronic devices and more particularly to a direct-current (“DC”) blocking capacitor integrated into the center conductor of a coaxial transmission structure.
BACKGROUND OF THE INVENTION
Blocking capacitors (commonly called “DC blocks”) are used in a variety of applications to couple alternating current (“AC”) of sufficient frequency across the capacitor while blocking DC current Blocking capacitors have a cutoff frequency, below which AC is not efficiently coupled across the capacitor, and a self-resonant frequency that typically limits the upper frequency of operation. Generally, a lower cutoff frequency can be achieved with a greater capacitance, and a higher self-resonant frequency can be achieved with a physically smaller capacitor.
Blocking capacitors are incorporated in electronic circuits, such as at the input of an amplifier or mixer in a series configuration, to keep DC from damaging the circuit. Incorporating a conventional DC block in a packaged microcircuit typically involves die attaching one plate of a capacitor to the microcircuit and then wire-bonding or mesh-bonding the other plate of a capacitor to another portion of the packaged microcircuit, such as a feedthru pin. This increases assembly time and occupies additional room inside the packaged microcircuit. The conventional DC block often disrupts the transmission characteristics of the circuit, so compensating for the disruption by manipulating the wire-bond or mesh-bond, or by adding tuning elements, such as poly-iron, inside the packaged microcircuit, is employed, which increases the assembly time of the microcircuit.
Coaxial DC blocks incorporate a capacitor along the electrical path of the center conductor of a coaxial structure, such as a coaxial transmission line, a coaxial connector, or a coaxial feedthru. Coaxial DC blocks can be integrated into a packaged microcircuit, an external device, such as a connector, adaptor, or bias-T, or integrated into a port of a test instrument, such as a network analyzer, spectrum analyzer, or signal generator.
FIG. 1
is a simplified cross-section of a first prior-art coaxial DC block
10
. A capacitor
12
is attached to the end face a first center conductor half
14
of a coaxial structure. A bellows
16
is held in a pocket
18
of a second center conductor half
14
′. The bellows are electrically conductive and press against the metallized plate
13
of the capacitor
12
. The other plate
11
of the capacitor is soldered or otherwise electrically and mechanically coupled to the first center conductor half
14
. The capacitance of the capacitor is a function of the area of the plates of the capacitor, the distance between the plates, and the dielectric constant of the material
15
between the plates.
The bellows
16
approximate the diameter of the center conductor to maintain the characteristic impedance of the coaxial structure. Both center conductor halves
14
,
14
′ must be supported to maintain contact (compression) and alignment. The gap that the bellows
16
occupies varies from assembly to assembly, and the bellows compensate for the manufacturing tolerance build-up of the other parts of the DC block by extending or compressing. The diameter of the capacitor
12
is less than or equal to the diameter of the center conductor, so if the center conductor is small the associated capacitor might have an undesirably low capacitance, resulting in a higher cutoff frequency.
The type of coaxial DC block illustrated in
FIG. 1
can achieve good results if the cross-sectional area of the center conductor is sufficiently large or the intended operating frequency is limited. For example, the type of coaxial DC block illustrated in
FIG. 1
can work well in an N-type coaxial connector because the center conductor is relatively large. In these N-type coaxial connectors the coaxial DC block can provide acceptable performance from about 10 MHz to about 18 GHz.
FIG. 2
is a simplified cross section of a second prior art coaxial DC block
20
that uses multiple capacitors. Two parallel-plate capacitors
22
,
24
are held in a special clip
26
that mechanically supports the capacitors and electrically connects the outer plates
28
,
30
of the capacitors to a first center conductor half
14
. Axially resilient coaxial connections
32
,
34
press against the clip
26
and the inner plates
36
,
38
of the capacitors to create a solderless electrical connection with a second center conductor half
14
′.
Unfortunately, the clip
26
and the capacitors
22
,
24
extend relatively far beyond the circumference of the center conductor halves
14
,
14
′. Creating a discontinuity, such as a change in the diameter in the center conductor that disrupts the characteristic impedance, affects the transmission of high-frequency electrical signals through the coaxial DC block. These discontinuities are especially difficult to compensate for at millimeter frequencies. In addition, transmission of high-frequency signals through the compressive contacts are susceptible to shock and vibration as the parts move relative to each other. Additionally, both center conductor halves
14
,
14
′ have to be firmly secured to maintain the compressive contact of the resilient coaxial connections against the capacitors.
BRIEF SUMMARY OF THE INVENTION
A DC block constructed according to the embodiments of the present invention includes a first conductor half and a second conductor half attached to plates of a capacitor extending along a longitudinal axis of the center conductor. The capacitor electrically couples alternating current between the first and second conductor halves and securely attaches the first conductor half to the second conductor half. In some embodiments, two or more capacitors are soldered to the first and second conductor halves in parallel, thus increasing the capacitance between the first and second conductor halves. In other embodiments, a radial capacitor, such as a cylindrical capacitor, is disposed within an outer conductor half.
A method according to an embodiment of the present invention fabricates a coaxial DC block from a center conductor of a coaxial structure, such as a center pin of a coaxial connector or feedthru. One or more parallel-plate capacitors are positioned in mounting slots formed in the center pin before the center pin is separated into first and second center conductor halves. This maintains alignment of the center conductor halves and length of the center pin during fabrication of the coaxial DC block.


REFERENCES:
patent: 2516529 (1950-07-01), Raymond
patent: 3617607 (1971-11-01), Williams
patent: 3980976 (1976-09-01), Tadama et al.
patent: 4399419 (1983-08-01), Dobrovolny
patent: 4586008 (1986-04-01), Raleigh
patent: 5276415 (1994-01-01), Lewandowski et al.
patent: 5327111 (1994-07-01), Gipprich
patent: 6496353 (2002-12-01), Chio
Gill, Ravinder, Bias Tee and DC Block Illuminate 65 GHz, Microwaves and RF, pp. 116-118 and 133 (Sep. 2001).

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