Electrical connectors – With coupling movement-actuating means or retaining means in... – Coupling part with relatively pivotable concentric...
Reexamination Certificate
2002-10-29
2004-08-31
Zarroli, Michael C. (Department: 2839)
Electrical connectors
With coupling movement-actuating means or retaining means in...
Coupling part with relatively pivotable concentric...
C439S332000
Reexamination Certificate
active
06783382
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention concerns the confluence of two issues related to high frequency test equipment, and particular, to test equipment where individual coaxial connectors are used to connect a detachable probe to that equipment. One example is present day high performance oscilloscopes.
The first issue concerns what series connector is used, especially for probes or the connections to signals to be measured by the test equipment, and that are not merely an ancillary part of a test set-up. It is customary for 'scopes to employ BNC connectors for their front and rear panel connections. The BNC connector has a number of attractive features that, so far anyway, have outweighed its disadvantages. These attractive features include ease of use (a quarter twist to mate or un-mate), small enough to not consume too much panel space but not so small as to be mechanically delicate, reasonable in cost and already widely in use with many manufacturers and mounting styles to choose from. It is also a controlled impedance connector, and is available in the commonly used values of 50 &OHgr; and 75 &OHgr;. Save for characteristic impedance, any BNC connector will (in theory, anyway) mate with one of the opposite gender, regardless of who the manufacturers were or what the mounting styles are. In many respects it is the workhorse of the general electronics industry; if it wasn't at hand we'd have to invent it. Nevertheless, and despite its longevity and venerable origin [the Bayonet Navy Connector (BNC) was developed for the US Navy during WW II] it has begun to reveal certain shortcomings. The following several paragraphs relating to the shortcomings of the conventional BNC connector, and an attractive solution therefor, have been abstracted from PRECISION BNC CONNECTOR.
Despite its popularity, the BNC connector has some significant drawbacks when used as an instrument grade connector for some electronic test equipment, such as top of the line high frequency oscilloscopes. It has reactive discontinuities at high frequencies. That is, above certain frequencies it fails to match the 50 &OHgr; characteristic impedance of the coaxial transmission line of which it is expected to be a part. Even the most carefully installed silver-plated mil-spec clamp type BNC connector is extremely visible as a discontinuity on a TDR (Time Domain Reflectometer) of even modest bandwidth. Next, it tends to “leak” (radiate from its mating surfaces) above, say, 500 MHZ. Finally, since it relies solely on internally supplied spring tension to draw its parts together, it can, when under externally applied tension, allow the mating parts to separate sufficiently to degrade the quality of the connection (greater discontinuity, more loss), sometimes to point where the connection is interrupted altogether (especially if the parts are worn from extended use).
Many of the problems of BNC connectors can be traced to aspects in the design of the male half, which is to say, the part that has the male center conductor pin and that is given the quarter turn twist while gripping a knurled shell we shall call a bayonet latch. Let us briefly take a closer look at the conventional BNC connector, the better to appreciate why it has these problems.
The female connector portion includes a female center pin that is centered and held in place by an enclosing Teflon female sleeve. The female sleeve has a reduced diameter portion in front, and toward the rear has a stepped diameter that engages a corresponding shoulder in a female shell. The female sleeve is secured in place from the rear in various ways, depending upon the style and manufacturer. The reduced diameter portion in front will be of interest, shortly.
Now consider the male connector half. As an assembly, it includes a Teflon male sleeve whose rear portion has a small diameter bore that centers and supports a male center pin, and whose front portion has a larger diameter bore sized to just slip over the reduced diameter portion of the female sleeve. When the connector halves are properly mated the two Teflon sleeves are not only in contact over adjacent cylindrical surfaces, but the female sleeve “bottoms out” inside the male sleeve. (The terms “male” and “female” are applied to component parts according to the connector halves as a whole, and its gender is determined by the shape of the center conductor pin. Viewed in isolation, the “male” Teflon sleeve might be thought to be “female”, as it surrounds the outside of the “female” sleeve when the connector halves are mated. But it is part of the male connector half. So it is that the male sleeve has a female shape, but is still called the male sleeve.) Potential gender confusion aside, the important thing is that when proper mating occurs there are edges and surfaces of the sleeves that “vanish” to form one (i.e., unitary) longer tube of Teflon that will be the dielectric material disposed between the center conductor and the outer shield forming the coaxial transmission line.
A similar thing happens to the center pins that they carry. The male pin has a reduced diameter tapered tip that enters a cavity, or socket, centered in the end of the female center conductor. The cavity is slightly undersize, but the end of the female socket is slit to allow a slight resilient outward motion that promotes good ohmic contact between the pins. The thus-expanded outer diameter of the female center pin is the same as that of the male center pin, so that when they are fully mated a shoulder on the male pin and the face of the female pin “disappear” as each of the two pins presses against the end of the other, and the pins appear to be one (unitary) longer cylindrical conductor.
The two sleeves and the two pins are supposed to fully mate simultaneously, for if one were to mate before the other it would prevent the further motion needed by the other to become fully mated.
Surrounding and carrying the sleeves are respective cylindrical connector shells, one male and one female. The male shell has a collection of slits so that they can bend inward slightly under compression as they enter a female shell of slightly insufficient diameter. This provides good ohmic contact for maintaining the outer shield of the coaxial system. Once again, the male shell is expected to bottom out against a stepped diameter within the female shell, so that (save for the slits) the mated pair of shells appears as a complete unitary cylinder of uniform inner diameter as the end of the male shell vanishes against the shoulder inside the female shell.
A pair of bayonet pins on the outside of the female shell engage detents at the end of a quarter turn spiral groove in a rotatable captive bayonet latch carried on the male shell. Depending upon the particular design, a spring located somewhere in the above described elements provides a resilient force that pulls the center pins, sleeves and shells together once the detents in the bayonet latch contain the bayonet pins. If everything is working correctly, no RF currents flow through the connection between the bayonet pins and the bayonet latch. Unfortunately, pulling on the cable, or otherwise inducing external tension urging the two connector halves apart, can overcome the internal spring tension keeping the connectors halves together. If a sufficient tension is applied the connector halves will draw apart slightly.
There are two basic aspects that we wish to point out. First, the tapered end of the male center pin enters a slitted socket in the end of female pin, and ordinarily spreads those slit portions apart slightly, for good contact. As the connector wears the diameter of the tapered end portion of the male center pin and the resilience in the slit female pin are both reduced, while the inner diameter of the female pin is increased, so that a slight withdrawal of the male pin can significantly decrease the ohmic quality of the connection. Equally as bad at higher frequencies, as the withdrawal occurs, there appears a short length over which there is a marked decrease
Agilent Technologie,s Inc.
Miller Edward L.
Zarroli Michael C.
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