Precision BNC connector

Electrical connectors – With coupling movement-actuating means or retaining means in... – Bayonet coupling part movable about its axis

Reexamination Certificate

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C439S578000

Reexamination Certificate

active

06609925

ABSTRACT:

BACKGROUND OF THE INVENTION
The venerable BNC connector is used in a great many instruments for a variety of purposes, ranging from connecting probes on equipment that uses probes (e.g., oscilloscopes) to general input and output of signals. It comes in various grades, ranging from the more expensive instrument grade clamp type to crimp on versions that are not expected to exhibit the full measure of performance or long term durability that is associated with the silver plated mil-spec top of the line versions. Aside from the variety of versions to choose from, one of the positive aspects of BNC is its ease of use. It is pushed on and then mated with a simple quarter-turn twist. This aspect of BNC compares favorably with other series connectors, such as TNC, N, SMA, APC-7 and APC-3.5, each of which involves a threaded nut or sleeve that must be given several turns to mate the connector.
BNC connectors are readily available, relatively inexpensive (as RF connectors go) and all the various versions (as long as the characteristic impedance is the same) inter-mate with one another. It is truly a workhorse of the electronics industry.
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 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).
Much (although not all) 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.
To begin, refer to
FIG. 1
, which is a side view
1
of a pair of mating BNC connector halves
2
and
3
. Connector half
2
is the female part (going by the pin for the center conductor, which is not visible), while connector half
3
is the male part. From the drawing we cannot tell what overall function the female part
2
performs; it might be part of a “tee,” the BNC part of a cross series adapter, be cable mounted or bulkhead mounted; such differences do not matter, the portion that is shown would be the same in all those cases. The male part is depicted as being a clamp-type cable mount part, and that, too, is simply one choice among many. Thus, a rear portion of a male shell
10
of the clamp variety is visible, as is the cable
11
it attaches to.
The female part
2
has a reduced diameter section
5
over which the male part slides. During this action the male center pin enters the female center pin (which is not visible in this view) while spiral grooves
7
engage pins
6
for the quarter turn (another groove
7
and pin
6
are on the back side of the part shown). A detent
8
in the spiral groove
7
engages the pin
6
, preventing spontaneous disengagement by requiring that some twisting force be applied to the connector to overcome the action of the detent. A spring action to be described below cooperates with the detent and pin
6
in producing this behavior. A knurled ring on bayonet latch
9
assists in performing the quarter turns needed for mating and un-mating the connector halves.
Refer now to
FIG. 2
, which is an exploded view of the BNC connector halves of FIG.
2
. While bearing in mind that BNC connectors are fabricated in different ways according to manufacturer's preference, we can nevertheless appreciate the basic mechanisms that account for certain of the BNC connector's troubles that we set out above.
Note that the female connector portion
2
includes a female center pin
13
that is centered and held in place by a Teflon sleeve
15
. The sleeve
15
has a stepped diameter in front that engages a corresponding shoulder in a female shell (
4
,
5
). The sleeve
15
is secured in place from the rear, either by a rolled edge with or without a washer, or, as is shown, by a clamp nut
14
. (In this figure we have chosen to let connector half
2
be a bulkhead mounted clamped-to-cable assembly, which is merely exemplary.) Teflon sleeve
15
has a reduced diameter portion
22
that will be of interest, shortly.
Now consider the male connector half
3
. As an assembly it includes a Teflon sleeve
20
whose rear portion has a small diameter bore that centers and supports a male center pin
19
, and whose front portion has a larger diameter bore
23
sized to just slip over the reduced diameter portion
22
of sleeve
15
.
Another aspect of male connector half
3
that is of interest are the washers
16
and
18
, between which are located spring washer(s)
17
. When assembled, the knurled male BNC bayonet latch
9
is made captive to male BNC shell
21
. BNC shell
21
has a collection of slits that make somewhat springy the end that enters the female shell
5
.
Here is how BNC shell
21
is made captive to the bayonet latch
9
. Washer
18
centers itself on and is retained by, a shoulder
24
of the male shell
21
(or the washer
18
is split, so that it may snap into a groove
24
), while the outside of washer
16
centers itself within a cavity in the back of the bayonet latch
9
, as its inside slides over the outer diameter of male shell
21
. A rolling of an edge in the back side of bayonet latch
9
makes washer
16
captive within the cavity. Furthermore, bayonet latch
9
cannot slide off the male shell toward the rear, owing to a stepped shoulder
24
. Between the two washers
16
and
18
are one or more spring washers
17
that push washers
16
and
18
apart. What they also do is push the bayonet latch
9
rearward in the direction of arrow
25
when the connector halves are mated. This is the internally supplied spring tension that keeps detent
8
engaged against pin
6
, and requires torque to overcome in order to unlatch the connector. Unfortunately, pulling on the cable
11
(see
FIG. 1
) in the direction of arrow
25
, or otherwise inducing external tension urging the two connector halves apart, can overcome the internal spring tension (from spring washers
17
) keeping the connectors halves together. A sufficient tension will compresses the springy washers
17
as they yield, and the connector halves draw apart slightly.
There are two basic aspects that we wish to point out. First, the tapered end of the male center pin
19
enters a slit end of female pin
13
, and ordinarily spreads those slit portions apart slightly, for good contact. As the connector wears the resilience in the slit female pin is reduced, so that a slight withdrawal of the male pin can 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 decrease in diameter. (That is, the male and female center pins have the same outer diameter, and when they are fully mated there are annular surfaces that touch, shoulder to shoulder. When that occurs there is no, or very little, effective change in the outer diameter of the combined center pins.) A similar increase in the effective diameter of the outer conductor occurs also, as the end of the male shell pulls away from the shoulder in the

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