Straddle-Mount Connectors

When the snow finally came this year, it really poured, so I decided to make a *big* snowman. I hooked up a 35-HP snow-blower to my tractor and used it to pile up a veritable mountain of snow, out of which emerged what has come to be known as the "Giant George Washington's Head Project". After all, if one is going to make a snowman, why not "do it right"?

The George Washington Project

I wish you all a warm and comfortable holiday season.


In response to my previous EDN column about "Tapered Transitions" I received numerous messages asking what is a "straddle mount connector".

A straddle mount connector is one designed to hang over the edge of a printed-circuit board, as opposed to a connector that sits up on one side of the board only.

Connectors that sit up on one side of the board are convenient for mechanical reasons, as they absorb almost no mechanical headroom below the plane of the board (beyond that required to clear the back sides of the through-hole pins, if they have them). The disadvantage of a one-sided connector is that the connector pins are necessarily displaced up above the plane of the board (and the plane of the circuits), meaning that your signals must travel UP from the board, then across through the connector, and then back DOWN on the other side. In doing so the signals will encounter some inevitable amount of parasitic inductance and capacitance.

A straddle-mount connector locates its pins down much closer to the plane of the printed-circuit board. It is used only at the edge of a board, as some part of the mating assembly of the connector is usually designed to hang off the edge of the board, protruding down below the plane of the board. This style of connector requires mechanical headroom below the plane of the board.

It is called a straddle-mount connector because when viewed from the side the connector assembly appears to "straddle" the edge of the board (as opposed to "riding on top" of the board.

The main advantage of a straddle-mounted connector is the reduction in the distance your signal must flow when traversing the connector. Because no long pins are required to bring your signals UP out of the plane of the board, the straddle-mount connector may achieve a much more intimate connection between the grounds of the two mated circuits than is otherwise possible using a connector that sits up on one side of the board only.

Figure 1 shows example of a straddle-mounted (a.k.a. edge-mounted) SMA connector. I don't mean to imply it's the best one; it's just an example. The four square ground pins on this connector straddle your PCB, two on the top and two on the bottom. The central signal pin comes off the connector co-planar with the two top grounds; it attaches to the top layer.

Straddle-mount RF connectorFigure 1—AMP makes this Straddle-mount SMA-style connector (p/n 449692-1).

Another question I got was, "What's an Eisenhart connector?" The Eisenhart connector was a very old N-type connector documented in Gupta and Garg's book, "Microstrip Lines and Slotlines", Artech House, 1996, ISBN 0-89006-766-X. I mentioned it as an example of where someone had successfully used the tapering principle. I didn't intend for anybody to rush out and buy one... and I can't find a supplier that has any. If you locate one that's still available, let me know. It may now go by a different name.

Best Regards,
Dr. Howard Johnson