Re: [SI-LIST] : SMA test connector PCB layout

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From: Bob Lewandowski (Bob.Lewandowski@Vixel.com)
Date: Wed Jan 03 2001 - 10:41:16 PST


Johnson Components has an "end launch" SMA that fits into a slot in the board
edge, with a pin to contact a top surface microstrip trace. It's application is
independent of board thickness. Johnson P/N 142-0721-88x. Their web site is:
http://www.johnsoncomp.com/sma.htm. A front view picture is shown as "End
Launch". The pin diameter is 30 mils, so some adjustment of the launch must be
made for trace widths less than 30 mils. Connection from the body to ground is
also a significant issue with this type of connector. An edge plated slot that
ties directly to the ground layers is best, both electrically and mechanically.

Your other choice, the 4 ground pin through hole type is difficult to match to
FR-4 or similar Er materials. The best possible application is to a back side
microstrip with minimum center conductor hole and pad diameters. No pads on
inner layers. Clear ground pads on all layers back as far as possible, and
drill holes after plating between the center pin and the corner pins to reduce
capacitance. With careful layout you can get a reflection coefficient of better
than 10%.

Incidentally, you should care about bad launches with a TDR because a large
launch discontinuity masks (attenuates) downstream reflections.

---Bob Lewandowski
    Vixel Corp.

Larry Miller wrote:

> I need to make some measurements on PC boards up to 5 GHz using a network
> analyzer. In the past I didn't care that much because I was doing TDR
> measurements.
>
> Unfortunately, the boards I will be looking at are too thick to use the "SMA
> edge connector" trick.
>
> Does anyone have a pointer to a good PCB footprint that will not have
> impedance discontinuities. The connector I want to mount is the familiar
> center pin + 4 ground pins vertical SMA style.
>
> Thanks in advance for any help,
>
> Larry Miller
>
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