Re: [SI-LIST] : SMA Impedance Matching

Michael A. Baxter (baxter@nesa.com)
Wed, 04 Mar 1998 15:28:04 -0500

Hi Bruce,

We use PCB-mount SMA connectors quite frequently in many different
types of test boards. One big advantage in using them in test boards
is that much of the equipment we use has SMA connectors and cables.

You do see quite a capacitive looking discontinuity with a TDR, which
looks worse with faster risetime as expected. This discontinuity is
less if say an SMA is located some distance down a PCB trace since the
TDR pulse risetime slows down as it propagates.

In our work, we really haven't seen SMA's affecting our results even at
high speeds and fast risetimes likely because the discontinuity is quite
short in duration in most cases. We have, however, experimented with
reducing the capacitive dip by cutting out some of the ground plane around
the ground locations. The ground pins can be connected with wide pieces
of trace and this has the effect of making the connection a little more
inductive. You can play around with this method a bit and reduce the
overall discontinuity by offsetting the capacitive dip. I must admit that
we haven't been real scientific in our approach to date. But, doing this
has the added benefit of easing the soldering of the SMA (less immediate
plane area to heat) which is much appreciated if you tend to solder and
de-solder SMAs in the lab by hand.

Regards,

- Michael Baxter

At 01:59 PM 3/4/98 EST, Bruce.Wallick@analog.com wrote:
>Hello:
>I installed a Straight Jack Receptacle (SMA female connector) on an
>FR-4 PCB. TDR measurements indicate that the impedance goes down to
>25-30 ohms where the connector pin actually goes through the board.
>
>I am concerned about this mismatch. Does anyone have any input as to
>how I can minimize the size of this discontinuity?
>
>Thanks for any and all help,
>Bruce Wallick
>
>

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