From: Bob Lewandowski (Bob.Lewandowski@Vixel.com)
Date: Fri Jun 09 2000 - 18:42:16 PDT
As the curve of the Smith Chart for reflection coefficient (S11) moves in from the outside of the circle it means that the impedance seen at the reference plane is becoming resistive. A "0" ohm reflection will always be just a dot at the "0" ohm point on the chart at all frequencies. If the thing being tested is supposedly
only lossless-reactive it should follow the outside circle of the chart. It is extremely difficult to establish a "short" at the reference plane to calibrate the location of "0" on the chart. A slight mis-adjustment of the electrical delay or length to the reference plane will turn "0" ohms into an inductor or capacitor.
You might try using .085" semi-rigid coax (solid copper jacket, teflon insulation, solid center conductor)
to make your connection to the board under test. Semi-rigid coax typically has close tolerance characteristic impedance and can be cut accurately. Prep the cable by cutting the outer jacket and dielectric off square, and have the center condutor extending out slightly longer than the thickness of the board. Drill a hole
through the board that is very slightly larger than the center conductor diameter, and counterbore the copper on one side to clear the center conductor, but not to the diameter on the outer jacket. Then you can solder the outer jacket to the ground plane on the cleared side and the center conductor to the opposite side. If
you calibrate the analyzer and adjust the reference plane using a short that goes between the center conductor and the outer jacket (a small piece of copper foil with a hole for the center conductor) before you solder the cable to the board you can get a good adjustment of the reference plane location. The inductance of the
center conductor going through the board is still there and not calibrated out, but it's pretty small compared to almost any other calibration method (<1 nH).
Barry Ma wrote:
> Thanks for your instructive input.
> Our simple task is to know what is the upper limit of operating frequency for the planecap. In S11 measurement, when the measuring point in the Smith chart moves from outside circle (nearly "short circuit") toward the center, indicating the impedance of planecap gets higher than 1 ohm, we can know that is the upper limit.
> The price for the simple S11 measurement is inaccuracy. But it still gives us a rough approximation.
> Barry Ma
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