High Frequency Measurements Web Page
Douglas C. Smith

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The Tapered Wall Cavity, or Maintaining Matched Impedance

The symmetric structure shown in the figure is the right half of a T-Pad attenuator manufactured by Barth Electronics. The input signal passes through a long horizontal resistor which forms a voltage divider with the short vertical resistor. The horizontal resistor on the other side is connected to the output. The two horizontal resistors have a value such that the resistance looking into each port equals 50 Ohms to match the cables attached to the input and output ports. The attenuation is set by the value of the short vertical resistor.

Along each horizontal resistor, the resistance to ground decreases along the resistor as one moves from the input/output port to the junction of the resistors. At either port, the impedance to ground of this particular attenuator is 50 Ohms and at the junction of the horizontal and vertical resistors it is about 15 Ohms. The walls of the attenuator are tapered so as to cause the characteristic impedance of the transmission line formed by the horizontal resistor and the cavity walls to be equal to the resistance to ground along each horizontal resistor. Such a design technique results in a device with flat frequency response from DC to tens of GHz.

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Copyright © 1999 Douglas C. Smith