Articles

RSS

Click Here To Download:
•Application Note: Impedance Matching Techniques For Mixers And Detectors

By Avago Technologies

Introduction

The use of tables for designing impedance matching filters for real loads is well known. Simple complex loads can often be matched by this technique by incorporating the imaginary portion of the load into the first filter element. This technique is rarely useful for matching diodes because the equivalent circuit for the diode must include several real and imaginary elements. A methodical technique for matching such complex loads to a transmission line will be described. Previous references to similar procedures were empirical in nature. No tables are used, but it is necessary to know the admittance of the diode in the frequency band of interest.

Mixer Diode Admittance

As an example, the Agilent Technologies 5082-2709 mixer diode will be matched from 8 GHz to 12 GHz. A common local oscillator power level is one milliwatt, but it is not useful to measure the diode admittance at this level. The admittance is a function of power absorbed, so the diode admittance would change when the matching circuit is added. A matching circuit designed for the diode admittance measured with one milliwatt incident would be incorrect for a diode absorbing one milliwatt. The correct measurement of diode admittance must use more power so that the diode absorbs one milliwatt. A convenient method monitors the rectified current. A level of 1.5 milliamperes is indicative of one milliwatt absorbed.

Click Here To Download:
•Application Note: Impedance Matching Techniques For Mixers And Detectors