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Document Title: Impedance Matching Techniques for Mixers and Detectors (AN 963)
Part Number: 5952-0496
Revision Date: August 1980
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Impedance Matching Techniques
for Mixers and Detectors
Application Note 963
Introduction
The use of tables for designing impedance matching filters for real
loads is well known [1]. Simple complex loads can often be matched by
this technique by incorporating the imaginary portion of the load into
the first filter element [2]. 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[3] 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 Hewlett-Packard 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
GHz
matching circuit designed for the diode admittance measured with one 12
milliwatt incident would be incorrect for a diode absorbing one 8
milliwatt. The correct measurement of diode admittance must use 4
2
more power so that the diode absorbs one milliwatt. A convenient 0.2 1 0.5 1.0 2.0 3.0 5.0 10.0
method monitors the rectified current. A level of 1.5 milliamperes is
indicative of one milliwatt absorbed.
MEASURED
Figure 1 shows the measured admittance of the 5082-2709 mixer diode.
CALCULATED
Some computer analysis programs can work with a load characterized
in this way. However, in many cases, the equivalent circuit must be
defined. The circuit shown in Figure 1 is commonly used for mixer
0.09 nH 5
diodes. The values shown were obtained by a computer optimization
program. The program changes the values until the calculated 0.02 0.11
pF 230
admittance at several frequencies is close to the measured admittance. pF
Although the impedance matching will be done at X band, a more
accurate equivalent circuit was obtained by using a broader band of
frequencies in the computer optimization procedure. Figure 1. Admittance of 5082-2709
Beam Lead Mixer. 1.5 mA Rectified
Current.
2
Narrow Band Matching 90