ELEC 477 - Facstaff Bucknell

Topics in Wireless System Design Lab
Spring 2008
Lab #2: LC Impedance-Matching Networks
Most wireless systems are comprised of subsystems, usually called stages, that are connected
together to accomplish a specific task. For example, a wireless receiver typically consists of
amplifiers, filters, mixers, and other circuits all operating together as a unit. An important
consideration in RF system design is to ensure that the input of each stage presents the proper
load impedance to the preceding stage and that the output impedance of the stage is appropriate
to feed the following stage. This might be necessary to ensure maximum power transfer between
stages. Also, most filters require very specific source and load impedances (sometimes very
different from the standard 50 Ω) in order to operate properly. The process of changing an
effective source or load impedance from one value to another one is called impedance
transformation, or impedance matching. One way to match impedances is to insert a simple
network of inductors and capacitors between stages. In this lab exercise, you will design one
such network, called an L network, to transform a load impedance to 50 Ω.
Experimental Procedure
Record the results of the following procedures on a separate sheet(s) of paper. Turn in your
notes at the end of the lab session. Only one set of notes is required from each group, but please
make sure that the notes are legible and well organized; otherwise, points will be deducted.
Design an L network to match a resistance of 10 Ω to the source impedance of the bench top
function generators (50 Ω) at an operating frequency of 7.1 MHz.
Calculate the input impedance of the matching network/load resistor design over the
frequency range 5-9 MHz using Mathcad, Matlab, or Excel, and use the results to plot the
VSWR (with respect to 50 Ω) over that range.
Build the L network using the components available to you. The inductor should be wound
on a toroidal iron powder core. Information on these cores is available at the Amidon
Associates web site, which is linked on the laboratory web page. The required capacitance
can be approximated using series and/or parallel combinations of standard capacitor values.
Use a quarter-watt 10-Ω resistor to serve as the load. The components should be soldered
together for best results.
Connect the L network/10- load combination to the output of a function or signal generator
set to a frequency of 7.1 MHz. Monitor the input voltage to the matching network using the
oscilloscope, and record its magnitude. Unfortunately, there is no way to measure the phase
shift between the source and load voltages. Remember that the cables commonly used in the
lab can introduce a significant amount of capacitance into the circuit and that the input to the
oscilloscope itself has a capacitance of approximately 13 pF. These loading effects can be
reduced significantly if the ×10 probes are used. Comment on your results; is the input
voltage close to the correct magnitude for a 50- load at 7.1 MHz, given the amplitude
setting of the generator?
Use the handheld impedance analyzer to plot the VSWR vs. frequency over the same range
for which you calculated the VSWR. Hand-copy the curve displayed on the analyzer’s
screen onto your mathematically predicted curve (i.e., plot both on the same graph). Besides
measurement error, what other factors might account for any differences you see?
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