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ChasW.
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Homework Statement
I am calculating matching impedances for a source and its load using 2 different L-network topologies.
One is:
source-parallel-to-C, load-series-to-L
and the other is:
source-parallel-to-L, load-series-to-C.
I go through all the calculations and notice that the 2 sets of answers from the quadratic part each yield a set of values that if I intuitively adjust signs for, I get a correct impedance match. Of course only 1 set of answers is appropriate for each of the above 2 L-network cases.
With signs adjusted by me, I verify the match by comparing the resulting impedance of the source paralleled with its element with the resulting impedance of the load series combined with its element. I've done this quite a few times and the results are always the same. Right values, wrong signs.
It would seem to me that if I am applying the formula correctly that at least one set of results from the quadratic equation should yield a correct set of values that are signed properly. Again, I am not sure and hope somebody can enlighten me on this point so I can know whether to continue pursuing a sign error on my part or correct my assumptions with the application of the formula.
Homework Equations
The representations of the above two L-network cases I am using are:
parallel L to series C
[tex]{R_T+jX_T \over 1+jB_L(R_T+jX_T)}-jX_C=R_A-jX_A[/tex]
where,
RT is the source resistance
XT is the source reactance
RA is the load resistance
XA is the load reactance
BL is the inductive susceptance of the parallel element
XC is the capacitive reactance of the series element
parallel C to series L
[tex]{R_T+jX_T \over 1+jB_C(R_T+jX_T)}+jX_L=R_A-jX_A[/tex]
where,
RT is the source resistance
XT is the source reactance
RA is the load resistance
XA is the load reactance
BC is the capacitive susceptance of the parallel element
XL is the inductive reactance of the series element
ZT=150+j75Ω and ZA=75+j15Ω and the frequency of interest is 2GHz.
Note that for both cases RA-jXA is the conjugate of the actual load impedance.
The Attempt at a Solution
The two sets of answers I get are:
BL-1 = 108.712, -258.712
XC = -76.8559, 106.856
BC-1 = 108.712, -258.712
XL = 76.8559, -106.856
For both sets above, the comma delimited values are each results from the positive and negative version of the quadratic.
but for the parallel L to series C, the correct values are:
BL-1 = 258.712
XC = -106.856
and for the parallel C to series L, the correct values are:
BC-1 = -108.712
XL = 76.8559
My questions are somewhat general in nature:
1) Am I wrong for expecting resulting values for the inductive reactance to be positive and the capacitive to be negative? Just to be clear here, I realize that the actual reactances, inductive or capacitive, are positive, but what I am referring to are the complex values as they may apply or not apply to what the result of the quadratic equation yields.
2) The book I am using says to use the positive case for the quadratic part when RT>RA, but that does not as of yet seem to be consistently be the right thing to do. Is there a rule of thumb for this?
3) For the first network I am subtracting the element in series because it is capacitive and the second one I am adding the element in series because it is inductive. Was this correct to do?
I did not post the long list of calculations because of the general nature of my questions, but will happily do so upon request.
The general overview of how I am solving can be stated briefly, however:
Separate the real and imaginary parts of the ZSOURCE. i.e. Solve for RT and XT. From the RT part, solve for the series element and substitute that into the XT part. This results in a quadratic equation for the parallel element. Plugging values into that equation yields two answers, and plugging the parallel element value into the series element equation yields the answer for the series element.
Thank you much in advance for any assistance and patience with the lengthy post.