Computing with Scattering Parameters
Sunshine Design Engineering Services
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Computing with Scattering Parameters
By Joseph L. Cahak
Copyright 2013 Sunshine Design Engineering Services
Complex Numbers and Parameters
A complex number is represented by two values X and
Y, as in X + iY = Z. X is the real component and iY is the imaginary component with Z being the complex representation.
The letter 'i' is used to designate the complex operator (√-1) by scientists and mathematicians, but in engineering
the letter 'j' is most often used. This is used to represent AC or RF signals. X may be the resistance and Y the reactance
in ohms of the component to an AC signal. A standard RF load for instance is represented by 50 ohms real resistance
and some reactance with good loads having reactances of close to 0 ohms. X + iY can also represent a voltage and current
as a function of time. Current is a function of the load and the voltage applied, so the resistances, voltages and
currents are all important in describing a network for DC and AC (or RF) signals. A 2-port network can be represented
by two equations with four parameters. The four parameters have different representations depending on what is known
and what operations the users desire to use them for. Some network parameters (ABDC, S-, Z-, Y-, h-, etc.) are better
than others depending on circuit types and the operations on them.
numbers are used in a variety of ways in calculations. Typical data structures are data clusters and groups of values
and the properties associated with them. Properties most often used are units of dB/linear, degrees/radians, rectilinear/polar.
With this data description we know enough about the state of the complex value to operate on it for RF operations.
Values in dB or dBv must first be converted to Linear and Rectilinear for most complex operations other than simple
addition or subtraction. S-parameter and any other complex number must first be to converted to the proper non-decibel
data format for the calculations, and then back to the final display format (dB-Polar, for instance).
Open circuit impedance parameters are used to represent the impedances of the
network. The values are complex and represent real resistance (R) and reactance (jX) of the elements of the network.
Short circuit parameters or admittance parameters are used to represent the inverse of impedance. The real and
imaginary parts are conductance (G) and susceptance (jB). The units are mhos or Siemens.
model commonly used to analyze BJT (bipolar junction transistor) circuits is the h-parameter model, closely related
to the hybrid-pi model and the y-parameter two-port model, but the h-parameter model uses input current and output
voltage as independent variables, rather than input and output voltages. In this case it is a hybrid of an open circuit
on the input and a short circuit on the output.
this circuit is selected when a voltage amplifier is wanted at the output. The off-diagonal g-parameters are dimensionless,
while diagonal members have dimensions the reciprocal of one another.
ABCD or T Parameters
ABCD-parameters are known variously as chain, cascade, or transmission line parameters. This is useful for cascading
2-port network responses.
ABCD' or T' Parameters
These are the inverse of the ABCD or T-parameters, respectively.
They are useful for de-embedding 2-port network responses when multiplied with the ABCD matrix for the overall network.
These are the matrix transform that are used to convert from one network description to another. Typically the –parameter,
Z, Y, h and ABCD are direct conversions. The conversion to a few of the others are after the base conversion then
the conversion from h to the inverse g, ABCD with the inverse ABCD-1 and S-parameter to the T (one of the
two types) or the anti-S parameter (S-1) . Note that the ABCD matrix is also known as a “T” matrix; this
is not to be confused with the other two descriptions of a “T” matrix connected with the S-parameters calculations
and will be presented in another paper. See Figure 1 for a graphical representation of the transformations available.
Figure 1. Network Description Transformations
Some of the
transmission line functions and parameters are complex in value. They are excited by RF signals with most having complex
modulation applied to them. Some basic measurement parameters need to be defined. A load can be represented at RF
frequencies as a real resistance and an imaginary reactance driven by the charge delay or advancement as related to
the driving voltage. This complex impedance is written as Z(ohms) = X + iY, with Z being complex value of the real
(X) and imaginary (Y) components. Y is positive for an inductive element and negative for a capacitive element.
Scattering Transfer or T-Parameters
While S-parameters are useful for measurements and
describing network response, it is not as useful for network embedding and de-embedding. To perform these operations
another network parameter better suited: either the T-parameter or ABCD matrix. The scattering transfer parameters
or T-parameters of a 2-port network are expressed by the T-parameter matrix and are closely related to the corresponding
S-parameter matrix. The T-parameter matrix is related to the incident and reflected normalized waves at each of the
ports as follows:
They can be defined
When I was researching this for the RFCalculator™ and S- parameters library, I made the discovery that there were
two different definitions for these T-parameters. The RF Toolbox add-on from MATLAB and several other references use
the last definition and the operations do not mix. The "From S to T" and "From T to S" in this article are both based
on the first definition. Adaptation to the second definition is trivial (interchanging T11 for T22, and T12 for T21).
The advantage of T-parameters compared to S-parameters is that they may be used to readily determine the effect of
cascading two or more 2-port networks by simply multiplying the associated individual T-parameter matrices together.
With T-Parameters, we can perform matrix multiplication for the cascading networks operations to get the total network
T-Parameters and transform back to S-parameters. See Figure 2.
Ttotal = TaTbTc
Figure 2 - Cascaded Network
T-Parameters are complex just like S-parameters and there are transform equations between the two parameter types
and this must be performed on all elements prior to the complex matrix multiplication. Then the Ttotal
must be transformed back to S-parameters for the final answer in S-parameters. Note also that matrix multiplication
is not commutative; that is, AB does not equal BA. This is very important to the cascading math.
To convert from S-parameters to T-parameters the following formulas apply:
T to S:
To convert from T-parameters
to S-parameters the following formulas apply:
There are the formulas that can be used directly with S-parameters to embed
the network response of two network components, N and N#, for the response as a singular network, S. The network embedding
equation will take both network S-parameter responses and compute the S-parameters as the combined response of the
two as a whole network. See Figure 3.
Network Embedding Equations
Figure 3 – Embedding Cascaded Network S-Parameters
S-Parameter Anti Network and De-Embedding
To perform de-embedding, we must first find the inverse network or anti-network. This is done by using the embedding
formula and the network to be de-embedded as one branch and the sum network to be ideal S11=S22=0
and S12=S21=1. Then, solve for the unknown sub-network to obtain the anti-network. This can
then be used to embed to the network to be de-embedded to remove that network element from the total response for
the sub-net response of the remaining component. See Figure 4.
S11 = S22=0 and
S21 = S12=1
Figure 4 - Anti-Network
Solving for N#xx gives:
The process used to de-embed a sub-network from the network to get the remaining sub-network result is to calculate
the anti-network of the sub-network you are removing and then cascading the result with the network to remove it from
the response. Doing so produces the desired sub-network.
Fixture Embedding and De-Embedding Process
This section describes the process of fixture embedding and de-embedding. The embedding function is included to
facilitate making simulated networks to test the fixture de-embedding process. This case is an extension of the de-embedded
AB network. Instead of just computing AB x A-1 = B , double up the math to compute AB and then (AB)C to
get the total. The operations must be performed in the order shown in Figure 5 to be correct.
Figure 5 - Fixture
Gamma In and Out
This value of gamma (reflection coefficient) represents the source and load impedance
compared to the measurement system impedance. It gives a measure of power reflected. The gamma in and out values are
both representative of the source or load looking through a network. So the gamma in is the combined response of the
source and an intervening network and similarly for gamma out on the load side.
The network S-parameters can be used to calculate the scalar gain of the network. The
gains are the operating power gain, transducer gain, unilateral transducer gain and the available gain.
Operating Power Gain
The operating power gain is the ratio of the power delivered to the load
and the power input to the network.
Transducer gain is the ratio of the power delivered to the load to the
power available from the source.
Unilateral Transducer Gain
The unilateral transducer gain is the ratio of the power delivered
to the load to the power available from the source for a device with little to no S12 reverse transmission
Available gain is the ratio of the power available from the 2-port network to the power available
from the source. This gain is useful to calculate the network gain (or loss) of an input network to a device being
tested for noise figure. This loss is the noise figure in dBF of the input network for the cascaded gain equation
when making a device noise figure measurement.
D = S11S22
M = S11 - DS'22
N = S22 - DS'11
When using a vector network analyzer (VNA), a spectrum analyzer
(SA), or vector signal analyzer (VSA), the measurement impedance is defined by the system hardware which is designed
to a specified system impedance. When a component must be designed and measured with an impedance not at the typical
50 ohm measurement system impedance, special accommodations must be made. In a scalar measurement system, this
can be accommodated with a simple voltage impedance conversion and gain adjustment. In a complex signal environment
this has to be done through the use of complex mathematical operations to account for all the complex impedances,
matches and gains. I found the following formulas (see references) for the conversion of 50 ohm or any other
impedance measurements to any device impedance like 75 ohms commonly found in commercial broadcast equipment
or the less common video 90 ohms. The newer model VNAs have these capabilities built in. In those cases where
you do not have the feature built in, these normalization formulas let you use a 50 ohm VNA to make measure an
arbitrary impedance a 75 ohm amplifier measurement, for instance. See Figure 6.
Figure 6 - S-Parameter Re-Normalization to Arbitrary Impedances
N21 = Zo[(1+S11)(1-S22Γ2)+S12S21Γ2]
D21 = Z1[(1-S11)(1-S22Γ2)-S12S21Γ2]
N12 = Zo[(1+S22)(1-S11Γ1)+S12S21Γ1]
D12 = Z2(1-S22)(1-S11Γ1)-S12S21Γ1]
Shown was that S-parameters can be used for a variety of network computations and can add value to measurements
where the equipment is limited in features. The reader can find these equations and more in my S-Parameter Library
(DLL & LLB) and my RF Calculator products.
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Posted August 11, 2013
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