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Computing with Scattering
Parameters
By Joseph L. Cahak
Copyright 2013
Sunshine Design Engineering ServicesComplex 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.
Complex ValuesComplex
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).
Z Parameters 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.
Y Parameters
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.
h Parameters
Another 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.
g Parameters
Often 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 ParametersABCD-parameters are known variously as chain, cascade,
or transmission line parameters. This is useful for cascading 2-port network responses.
ABCD' or T' ParametersThese 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.
Network Transforms 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
Transmission Lines
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-ParametersWhile
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:
b
_{1}=T
_{11}a
_{2}+T
_{12}b
_{2}
a
_{1}=T
_{21}a
_{2}+T
_{22}b
_{2}
They can be defined another way:
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.
T
_{total} = T
_{a}T
_{b}T
_{c}
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 T
_{total} 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.
S to
T: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:
Embedding
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 S
_{11}=S
_{22}=0 and S
_{12}=S
_{21}=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.
S
_{11}
= S
_{22}=0 and S
_{21} = S
_{12}=1
Figure 4 - Anti-Network Computation
Anti-Network EquationSolving
for N#
_{xx} gives:
De-Embedding
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 ProcessThis 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 De-embedding Process
1.1.1
Gamma In and OutThis 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.
1.1.2
Gain EquationsThe 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 GainThe operating
power gain is the ratio of the power delivered to the load and the power input to the network.
Transducer
GainTransducer gain is the ratio of the power delivered to the load to the
power available from the source.
Unilateral
Transducer GainThe 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 S
_{12}
reverse transmission (high isolation).
Available GainAvailable 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.
with
D = S
_{11}S
_{22}
- S
_{21}S
_{12}M = S
_{11} - DS'
_{22}N = S
_{22}
- DS'
_{11}S-Parameter Re-NormalizationWhen 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
Forward Parameters
N
_{21 }= Z
_{o}[(1+S
_{11})(1-S
_{22}Γ_{2})+S
_{12}S
_{21}Γ_{2}]
D
_{21} = Z
_{1}[(1-S
_{11})(1-S
_{22}Γ_{2})-S
_{12}S
_{21}Γ_{2}]
Reverse
ParametersN
_{12 }= Z
_{o}[(1+S
_{22})(1-S
_{11}Γ_{1})+S
_{12}S
_{21}Γ_{1}]
]
D
_{12} = Z
_{2}(1-S
_{22})(1-S
_{11}Γ_{1})-S
_{12}S
_{21}Γ_{1}]
ConclusionShown 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.
Sunshine Design Engineering Services is located in the sunny San Vicente Valley
near San Diego, CA, gateway to the mountains and skies. Are you looking for new things to design,
program or create and need assistance? I offer design services with specialties in electronic hardware,
CAD and software engineering, and 25 years of experience with Test Engineering services in RF/microwave,
transceiver and semiconductor parametric test, test application program development, automation
programs, database programming, graphics and analysis, and mathematical algorithms. |
See also: |
- Searching for the Q -
Hybrid Heaven -
Noise and Noise Measurements -
Solace in Solar -
Measuring Semiconductor
Device Input Parameters
with Vector Analysis
- Computing with Scattering Parameters
- Measurements with Scattering Parameters
- Ponderings on Power Measurements -
Scattered Thoughts on Scattering Parameters |
Sunshine Design Engineering Services 23517 Carmena Rd Ramona,
CA 92065 760-685-1126 Featuring: Test Automation Services, RF Calculator and S-Parameter
Library (DLL & LLB) www.AstroCalculator.com
SunshineDesign@cox.net |
Posted August 11, 2013