HP8510

Lecture 10

Vector Network Analyzers and Signal Flow Graphs

Sections: 6.7 and 6.11

Homework: From Section 6.13 Exercises: 4, 5, 6, 7, 9, 10, 22

Acknowledgement: Some diagrams and photos are from M. Steer’s book “Microwave and RF Design”

Vector Network Analyzers

port 1 port 2

DUT

HP8510

Nikolova 2012 2LECTURE 10: VECTOR NETWORK ANALYZERS AND SIGNAL FLOW GRAPHS

R&S®ZVA67 VNA2 ports, 67 GHz

Agilent N5247A PNA-X VNA, 4 ports, 67 GHz

Agilent 8719ES

Test

Instruments

MICROPROBERS

UnderDevice

Vector Network Analyzer and IC Probes

HP8510

Nikolova 2012 3LECTURE 10: VECTOR NETWORK ANALYZERS AND SIGNAL FLOW GRAPHS

measurements of circuits with non-coaxial connectors (HMIC, MMIC)

NETWORKANALYZER

S PARAMETERTEST SET

SYNTHESIZER

CONTROLLER

PROBES

RF

HF

GPIB

GSG Probe

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2-Port Vector Network Analyzer: Schematic

1b1a

2a

2b

1a 1b

2b2a

1a

2aLO1

LO2

[Pozar, Microwave Engineering] 4

N-Port Vector Network Analyzer: Schematic

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[Hiebel, Fundamentals of Vector Network Analysis]

1a 1a

1b1b

Vector Network Analyzer: Directional Element

reversed directional coupler

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port 3 terminated with a matched load (power is absorbed, not used)

measure scattered wave bb

a

Signal Flow Graphs used to analyze microwave circuits in terms of incident and

scattered waves used to devise calibration techniques for VNA measurements components of a signal flow graph

nodeseach port has two nodes, ak and bk

branches• a branch shows the

dependency between pairs of nodes

• it has a direction – from input to output

example: 2-port network

1 11 1 12 2

2 21 1 22 2

b S a S ab S a S a

Nikolova 2012

Signal Flow Graphs of Two Basic 1-port Networks

11l S lb a

1bsV

0Z

0

0( )s

ZZ Z

8

a

b

0

0( )s ss

Zb a VZ Z

0

0 0

0

0

, ( )s

s

ss

s

Z VV V bZ Z Z

Z ZZ Z

(a) load

0Z

0

0

ll

l

Z ZZ Z

(b) source

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Decomposition Rules of Signal Flow Graphs

(1) series rule

(2) parallel rule

(3) self-loop rule

(4) splitting rule

9

Nikolova 2012 LECTURE 10: VECTOR NETWORK ANALYZERS AND SIGNAL FLOW GRAPHS

Signal Flow Graphs: Example

Express the input reflection coefficient Γ of a 2-port network in terms of the reflection at the load ΓL and its S-parameters.

21

221 L

ss

2

rule #4at a

2rule #3 at b

rule #1

10

1 12 2111

1 221L

L

b s ssa s

VNA Calibration for 1-port Measurements (3-term Error Model)

• the 3-term error model is known as the OSM (Open-Short-Matched) cal technique (aka OSL or SOL, Open-Short-Load)

• the cal procedure includes 3 measurements performed before the DUT is measured: 1) open circuit, 2) short circuit, 3) matched load

• used when Γ = S11 of a single-port device is measured

• actual measurements include losses and phase delays in connectors and cables, leakage and parasitics inside the instrument – these are viewed as a 2-port error box

• calibration aims at de-embedding these errors from the total measured S-parameters

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3-term Error Model: Signal-flow Graph

Nikolova 2012 12LECTURE 10: VECTOR NETWORK ANALYZERS AND SIGNAL FLOW GRAPHS

error box

S-parameters of the error box contain 3 unknowns

00

10 01 11

1E

ee e e

S

10e

01e

00 01

10 11E

e ee e

S

M

[Rytting, Network Analyzer Error Models and Calibration Methods]

Note: SFG branches without a coefficient have a default coefficient of 1.

3-term Error Model: Error-term Equations

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Using the result from the example on sl. 10 and the signal flow graph in sl. 12, prove the formula

00M

111ee

e

error de-embedding formula(see sl. 10)

3-term Error Model

• for accurate results, one has to know the exact values of Γo, Γs and Γm – use manufacturer’s cal kits!Nikolova 2012 14LECTURE 10: VECTOR NETWORK ANALYZERS AND SIGNAL FLOW GRAPHS

• ideally, in the OSM calibration,

1 o

2 s

3 m

11

0

• the 3 calibration measurements with the 3 standard known loads (Γ1, Γ2, Γ3) produce 3 equations for the 3 unknown error terms

00 11( , , )ee e M 00

M 11 e

ee

error de-embedding

linear system for 00 11[ , , ]Tee e x

2-port Calibration: Classical 12-term Error Model

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consists of two models:• forward (excitation at port 1): models errors in S11M and S21M• reverse (excitation at port 2): models errors in S22M and S12M

port 1

port 2

[Rytting, Network Analyzer Error Models and Calibration Methods]

12-term Error Model: Reverse Model

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port 1

port 2

12-term Error Model: Forward-model SFG

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( )

12-term Error Model: Forward-model SFG

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Using signal-flow graph transformations derive the formulas for S11Mand S21M in the previous slide.

( )

12-term Error Model: Reverse-model SFG

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( )

12-term Calibration Method

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Step 1: Port 1 Calibration using the OSM 1-port procedure.Obtain e11, e00, and Δe, from which (e10e01) is obtained.

Step 2: Connect matched loads (Z0) to both ports (isolation). (S21 = 0)The measured S21M yields e30 directly.

Step 3: Connect ports 1 and 2 directly (thru). (S21=S12=1, S11=S22=0)

Obtain e22 and e10 e32 fromeqns. (*) usingS21 = S12 = 1, S11 = S22 = 0.

• All 6 error terms of the forward model are now known.

• Same procedure is repeated for port 2.

12-term Calibration Method: Error De-embedding

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2-port Thru-Reflect-Line Calibration

• TRL (Thru-Reflect-Line) calibration is used when classical standards such as open, short and matched load cannot be realized

• TRL is the calibration used when measuring devices with non-coaxial terminations (HMIC and MMIC)

• TRL calibration is based on an 8-term error model

• TRL calibration requires three (2-port) calibration structures

thru: the 2 ports must be connected directly, sets the reference planesreflect: load on each port identical; must have large reflectionline (or delay): 2 ports connected with a matched (Z0) transmission line (TL must represent the IC interconnect for the measured DUT)

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Thru, Reflect, and Line Calibration Connections

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(a) thru

(b) reflect

(c) line

Thru-Reflect-Line Calibration Fixtures

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DC needle probes

GSG probes

2-port Calibration: 8-term Error Model

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[Rytting, Network Analyzer Error Models and Calibration Methods]

port 1

port 2

Signal-flow Graph of 8-term Error Model

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port 1

port 2

XT

YT

T

Signal-flow Graphs of the 3 TRL Calibration Measurements

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Signal-flow Graphs of the 3 TRL Calibration Measurements (2)

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Scattering Transfer (or Cascade) Parameters

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• when a network is a cascade of 2-port networks, often the scattering transfer (T-parameters) are used

1 11 12 221 22 21

V VT TT TV V

• relation to S-parameters

11 12 11111 22 12 2121

21 22 22, 1

SS

T T SS S S S ST T S

A BT T TAT BT

1 11 12 2

1 21 22 2

b T T aa T T b

or

8-term Error Model in Terms of T-parameters for TRL Calibration

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error de-embedding

Nikolova 2012 31LECTURE 10: VECTOR NETWORK ANALYZERS AND SIGNAL FLOW GRAPHS

8-term Error Model for TRL Calibration• the number of unknown error terms is actually 7 in the simple

cascaded TRL network (see sl. 26)

1 110 32 M( )e e T A T B

• TRL measurement procedure

M X Y

M1 X C1 Y

M2 X C2 Y

M3 X C3 Y

(1) measured with DUT(2) measured with 2-port cal standard #1(3) measured with 2-port cal standard #2(4) measured with 2-port cal standard #3

T T TTT T T TT T T TT T T T

Nikolova 2012 32LECTURE 10: VECTOR NETWORK ANALYZERS AND SIGNAL FLOW GRAPHS

8-term Error Model for TRL Calibration

• measuring the 3 two-port cal standards yields 12 independent equations while we have only 7 error terms

• thus 5 parameters of the 3 cal standards need not be known and can be determined from the calibration measurements

• which 5 parameters are chosen for which cal standards is important in order to reduce errors and avoid singular matrices

cal standard #1 TC1 must be completely known – thru cal standard #2 TC2 can have 2 unknown transmission terms – line cal standard #3 TC3 can have 3 unknowns; if its reflection

coefficients satisfy S11 = S22 (it is best if S11 = S22 are large!) then its 3 coefficients can be unknown – reflect

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• errors are introduced when measuring a device due to parasitic coupling, leakage and imperfect connections

• these errors must be de-embedded from the overall measured S-parameters

• the de-embedding relies on the measurement of known or partially known cal standards – calibration measurements, which precede the measurement of the DUT

• 1-port calibration uses the 3-term error model and the OSM method• 2-port calibration may use 12-term or 8-term error models• the 12-term error model requires OSM at each port, isolation, and

thru measurements• the 8-term error model with the TRL technique is widely used for

non-coaxial devices• there exists also a 16-term error model, many other cal techniques

VNA Calibration – Summary