Network Analyser Basics

Network Analyser Basics

Martin J. SiebersLab Manager

msiebers@sun.ac.zahttp://services.ee.sun.ac.za/wiki/index.php/High_Frequency_Lab

Network Analyser Basics

Overview• Network analysers fall into two categories

– The Vector Network Analyser (VNA) is capable of measuring complex reflection and transmission

– The scalar analyser can only measure magnitude • A VNA can measure S-parameter and VSWR,

loss, gain, isolation, and group delay of any two (or more) ports of a given network

• Key manufacturers of VNAs are Agilent (former HP), Anritsu (Wiltron), and Rohde & Schwarz.

• VNAs also called "ANA“ automated network analyser.

Network Analyser Basics

History• The original network analyser (HP 8409)

– No Error Correction build in– Error correction was done by hand

• Then the first automated network analysers were introduced.– A minicomputer processed the vector data– delivered error corrected and accurate magnitude and

phase of the four S-parameters• The next step was to build the error correction

into the TE and display the error-corrected measurements in nearly real time (the original HP 8510, circa 1982).

Network Analyser Basics

The 8510 VNA• The "classic" vector

network analyser is the Agilent (HP) 8510.

• Depending on how much one spend, this analyser can make measurements from 45 MHz to 110 GHz.

• E&E has two 8510C– 45MHz to 20GHz– 45MHz to 50GHz

Network Analyser Basics

Simplified VNA Block Diagram

RECEIVER / DETECTOR

PROCESSOR / DISPLAY

REFLECTED(A)

TRANSMITTED(B)

INCIDENT (R)

SIGNALSEPARATION

Incident Transmitted

Reflected

DUT

SOURCE

TransmittedIncident

TRANSMISSION

Gain / Loss

S-ParametersS21,S12

GroupDelay

Coefficient

Insertion Phase

ReflectedIncident

Transmission

REFLECTION

SWR

S-ParametersS11,S22 Reflection

Coefficient

Impedance, Admittance

R+jX, G+jB

ReturnLoss

Γ, ρΤ,τ

Incident

Reflected

TransmittedR BA

AR

=BR

=

Characterizing Devices

Network Analyser Basics

Network Analyser Basics

Incident TransmittedS 21

S11Reflected S22

Reflected

Transmitted Incidentb 1

a1 b2

a 2S12

DUT

b1 = S11 a1 + S12 a 2b2 = S21 a1 + S22 a 2

Port 1 Port 2

S-Parameters• Relatively easy to obtain at high frequencies

– Measure voltage traveling waves with a vector network analyzer– Don't need shorts/opens which can cause active devices to oscillate or

self-destruct• Relate to familiar measurements (gain, loss, reflection coefficient ...)• Can cascade S-parameters of multiple devices to predict system

performance• Can easily import and use S-parameter files in our electronic-

simulation tools

Network Analyser Basics

Measuring S-Parametersb

S 11 = ReflectedIncident

=b1a 1 a2 = 0

S 21 =Transmitted

Incident=

b2

a 1 a2 = 0

S 22 = ReflectedIncident

=b2a 2 a1 = 0

S 12 =Transmitted

Incident=

b1

a 2 a1 = 0

S 2Incident Transmitted21a

S 11Reflected

Z 0Load

a2 = 0DUT

1

Forwardb 1

IncidentTransmitted S 12

S 22Reflected

b 2

a2b

a1 = 0DUTZ 0

Load

1

Reverse

Network Analyser Basics

S-Parameters Relate to Common Measurement Terms

• S11 = forward reflection coefficient (input match)• S22 = reverse reflection coefficient (output match)• S21 = forward transmission coefficient (gain or loss)• S12 = reverse transmission coefficient (isolation)

Remember, S-parameters are inherentlylinear quantities -- however, we oftenexpress them in a log-magnitude format

Network Analyser Basics

ErrorsSystematic errors

due to imperfections in the analyzer and test setupare assumed to be time invariant (predictable)can be characterized (during calibration process) andmathematically removed during measurements

Random errorsvary with time in random fashion (unpredictable)cannot be removed by calibrationmain contributors:

instrument noise (source phase noise, IF noise floor, etc.)switch repeatabilityconnector repeatability

Drift errorsare due to instrument or test-systemperformance changing after a calibration has been doneare primarily caused by temperature variationcan be removed by further calibration(s)

Unknown Device

Measured Data

SYSTEMATIC

RANDOM

DRIFT

Errors:

Network Analyser Basics

Systematic Measurement ErrorsA B

SourceMismatch

LoadMismatch

CrosstalkDirectivity

DUT

Frequency responsereflection tracking (A/R)transmission tracking (B/R)

R

Six forward and six reverse error terms yields 12 error terms for two-port devices

Network Analyser Basics

What is Calibration?

• Measure known device• Note measured value• Generate error table based on the

difference between known and measured values

• Subtract error from subsequent measurements

Network Analyser Basics

Standards Definitions

• Definitions are the parameters of an equivalent circuit network that represents the physical network– A standard may have a combination of a delay, a

loss, a capacitance, an inductance, a frequency range and a characteristic impedance

• Standards definitions must be known to the VNA– Enter manually – Load from disc

• A Cal Kit comes with the definitions of its standards.

Network Analyser Basics

Calibration Kits• Coaxial calibration kits come in type N, 7 mm, 3.5 mm,

2.92 mm, 2.4 mm, 1.0 mm and skripsie. (Bold available at E&E)

• Be sure not to exceed the frequency capability of the cables, adapters and calibration kit (remember session on connectors).

• There are waveguide calibration kits for every waveguide band. E&E has one for S-Band and one for X-Band.

• Remember that cal kits are expensive, and pieces of the cal kit should never be used as adapters or terminations.

• Always put the little plastic covers onto the cal kit pieces.

Network Analyser Basics

Vector-Error Correction• Process of characterizing systematic error

terms– Measure known standards– remove effects from subsequent

measurements.• 1-port calibration (reflection measurements)

– only 3 systematic error terms measured– directivity, source match, and reflection

tracking• Full 2-port calibration (reflection and

transmission measurements)– 12 systematic error terms measured– usually requires 12 measurements on four

known standards (SOLT)

Network Analyser Basics

Response calibration• This type of calibration is the quickest to perform

but it has its limitations because ports are not matched and:

• A transmission response cal will merely measure the magnitude and phase of the through connection, which will be subtracted from all subsequently measured data. But you won't know anything about the magnitude and phase of the DUTs reflection coefficients.

• A reflection response cal will provide magnitude-only information for reflection coefficients.

Network Analyser Basics

Full 2 Port Calibration

• Calibrate Reflection:– Standards used: Open, Short and Load

• Calibrate Transmission:– Standard used: Thru

• Calibrate Isolation– Standards used: 2 X Load– or "omit isolation“. To measure the loss of some test

cables and not expect to see transmission data below -80 dB, go ahead and omit the isolation. What is recommended in general!

Highest accuracyRemoves these errors:

DirectivitySource & load matchReflection trackingTransmission trackingCrosstalk

UNCORRECTED RESPONSE 1-PORT FULL 2-PORT

ConvenientGenerally not accurateNo errors removed

Easy to performUse when highestaccuracy is not requiredRemoves frequencyresponse error

For reflection measurementsNeed good termination for

high accuracy with two-port devicesRemoves these errors:DirectivitySource matchReflection tracking

DUT

DUTDUT

DUT

OPEN

LOAD

OPEN

LOAD

OPEN

LOADthrough

through

ENHANCED-RESPONSECombines response and 1-portCorrects source match for transmission

measurements

SHORT SHORT SHORT

Errors and Calibration Standards

Network Analyser Basics

Network Analyser Basics

Verification - Validation

• Verification means to measure known standards which haven’t been used for the actual calibration, a verification kit is used.

• To validate the calibration and the general health of the test equipment, look at a few things after calibration– Check the residual error in a "through" connection. Ideally it

should be 0 dB. The phase should be 0.0 degrees as well.– The return loss of both ports should be at least 40 dB but can be

better than 60 dB.– Depending on the cable quality and the frequency range a

variation of the transmission and reflection parameters will be noticed upon the movement of the cables.

• If there is an issue with the calibration, figure out the source of the problem before performing another calibration. Otherwise you will be wasting your time.

Network Analyser Basics

General Considerations

• Before performing a calibration, answer the following questions:– Frequency range? – Do the cal kit, the cables and any adapters operate

over the desired band? – Are the cables fit for this application?

• Notice the effect of gently bending cables.• Notice the effect of moving cables. • Will the cables reach the DUT without unnecessary bending?• Perform calibration with cables in the position needed to be

connected to DUT

Network Analyser Basics

General Considerations• Averaging

– Performing averaging will improve the accuracy of your data, as long as you do it during the calibration as well as the actual measurement.

– It will slow down the measurement process• Smoothing

– Smoothing is cheating!– Smoothing reduces the "bumpiness" of a frequency

response by averaging data across a couple of frequency points and using the result at one frequency.

Network Analyser Basics

General Considerations

• Electrostatic Discharge– Always discharge yourself!– Use the Static Control Wrist Straps provided.– Be extremely careful when executing antenna

measurements (centre conductor is often exposed)• Torque

– The torque of the connections will have an influence on the quality and repeatability of the calibration.

– Always apply the right torque using a torque wrench.

Network Analyser Basics

Any Questions?

Thanks for your attention!See you upstairs R551 for the

VNA Demonstration!