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Cable Testing & Time Domain

VNA Roadshow – Budapest 17/05/2016

VNA Roadshow 2016

Copyright© ANRITSU2

Content

Coax Cable Measurements are simple and easy?

Yes, BUT.... you need to understand some basic terms:

• Return Loss, VSWR

• Insertion Loss

• Time Domain, Distance to Fault

+ + Background infos and measurement tips


Why characterize a RF cable?Communications systems need good integration of all components from the radio to the antenna

The goal of such a system is to transfer the maximum amount of RF energy.

To accomplish this, the system components must Match from an RF standpoint

Otherwise, reflections occurReflections are bad for the performance of our syst em

The best results happen when every system component is exactly 50 Ohms (or well matched)


The ideal world of transmission lines• Ideally a device has exactly it‘s characteristic impedance (e.g. 50 Ohm)

• Transmission Line• Transmitter

• Receiver

• Impedance matching enables maximum power transmission

Input Output

50 Ohm

50 Ohm

TransmissionLine

TransmitPower

ReceivedPower

≈

Maximum power transmission


The real world of transmission lines• DUT (Device under Test) has an impedance different to the characteristic impedance

• Reflection coefficient r = V refl / Vtrans

• Transmission coefficient Pout / Pin

ReflectedPower P refl

Input Output

TransmissionLine Received

Power P out

TransmitPower P in

≈

Z ≠ 50 Ohm


Coaxial cablesImpedance of coaxial transmission lines

• Relationship between inner- and outer diameter defines impedance of a coaxial cable

• Dielectric material

• Propagation velocity

Examples of coaxial cables used in communication systems

• Aircore dielectric

• Extremely low loss

• Coaxial cables

• Polyethylene solid

• Polyethylene foam

• Teflon (PTFE)

• Radiating cables

• Underground radio communication

• Impedance mostly 50 Ohm

• 75 Ohm for video cables and satellite IF cables

• 100 Ohm (Twisted Pair, Ethernet, symetrical transmission lines)

Inner diameter D ofouter conductor

Outer diameter d ofcenter conductor


Why Z = 50 Ohms?

Good compromise


Upper useable frequencyDominant propagation mode for electromagnetic waves in coaxial cable is TEM mode

Since a coaxial cable supports TEM, TE and TM electromagnetic wave propagation modes, the TE and TM modes will come into existent for sufficiently high operating frequency


R e t u r n L o s s a n d V S W R


The basic reflectometerReturn Loss measurement require a reflectometer

• Distinguish between transmitted and reflected waves

Generator

Vefl

DUTS11

Vtransmit

Directional Coupler,Bridge

≈

r = Vrefl / Vtransmit

Z ≠ Z0

Vrefl

Vtransmit

No reflectometer is ideal → Calibration necessary


The meaning of VSWR

Voltage Standing Wave Ratio (VSWR)

Ratio of maximum voltage to minimum voltage

Some users prefer VSWR while others prefer Return Loss


Return Loss or VSWR: What is better?

Reflection Coefficient: r = Vrefl / Vforward

r = (VSWR-1) / (VSWR+1)

Return Loss: RL[dB] = - 20 x log ( r )

(V)SWR (Voltage Standing Wave Ratio) VSWR = ( 1 + r ) / ( 1 - r )

Rho (linear reflection) ρ = Z - Z0 / Z + Z0

What do you prefer?Both gets the job done


VSWR vs Return Loss

VSWR Return Loss (dB) Description 1.0:1 ∞ Perfect match. Cannot be done.

1.2:1 21 Very good match, very little reflection. (Often a design goal)

1.4:1 15 Acceptable match, acceptable amount of reflection.

3:1 6 Terrible match, significant reflections. Something is definitely wrong.

∞:1 0 Total Reflection. No signal is getting past the reflection point.

Comparison of VSWR and Return Loss


Typical coaxial cable parameters


Return Loss: Just connect the cable?

So you have bought a nice Anritsu VNA and now want to measure your cable...


Return Loss: Just connect the cable?

Is this the correct Return Loss of a 50m cable?


Return Loss: Terminate far end of the cable!

Looks better – but cable manufacturer specifies >25dB up to 1200MHz

Is the cable defective?


Return Loss = Sum of all reflection vectors

+ +

=

Connector ConnectorCable

Complete Assembly


Cable without connectors: Distance to Fault & Gatin g

Cable is in Spec! We have to remove influence of connectors to see

true behaviour of the cable Complicated measurement with Time Domain and Gating


What‘s this noise on the RL measurement? No noise! True behavious of a long cable...

You even can calculate the lenght of the cable by the distance of the ripples


Measured S11 as a function of frequency


Smith Chart: Another way to display Impedances

M1: 3.50MHz, Z = 23.3 Ω – j2.6 Ω

M1: 3.50MHz, Z = 59.8 Ω – j85.7 Ω

Smith Chart is often used for Impedance Matching


C a b l e L o s s

Slide Title or URL23


Why measure Cable Loss? Is Return Loss not good enough?

50m cable with WiFi antenna connectedAntenna specified VSWR = 1.5Looks OK – isn‘t it?

Mind the cable loss!


Transmission (2 Port): The traditional wayWhat do you need?

• 2 Port Benchtop VNA

• 2 Port Handheld Site Master

• SPA with Tracking Generator

• CW Signal Generator and Power Meter

• Scalar Network Analyzer


Transmission (2 Port): The traditional wayWhat do you get?

• Normally excellent results for cable loss

• ... and Return Loss (1 Path 2 Port Cal)


Transmission (2 Port): The problem!

Can‘t reach the far end of the cable• Cable already installed in a building, vehicle, aircraft,

tower, ship, etc.

What to do?• Buy a looooooong testport cable?

• Use another measurement methode?


Method I - 1 Port Cable LossMain application: Insertion Loss of low loss cables in antenna systems

• Calculation based on Return Loss measurement

• Divides Return Loss value by factor 2

• Far end of cable must be terminated with Short (or Open)

Urefl

Short

S11Uhin

Directional Coupler,Bridge

Generator

≈ Cable

Insertion Loss

Insertion Loss

S11 = RL = 2 x Insertion Loss


1 Port Cable Loss vs. 2 Port Transmission

1 Port Cable Loss: Mainly used for low loss feeder cables from BTS to Antenna


1 Port Cable Loss

Works best with low loss cables

• If return loss values (with short) become similar to terminated return loss than „noise“ is superimposed to cable loss measurements

Cable terminated with load

Cable terminated with short


Method II: External Transmission Sensor

Like in the good old days of Scalar Analyzers...


Method II: External Transmission SensorComparable results to a traditional 2 Port measurements


T i m e D o m a i n

Slide Title or URL33


Time DomainAdditional measurement to VSWR/Return Loss

• Can‘t replace a Return Loss measurement!

Information where the reflection happens

Different names are often used:

• Time Domain• TDR Measurement• Distance to Fault (DTF)• FFT Measurement• FDR

Time/Lenght

Am

plitu

de (

dB)

0dB = total reflection

-30dB = input connector


Limitations of VSWR/RL measurementVSWR/RL measurement does not show the location of reflexions

Connector tight Connector loose

Example: 2 x 6m cable with connector in the center


Additional informations with Time DomainTime Domain gives us informations, not available in the frequency domain


TDR versus FDRAnritsu VNAs are using FDR technique (Frequency Domain Reflectometry)

Frequency

SourceSpectralDensity

f1 f2

FDR

TDR

Less than 2% of the TDR pulse power is in the operation frequency range


Frequency Domain versus Time Domain

Each linear and time independent network can be described as:

• In Frequency Domain by its Transfer Function H(f)

• In Time Domain by its Impulse Response h(t)

The relationship between both forms is described by the Fourier Transformation

• = ℎ ×

Via Fourier Transformation the Impulse Response can be transformed into Frequency Domain


Fourier Transformation and VNAs

Data in the Frequency Domain (S-Parameter) can be transfered by Inverse Fourier Transformation into Time Domain h(t)

• ℎ = ×

Step Response is the Integral of the Impulse Response h(t)

A network analzer uses a special kind of FFT (Fast Fourier Transformation) called Chirp-Z transformation


Time Domain: The data flow

RL / VSWR DataFrequency

Domain

WindowFunction

Inverse Fourier-transformation

( IFT )

Data inTime Domain

f1 f2

VS

WR

Time Domain

= Frequency Domain


Low Pass Mode vs. Band Pass ModeMost VNA‘s offer the choice of two different Time Domain modes

Bandpass Mode

• Can be used for any DUT

• Coax, Waveguide, Stripline, etc.

• Independent from DUT‘s DC response

• Standard mode in Site Master (Handheld VNA)

Low Pass Mode

• DUT must have a valid DC term

• Coaxial cable, most transmission lines

• Not true for Waveguide or DUT with highpass characterisitics


Impulse Response vs. Step ResponseLow Pass Mode offers two different display modes

• Impulse Response (answer to a short impulse)

• Step Response (answer to a voltage step)

• Integral of Impulse Response

• Some information of the DC term necessary

• Data points need to be distributed evenly (harmonic sweep)

Integration ofImpulse Response

Integration ofImpulse Response

Impulse Response Step Response


Low Pass Mode (LP)• Impulse Response

• Display mode „Real“

• Display mode „LogMag“ (similar to BP Mode)

• Step Response

• Display Mode „Real“ (similar to a TDR in mRho)

• Display mode „Impedance“ (in Ohms)

Bandpass Mode (BP)• Impulse Response only

• Display mode LogMag, Real oder VSWR

• Location and amplitude of a discontinuity

• No information about true impedance• Exception„Phasor Impulse“

(e.g. For Waveguide)

Summary of Time Domain Modes


Beatty Standard (Stepped Airline)

25Ω50Ω 50Ω

• Often used as example to demonstrate time domain functionality


Real Display – Impulse vs. Step Response

Example: Beatty Standard with Load

Impulse Response

Step Response


Real Display vs. LOG Display – Impulse Response

Impulse Response LOG MAG



Impedance display – Step Response

Step Response withdirect Impedance reading



Time Domain: Practical hints for coax cables

Important parameters for fault location on cables

• Cable parameter• Propagation Velocity: Vrel• Loss: dB/m

• Frequency range

• Windowing


DTF: Just connect the cable?Perform a Full Reflection Calibration

(OSL)

Switch to DTF Return Loss/Time Domain

Connect the cable....

Is this my cable?Looks weird?

Distance (0.00 – 60.0m)

DT

F-R

L (d

B)


DTF Step 1: Find the end of your cableCheck max. distance with DTF Aid (here 154m)

• 1000MHz Span and 1033 data points

Use a D2 (stop distance) longer than the physical cable lenght• But not more than Dmax

Connect a short at the far end (or leave it open)

Large reflection indicates cable end

But why at 60m?

The cable is just 50m long....

60m ??


Propagation VelocityPropagation speed in cable, relativ to speed of light

Correct lenght measurement needs Vrel

-50

-40

-30

-20

-10

0

9,00 9,25 9,50 9,75 10,00 10,25 10,50 10,75 11,00

Vergleich

DTF ZOOM 3

M1 M2

Distance (9,0 - 11,0 Meter)

M1: ,03 dB @ 9,84 Meter M2: -10,96 dB @ 9,96 Meter

= 0,81 = 0,82

∆ = 15∆ = 15Example:Short at the end of a 10m cable

= 1√ε = 1√ε


DTF Step 2: Set the Prop VelocityInstrument sets propagation velocity to 1 (air)

• Propagation in cable is always slower than speed of light

• Use cable from cable list

• Enter Prop. Velocity manually (here 0.83)

Cable end now @ 50m

But why is amplitude at 13dB (short = 0dB)?

-13dB ??


Cable LossCable loss compensates the losses introduced by the cable under test

A short at the cable end should be at 0dB (total reflection)

0dB/m 0,225dB/m

Without entering the correct cable loss data, faults at the far end of the cable will look better than they are!

M1: -4,5dB @ 10m

M1


DTF Step 3: Set the Cable LossInstrument sets cable loss to 0dB/m

• Cable attenuation needs to be taken into account

• Use cable from cable list

• Enter cable loss manually (here 0.13dB/m)

Cable end @ 50m

Short @ 0dB

0dB @ 50m !!!


DTF Step 4: Now we can measure...Connect a load at the far end

• Not always necessary but often required

3 significant peaks visible

- 0m : N-Connector- 15.7m : Cable damaged- 50.2m : N-Connector


Time Domain: What if I change the frequency span?Frequency span defines distance resolution

• 1000MHz appr. 25cm (1MHz – 1001MHz)

• 100MHz appr. 2.5m (450MHz – 550MHz)• Also depends on Prop. Velocity

Significant peaks still visible


Frequency range determines distance resolution

!"#$%&#'()"'% ≈ 1

+$%,"!"#$%&#'()"'% ≈

1

+$%,"

6GHz Span

1,5GHz Span

3GHz Span


Frequency Domain

Time Domain


Distance Resolution

-#'()"'% ≈ 150

/&012$%34-#'()"'% ≈ 150/&012$%34

-')%5&"2 -("'% -#'()"'% = 0,577∆/-')%5&"2 -("'% -#'()"'% = 0,577∆/

= 37109∆/ = /: ;< − /: >

= 37109 m/s∆/ = /: ;< − /: > [Hz]

z.B. Span = 6GHz → 25mm Resolution (LP Mode, Air)


Time Domain: What does Windowing mean?Window function supresses unwanted side lobes

• Produced by IFT process

• Nominal Window supresses side lobes by 40dB

• Example: Short at 0m (calibration plane)

No Window (rectangular) Nominal Window


Which Window Function to choose?Compromise between distance resolution and sidelobe supression

• Typical Window functions in a VNA

Rectangular Window

Nominal Sidelobe

Low Sidelobe


Which Window Function to choose?Side Lobes with rectangular window could be seen as discontinuities......

Rectangular Window

Nominal Sidelobe

Example: Beatty Standard withLoad

Nominal Window is best compromise


Data PointsNumber of data points does not have influence of distance resolution!

200 Data Points2000 Data Points

Example: 20dB Offset Load at end of 10m cable

∆F = 2000MHz

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