Design Con VNA

1

Signal Integrity Testingwith a Vector Network Analyzer

Neil Jarvis

Applications Engineer

Agenda RF Connectors

A significant factor in repeatability

and accuracy Selecting the best of several types

for application

Compatibility

Connectors are consumables

limited lifetime

damaged connectors are costly

proper care maximizes lifetime

VNA What is a Vector Network

Analyzer? How will a VNA help with Signal

Integrity? Calibrating and (De)Embedding What is TDR? The VNA approach to TDR

Connector Considerations

A significant factor in repeatability and accuracy Selecting the best of several types for application Compatibility Connectors are consumables

o limited lifetimeo damaged connectors are costlyo proper care maximizes lifetime

Amplitude and Phase Error from VSWR εA = 20 * log (1 ± |ΓA * ΓB|) dB

εΦ = (180 / π) *| ΓA| * |ΓB|

Performance of a standard type-N connector

Performance of a mated pair SMA vs. 3.5 mm

-Introduction

-Detailed Views

-RF Connector Types

-Connector Grades

-Comparison of SMA and 3.5mm

-Connector Summary

-Cleaning

Mating SMA with 3.5 mm

SMA/SMAConventionalMated Pair

3.5mm/SMAConventionalJunction

FREQUENCY in GHz

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

1.05

1.10

1.15

SW

R

3.5mmMatedPair

Performance of a mated pair 2.92 mm vs. 3.5 mm

Typical Connector Cross Section

Male Female

Center Conductor

Outer Conductor

Outer Conductors

Outer conductor mating surfaces definemeasurement reference plane

Center Conductors

Slotted Female Center

Slotless Female Center Conductor

Connector Grades

Metrology

Instrument

Production (Field)

Metrology Grade

Used on calibration standards Highest performance slotless contacts Tightest tolerances Air dielectric interface Long life Highest cost

Instrument Grade

Used for test ports Economy calibration kits Good performance Tight tolerances Dielectric supported interface Long life

Production (Field) Grade

Systems and device connector Low performance Loose tolerances Dielectric supported interface Limited number of connections Lowest Cost

Always Inspect Before Connecting

Connector Examples

Type F

BNC

SMC

Type N

APC 7

SMA

3.5 mm

2.92 mm or K

2.4 mm

MaleFemale FemaleMale

3.5 mm Connectors

dmdDA

B

C

MP

OUTER CONDUCTORMATING PLANE

A D d

C

FP

OUTER CONDUCTORMATING PLANE

3.5mm Connector Detail

SMA Connectors

Coaxial Connectors – Operating Frequency

Connector Torque Limits

Connector Compatibility

3.5mm and 2.92mm connectors will mate, but have mismatch

SMA will mate with 3.5mm and 2.92mm

Be careful with low quality SMA male connectors – pin on high side of tolerance range can damage precision 3.5 / 2.92 female

2.4mm and 1.85mm connectors will mate, but have mismatch

Cleaning

Apply a mild blast of dry compressed air or Nitrogen

Use the minimum amount of pure alcohol

Use lint-free cleaning tools (swab or brush)

Do not use acetone, methanol, or CFCs (Freon).

Further Reading….

lResources:

lhttp://www.npl.co.uk/electromagnetic/clubs/anamet/connector_guide.pdf

Agenda RF Connectors

A significant factor in repeatability

and accuracy Selecting the best of several types

for application

Compatibility

Connectors are consumables

limited lifetime

damaged connectors are costly

proper care maximizes lifetime

VNA What is a Vector Network

Analyzer? How will a VNA help with Signal

Integrity? Calibrating and (De)Embedding What is TDR? The VNA approach to TDR

28

Why use Vector Network Analysis? Very low level signals can be measured more accurately with

narrow bandwidths

Can measure very fast Rise Times

The 4-port, single-ended S parameters have become a de-facto standard for describing the electrical properties of any 4-port interconnect.

ı For example, IEEE P802.3ap Task Force uses measured S-parameters as test cases[9] for proposed solutions to the problem of 10 Gbit/s Ethernet over backplanes.

What are the numbers in S.. o the first index being the going out port o the second index is the coming in port.o Example: Gain or Loss of Device is S21

29

What are we Actually Measuring?

A network analyzer is an instrument that measures the network parameters of electrical networks

Network analyzers commonly measure s–parameters because reflection and transmission of electrical networks are easy to measure at high frequencies

Measures Amplitude and Phase into and out of each DUT port

Typical Device Behaviour

DeviceIncident

Reflected

Transmitted

Transmission Line Model

Signals are Complex Quantities

Magnitu

de

Phase

Vector Ratios :

Vector Representation

Calibration and Reference Plane

Defines Measurement Reference Linear Magnitude = 1.0 (0 dB) Phase = 0 Degrees (Reflection and Transmission)

Establishes Characteristic Impedance, Z0

Port 1 Port 2

Port 1Reference Plane

Port 2Reference Plane

What is a Vector Network Analyzer?

A Vector Network Analyzer (VNA) is an instrument that measures the amplitude and phase of an electrical network

A VNA typically displays S-parameter.

ZNB ZVA

What is a Vector Network Analyzer?PROCESSOR / DISPLAY

INCIDENT (R)

Incident

Reflected

Transmitted

a1

b1

a2

b2

Port 1 Port 2

SIGNALSEPARATION

What is a Vector Network Analyzer?

Dual Directional Coupler

Directivity is a measure of how well a coupler can separate signals moving in opposite directions

Test port

(undesired leakage signal)

(desired reflected signal)

Directional Coupler

b a

What is a Vector Network Analyzer? Each port of a VNA contains a stimulus along with forward and

reverse measurement

Typical Measurement example: Stimulate Port 1

PORT

Meas. Receiver “b”

Ref. Receiver “a”

Reflectometer

Incident(“a1” receiver)

Reflected(“b1” receiver)

Transmitted(“b2” receiver)

Port 1 Port 2

DUT

S-Parameters of a 2 port network

S11 (b1/a1)Forward reflection coefficient (input match, return loss, VSWR)

S21 (b2/a1)Forward transmission coefficient (gain or loss)

S12 (b1/a2) Reverse transmission coefficient (reverse isolation)

S22 (b2/a2)Reverse reflection coefficient (output match, return loss, VSWR)

Pin-refl

PoutPin

Prev-refl

Prev

What is a Vector Network Analyzer?

What is a Vector Network Analyzer?

TransmittedIncident

TRANSMISSION

Gain / Loss

S-ParametersS21, S12

GroupDelay

TransmissionCoefficient

Insertion Phase

ReflectedIncident

REFLECTION

Standing Wave RatioSWR

S-ParametersS11, S22

ReflectionCoefficient

Impedance, Admittance

R+jX, G+jB

ReturnLoss

Γ, ρΤ,τ

Incident(“a1” receiver)

Reflected(“b1” receiver)

Transmitted(“b2” receiver)

b1a1

=b2a1

=

Port 1 Port 2

DUT

Measuring the Step Response In practice, it is easier to generate a step response as

compared to an impulse response Implementation of a TDR measuring device is shown

below:

This device can measure the measure the Reflected Step and Impulse responses, ГΘ(t) and Гh(t) respectively

Impulse and Step Response Examples

OPEN

SHORTImpulse

Step

Impulse

Step

The VNA approach to TDR The VNA measures in the Frequency domain In many instances, the process of converting to the Frequency

Domain, performing the analysis, and then converting back to the Time Domain is easier.

It is often times advantageous to measure in the Frequency Domain instead of the Time Domain in order to get the Impulse Response.

Applying the Inverse Fourier Transform converts the frequency response to the time domain.

Advantages:• Higher Dynamic Range: Lower instantaneous BW required, than a time

domain measurement• Analog to Digital Converter in the Time Domain measurement limits the

frequency response.

Applications of TDR Examination of faults in transmission lines RF imaging for nondestructive evaluation Separation of echo from the wanted signal in case of

multipath propagation Moving the reference plane across unknown

irregularities

Measurements with VNAs on Cables

Classical VNA measurements S-parameters

Transmission, Reflection, cross coupling Fext, Next Group delay Electrical length

TDR measurements Fault Location Rise Time Skew (interpair and intrapair) Impedance Quality of connectors

Relationship between Frequency Domain and Time Domain

f 2f 3f 4f 5f 6f 7f freq

time

Inverse Fouriertransformation

Fouriertransformation

Examples for Rise Times and Resolution

ZNB 8: 60 psZVA20: 25 psZVA40: 13 ps

Time Domain Measurements

Fault Location Skew Impedance vs Distance Gating Connector, Junction or Solder Charecteristics Resolution Enhancement

Time Domain Measurement Results

Board: Micro stripe Line, Length of the Line: 49mm, Er ~ 3, SMA Connector and FarEnd SMA connector:

Open Low Pass Impulse Response:

1

1

23

4

Reflection SMA connector

2 Reflection SMA connector

To PCB

3 Reflection PCB to SMA

connector

4 Reflection SMA connector

(OPEN)

Time Domain and Frequency Domain

l Impedance l Insertion Loss

Cables can be verified by using the best suitable Domain

(time domain and/or frequency domain)

Measurement of a Connector with Time Domain

Gating functionality can be used to suppress unwanted reflections

Gated time domain measurements can be re transformed into frequency domain

Typical application

o Test of the quality of a cable connector

Test of the Quality of a Connector

Problemo Test the quality of a connector soldered to a cable

o The other end of the cable has no connector at the other end to solder it directly to a module

Solutiono Isolate the connector by gatingo Measurement of the gated S11

52

Time Gating Frequency Domain Analysis does not always provide

insightful analysis of devices with multiple reflections -> Time Domain Reflectometry can provide this

Example below of Time Domain vs. Frequency Domain measurements: Which is easier to interpret??

OR

Time Gating (con’t)

Result

S11 with perfectly matched Port 2

Reslution and Resolution Enhancement Factor

0,00E+00

5,00E-02

1,00E-01

1,50E-01

2,00E-01

2,50E-01

3,00E-01

3,50E-01

-2,00E-09 0,00E+00 2,00E-09 4,00E-09 6,00E-09 8,00E-09 1,00E-08 1,20E-08

time [ns]

Ref

lect

ion

Fac

tor

[U]

ZVA: 1 GHz - 20 GHz ZNB: 1 GHz - 8 GHz REF: 2.5

ZVA: Start: 10 MHz - 20 GHzZNB: Start 10 MHz - 8 GHz, REF: 2.5fstep: 10 MHz