limitations and accuracies of time and frequency …€¦ · limitations and accuracies of time and...

Accuracies of Time & Freq Domain PHY Meas 1

Limitations And Accuracies Of Time And Frequency Domain Analysis Of Physical Layer

Devices

Page 2

Outline

• Short Overview• Fundamental Differences between TDR & VNA Instruments• Calibration & Normalization• Measurement Accuracies• Measurement Comparisons


Page 3

TDR and VNA

86100 DCA and 54754A TDR Modules E8364B PNA with 2-port Testset

N1930B Physical Layer Test Software

Page 4

TDR Set-up (Configuration Showing 1=>3, 2=>4)

Channel 1

Channel 2

Channel 3

Channel 4

Thru Adapters

BTL Board


Page 5

Appreciating the Complexity of it all

InstrumentArchitectures

ReciprocityRepeatability

Calibration

Normalization

Risetime

DynamicRange

Noise Floor

RcvrBW

SourceError

Source Drift SourceStability

Voltage &Temp Drift

SOLT

Time Base

IFBW

Averaging

Numberof Points

Signal-to-NoiseRatio

FilterRoll-off

Time Step

DeviceLength

Device Complexity

Accuracy

FrequencyRange

Fundamental Differences between

TDR & VNA Instruments

• Measurement Domains

• Sources

• Receivers

• Architectures and Sources of Errors

• Calibration and Measurement

• Summary of how TDR and VNA Measurements Differ


Page 7

Time and Frequency DomainsAll frequencies make up each time point

Device Under Test

tm tm= phase offsetVNA – Frequency Domain

Device Under Test

t0 t1=delayTDR – Time Domain

Δφ=2πf*time delay Group delay = dφ/df = time delayPhase

Group DelayFrequency

deg nsec

Page 8

Jitter in Time Domain is Phase Error in the Frequency Domainphase(radians) =2*pi*f*delay


Page 9

TDR and VNA Measurement Techniques

DUTIncident wave

Reflected wave

Transmitted wave

S11

S21

Incident wave

Reflected wave

Transmitted wave

TDR

TDTt

t

DUT

CH1 S11

log MAG 10 dB/ REF 0 dB

START 0.099 751 243 GHz STOP 20.049 999 843 GHz

C2

MARKER 1

1.452378096 GHz

1

1 _:-2.145 dB

1.452 378 096 GHz

CH 2 S21

log MA G RE F 0 d B10 dB /

S TAR T 0. 099 75 1 243 GH z STO P 2 0. 049 99 9 8 43 GH z

M AR KE R 1

1.017462619 GHz

1

1 _: -3 .08 9 dB

1 .01 7 462 61 9 GHz

Page 10

TDR Block Diagram

DUT

Clock Trigger

Front Panel

Step Generators

Device Reference Planes

ADCChannel 1

Samplers& ADCs

ADCChannel 2

2 Sources (Step Generators), 2 Samplers, and 2 ADCsTrigger TriggerTrigger

TDR TDT


Page 11

Network Analyzer Block Diagram

Reflected SignalS11 = b0/a0

Transmitted SignalS21 = b3/a0

Page 12

TDR and VNA Sources

TDR

VNA

Power decreases across frequency band – causes loss of accuracy at higher

frequencies

Power constant across entire frequency band –no loss in accuracy at

higher frequencies


Page 13

TDR and VNA Receiver Bandwidths

Agilent TDR has 4 Wide-Band Receivers

with 2 choices for cut-off Frequency

VNA has 4 Narrow Band Receivers

(definable by setting IF BW)that are swept across the

frequency range of interest

Loss of gain inthe high region

with TDRNo Loss of gain

with VNA

Noise Floor

IF Bandwidth

Page 14

Sources of Error in TDR Instruments- All significantly reduced with Normalization• Oscilloscope

• Finite bandwidth restricts it to a limited measurable risetime• Small errors due to trigger coupling into the channels & channel

crosstalk• Clock stability causes trigger jitter in the measurement

• Step Generator• Shape of Step stimulus (risetime of the edge, aberrations on the

step, overshoot, non-flatness)• Cables & Connectors

• Introduce loss and reflections into the measurement system

System_Risetime = sqrt[ScopeRisetime^2+StepRisetime^2+TestSetupRisetime^2]


Page 15

Sources of Error in VNA Instruments (2 Port)• Random Errors: Instrument Noise, Switch Repeatability, and

connector repeatability• Systematic Sources of Error

• Directivity & Crosstalk errors relating to signal leakage• Source & Load Impedance mismatches relating to reflections• Frequency Response errors caused by reflection & transmission

tracking issues within receivers

DUT

a0 b0

Directivity

Crosstalkb3

SourceMatch

LoadMatchReflection Tracking (b0 / a0)

Transmission Tracking (b3/ a0)

6 Forward & 6 Reverse Error Terms12 Terms total for 2 Port Device &48 Terms for a 4 port Device

Page 16

Loss of Source Power Affects Accuracy of TDR as Compared to the VNA

VNATDR Norm@20pSError: VNA/TDR

• The higher the frequency band the less accurate the TDR measurement

• Measured with normalization and additional correction

~1 dB diff ~2 dB diff ~4-6 dB diff


Page 17

VNA & TDR Rough Comparison of Dynamic Range

VNADynamicRange

Approx.TDR

DynamicRange

• Dynamic Range = MaxSignal-Noise Floor

• VNA with wider Dynamic Range allows measuring signals that are 40-80dB down

TDR Noise Floor

VNA Noise Floor

Page 18

VNA & TDR Attributes at a Glance

10MHz-20GHz55-110dB90dB-5 dBm0.2-1%Depends on calibration

VNA

0 -12.4GHzor 0 -18GHz

30-40dB45dB200mV0.5-5%Depends on calibration & freq range

TDR

Receiver Bandwidth

Noise FloorDynamic Range

Source Power

Accuracy

Disclaimer: Values appearing in this table are estimates and do not inferor imply any guarantee by Agilent Technologies as to actual results


Calibration & Normalization

• Overview of TDR & VNA Calibration• Calibration File & Data Storage Differences• Levels of Calibration with TDR• TDR Normalization vs. VNA Calibration• Risetime Effects on Accuracy • The Normalization Process• Advantages of Normalization

Page 20

MostAccurate

Easiest

S-Parameter De-embedding

Port ExtensionTime Domain Gating

NormalizationReference Plane Calibration

Thru-Reflect-Line (TRL)Line-Reflect-Match (LRM)

Short-Open-Load-Thru (SOLT)

Fixture Error Correction Techniques

= Pre-measurement error correction= Post-measurement error correction


Page 21

Levels of Calibration with TDR• NONE: No calibration is performed

• Hardware Service Calibration assumed

• MIN: Module Calibration & RPC (Reference Plane Calibration)• Module Calibration calibrates the gains and offsets of the data

acquisition channel.• RPC removes the delay of the cables and de-skews edges for the Diff &

Common Mode launches.

• MAX: Normalization – (Agilent only)• Normalizes to standards placed at the Reference plane. Takes out delay

and the loss associated with the cables, the reflections due to mismatch of the source, and improves the pulse edge and flatness. Allows for maximum accuracy and is heavily risetime dependent.

Page 22

Comparing TDR Calibration Methods with a VNA SOLT Calibration of a Thru Adapter

VNA

Norm@20pS

Norm@30pS

RPC

RPC

@30pS

@20pS & VNA

Magnitude

Phase

TDRWaveforms


Page 23

Improved TDR Calibration

Norm@20ps

With additional CorrectionAdditional correction• Save the frequency response of thru and reflect calibration standards• Correct the measured response with the frequency response of calibration standard.• The frequency and time domains are not simply related with FFT and IFFT

Page 24

How Risetime Affects Noise in the Time Domain

• Noise in the TD increases with faster risetimes

• Normalization at 10pS can be achieved but it is extremely noisy

• Normalization at @20pS (2046 pts) is a good balance between frequency domain accuracy and low TD noise

TDR Normalized @Tr=15pSTDR Normalized @Tr=20pSTDR Normalized @Tr=25pS


Page 25

Why Risetime Increases Noise in the Time Domain

• Risetimes determine the bandwidth (BW) of the Normalization filter

• Sharpening the risetime increases the frequency response but also the noise in the higher band

• This results in a distorted noise floor and a limit to your risetime beyond which there will be diminishing returns

fc

0dB-3dB

Log

(Am

plitu

de)

Log (Frequency)

Basic SystemResponse

Noise Floor

0dB-3dB

Log

(Am

plitu

de)

Log (Frequency) Initialfc

Noise Floor

New fc

Basic System ResponseNormalized

SystemResponse

Page 26

Summary of Good TDR Calibration

• The real advantage of Calibration (and more particularly Normalization) is that you can remove unwanted effects of cablesand connectors leading up to your device.

• Magnitude and Phase (S-parameters) of thrus will show error increasing as a function of frequency.

• A good calibration at a reasonable risetime will show acceptablenoise in the time domain.

• Checking Reciprocity is a way to verify a good calibration has been performed.


Measurement Accuracies: Reciprocity, Repeatability, Drift

•Using Reciprocity to Check Measurement Credibility •Reciprocity with a TDR & VNA•Repeatability with a TDR & VNA•Drift with a TDR & VNA •Summary of Calibration & Measurement Accuracy

Page 28

Using Reciprocity to Assure Good Calibration• Reciprocity is the constraint that for passive devices S12=S21.• In VNA measurements S12 virtually overlays S21 when a Thru path is

measured. For TDR measurements the alignment is not as good. • Be aware when exporting data that some tools may require a certain

level of reciprocity (eg. HSPICE).

to Port 1

to Port 2

to Port 3

to Port 4S13(Mag) = S31(Mag); S13(Phase) = S31(Phase)

S24(Mag) = S42(Mag); S24(Phase) = S42(Phase)

Reciprocity on a Thru Adapter SE Measurement


Page 29

TDR Reciprocity of BTL Board (Normalized @20pS)

SDD12/SDD21

TDR Reciprocity:+/- 25 degrees Phase+/- 4 dB Magnitude

SDD12/SDD21

Page 30

VNA Reciprocity of BTL board

SDD12/SDD21

VNA Reciprocity:+/- 2 deg Phase

+/- 0.25 dB Magnitude

SDD12/SDD21


Page 31

Repeatability with Normalized TDR measurements

2 measurementsNormalized at 20pS

(with new calibration) 5 days apart

Excellent Good

Cumulative Error

Magnitude Repeatability

Phase Repeatability

Difference

Difference

TDR Repeatability:+/- 60 degrees Phase+/- 4 dB Magnitude

Page 32

Repeatability with VNA with SOLT cal

2 VNA measurementsalso 5 days apart

Magnitude Repeatability

Phase Repeatability

Excellent across entire range

Difference

Excellent across entire range

VNA Repeatability:+/- 2 deg Phase

+/- 0.5 dB Magnitude


Page 33

TDR Source Drift Over the Course of a Day

12pm

13pS of drift corresponds to95degs phase difference(drift is always bounded)

Thru adapterover a 12 hr period at

4 hr intervals withthe same calibration

8am

8am

12pm

4pm

8pm

8pm

4pm

12

3

Sequence

Range of Drift

Page 34

VNA & TDR Attributes at a Glance

+/- 0.5dB for Magnitude+/- 2 degrees

3-6dB dependent on calibration+/- 60 degrees

Repeatability

Magnitude within noise of instrument< 5 degrees

Magnitude within noise of instrument210deg @ 20GHz

Drift

0.25dB Magnitude+/- 2 degrees

VNA

2-4dB dependent on calibration+/- 25 degrees

TDR*

Reciprocity

Disclaimer: Values appearing in this table are estimates and do not inferor imply any guarantee by Agilent Technologies as to actual results.

* TDR values may seem larger than expected. It should be noted that these values are at the high end of the frequency range


Measurement Comparisons

• Single Ended Comparisons of TDR & VNA Measurements

• Balanced (Differential) Comparisons of TDR & VNA Measurements

Page 36

25 Ohm Mismatch Airline (3.5 mm connectors)

Device Characteristics:

• Device that is traceable to NIST

• Insertion Loss – Low loss with known variations

• Return Loss – Known resonance pattern over wide dynamic range


Page 37

25 Ohm Mismatch Airline: Time Domain Comparisons

VNATDR Norm@20pSTDR with RPC

mVolts

Very good agreement forlength of the device

(regardless of calibration)

Normalization corrects for cable

losses in RPC measurement

• Typically Ref. Plane Calibration will not show correct voltage or impedance values –Normalization will need to be used.

• Since Ref. Plane Calibration does remove the DELAY of the cables (but not the loss) it can predict approximate device length

Page 38

25 Ohm Mismatch Airline: Insertion Loss Comparisons


Magnitude

Phase

VNA

Norm@20pSwith correction

RPC Only

Mismatch causes this standing wave pattern

Comparison of VNA - TDR measurementsusing different calibration methods


Page 39

25 Ohm Mismatch Airline: Time Domain Comparisons


Excellent agreement forlength of the device

• Good agreement on length of device for low loss controlled impedance DUT

• 10% variance (2.5 Ohms) on impedance between VNA and TDR with Reference Plane Calibration

• < 1 ohm discrepancy between VNA and TDR calibrated at 30pS risetime

Impedance (ohms)

Impedance discrepancy w/RPC

Page 40

25 Ohm Mismatch Airline: Return Loss Comparisons


Magnitude

Good correlation with Normalized TDR and VNA across a wide dynamic range:

•Resonances (due to length of mismatch line)

•Magnitude for normalized measurements

Resonance


Page 41

Single Ended Summary

• With Normalization and correction the TDR can be an effective strategy for measuring impedance and obtaining frequency domain characteristics of moderate loss devices at frequencies below 10GHz.

• Reference Plane Calibration without correction should only be used for estimating the length of a device and getting and idea of the response of the device.

Page 42

BTL Example: Differential Return Loss Details

~ 1 dB diff2-6 dB diff

~ 2 dB diff

VNANorm@20pS correctedDifference

• First 3 divisions -look very good• Mid 4 divisions -errors increase• Top 3 divisions -getting worse


Page 43

BTL Example: Differential Insertion Loss Comparisons

VNA

Norm Tr=20pS

Norm Tr=30pS

RPC weakmidband

Start at 10GHz

Highband is poor for TDR

Without additional correction

Page 44

BTL Example: Differential Insertion Loss Details

~1 dB diff ~2 dB diff ~4-6 dB diff

VNANorm@20pS correctedDifference

• First 3 divisions -look very good• Mid 4 divisions -errors increase• Top 3 divisions -getting worse


Page 45

Overall Summary• The simplicity of the TDR is useful for lower data rates and for

obtaining an intuitive understanding of signal integrity effects• The Vector Network Analyzer provides more accuracy and

repeatability than the Time Domain Reflectometer• TDR normalization provides data closer to a VNA than that derived

from a TDR utilizing only RPC calibration• Frequency domain data derived from TDR data is less accurate at

higher frequencies• This inaccuracy leads to error that can be interpreted as pessimistic

insertion loss data and optimistic return loss data for frequencies greater then 10-12 GHz.

• The accuracy provided by VNA data will be required for data rates above 6.25 GB

Page 46

Resources Websites

•www.agilent.com/find/plts•www.agilent.com/find/eesof-eda•www.agilent.com/find/si•www.agilent.com/find/sigint

Software•Physical Layer Test System (PLTS)•Advanced Design Software (ADS)

Hardware •N5230-245 Vector Network Analyzer•86100C Time Domain Reflectometer

PLTS Studio - Analysis Only Software • N1930B-1NP networked license• N1930B-1FP fixed license• N1930B-1TP USB key license

Physical Layer Test System (PLTS) Configurations

limitations and accuracies of time and frequency …€¦ · limitations and accuracies of time and...

Documents