distortion and modulation quality for wideband communications · analog i and q signals are...

© 2012 Agilent Technologies

Aerospace & Defense Symposium

Distortion and Modulation Quality for Wideband Communications

Presented by: Richard Overdorf, Agilent Technologies



2

Agenda

• Nonlinear Distortion

• Methods to Characterize Nonlinear Distortion

• Two-Tone Measurements

• Multitone Measurements

• CCDF Measurements

• Noise Power Ratio (NPR) Measurements

• Making Wideband High Frequency Measurements

• Making Vector Based Measurements



Linearity Concerns

Signals can interact

Non-linear power amp

Spectral re-growth

Unusable bandwidth

Need to characterize

4

HV

HV



5th order

IMD

7

Nonlinear Distortion: Intermodulation

Amplifier

In Out

(3A-2B)

(2A-B)

(3B-2A)

2nd

harmonics

3rd

harmonics

2B2A

fA B

3rd order

IMDtypical

channel

bandwidth

(2B-A)

3A 3B

A B

2nd order

IMD

A B

(B-A)

Intermodulation

distortion

3rd order

IMD

5th order

IMD



Non-Linear Distortion: Spectral Re-Growth

Pt

Pi

f

Spectral

‘Re Growth’

Non-Linear

Effect

8





IMD and Spectral Re-Growth

Pt

f

AB

A+

AA

+B

B+

B

A+

A–B

B+

B–

A

Pt

Pi

IMD3

Distortion

TOI or IP3

IMD3

Fundamental

Pt

f

A B

A+

A–B

B+

B–A

IMD3

1st, 2rd, 3th, etc.

harmonics mix together

forming IMD





Two Tone or TOI Test

Advantages

• Quick test

• Simple to perform

• Uses common test equipment

• High signal to noise

Disadvantages

• Essentially narrow band

• Requires wideband matching

• Does not simulate correct loading conditions

• Probably needs components to increase source

to source isolation

10



PXA performance

spectrum analyzer

PSG

CW signal generators

11

Traditional Two-Tone Measurements

DUT

LPF

Combiner

Isolator AMP

Attenuator

2nd & 3rd

harmonics

IMD

products

+





Three Tone Testing

Advantages

• Reasonably quick test

• Simple to perform

• Uses common test equipment

• Better Intermodulation test

Disadvantages

• Essentially narrow band

• Requires wideband matching

• Open to interpretation

• Averaging required

12

3rd5th 7th

Note:

Uses uneven spacing for tones





>10 Tone Testing

In practice, using analog signal sources and combining techniques

proves impractical for the majority of users.

Why is this?

• < +20 dBm is typically maximum out of signal sources, but usually not

able to use this due to distortion due to the source levelling circuits

• Combiners with good port isolation have high attenuation

• Use of filters and isolators creates limited flexibility

• Post combiner amplification either not available or too expensive.

• Cost and physical size of signal sources

13



14

Modern Multi-tone IMD

• For wideband components two-tone measurement

results vary depending on tone spacing

• Simulate real-world operating conditions

• Stress device with higher peak-to-average ratio

• Test with multiple phase sets

Why use multi-tone test signals?



• Well established test procedure

• Common test equipment

• Complicated test setup

• Signal parameters are not easily modified

• Manual tuning

• Difficult to generate random phase sets

• Equipment and capital intensive

15

Conventional Analog Test Stimulus

Disadvantages of analog test approach

Advantages of analog test approach





Analog I/Q modulation vs. digital I/Q up-

conversionConventional I/Q

modulation -

Analog I and Q

signals are

generated using an

AWG. An I/Q

modulator

generates the IF or

RF signal

Digital up-

conversion –

I/Q modulation is

performed digitally

- either in real-time

(in hardware) or

up-front in software

Mixer /

Multiplier

/ LO

AWG

D/A

D/A

Memory

Memory

Analog IQ Modulator

X

X

+~

90

AWG

Digital signalAnalog signal

D/A

X

X

+~

90

Memory

Memory

X

~





Vector Test Stimulus up to 1 GHz

17

I Q

DUT

Isolator

Dual LXI arbitrary

waveform generator

(N8241A)

(Differential)

PSG vector

signal generator

(E8267D)

PXA performance

spectrum analyzer

(N9030A)



18

Vector Test Stimulus

Advantages of vector test approach

• Simple test setup and procedure

• Easily modify signal parameters

• Apply pre-distortion to improve signal quality

• Repeatable and accurate test results

• Save time and capital equipment cost

Disadvantages of vector test approach

• Available output power

• Carrier feed through

• Images



• Up to 4097 tones

• Vary tone power

• Change phase settings

• Randomly spaced tones

• CCDF plot

• COM-based API

LAN/GPIB

PSG PXA

N7621A Signal Studio for Multi-tone Distortion

19

…and After

• Improved IMD

suppression

• Correct with additional

devices in the loop

• 80 MHz correction BW

(internal AWG)

• 1 GHz correction BW

(Agilent’s WB AWG)

Before…

Option 203





Achieving the Right Signal Statistics

22 Pv

Average random power?

Noise vs. pseudo noise

Peak to average ratio

CCDF signal statistic

PSG NPR adjustable

Generator seed selectable

20

t

Average

v

Power

% Time

Power

% Time

Power

% Time

Power

% Time

2

cdf1

cdf1log10

CCDF





Complementary Cumulative Distribution

Function

Link- CCDF demo video

http://www.youtube.com/watch?v=NlbuDgNj6ew

http://www.youtube.com/watch?v=NlbuDgNj6ew





IMD products

from DUT

Low IMD reduces

test uncertainty

Tone correction

Minimize test stimulus IMD…

even at the output of an

external power amplifier!

Non-linear distortion measurement

E8267D PSG

Enhanced Multi-tone Measurements

22

DUT

N9030A PXA





Complicated Loading Scenarios: NPR

Multiple signal bandwidths, amplitudes & modulation types

3rd’s, 5th’s, etc… Can add together

Difficult to predict, so measure with NPR test

?

f





NPR Measurement Overview

Notch

Depth

Notc

h

Frequency

Amplitude

Broadband Noise

Notch

Depth

Frequency

AmplitudeNotch Bandwidth

Spectral Re-Growth

NPR





Traditional NPR Test Setup

LO

RFIF

Up converter

Noise Source

Band stop

filter

Spectrum analyzer

CW signal generator

DUT





Traditional NPR Test Problems

Notch filter shape issues

Depth & bandwidth

Filter Q requirement

Cavity filters

Fixed center frequency

Noise flatness

Test station correlation

Notch

Depth

Frequency

PowerNotch Bandwidth

NPR

Band Stop

Filter





PXA Performance

Spectrum Analyzer

(N9030A)

I Q

DUT

(Differential)

27

Arbitrary Waveform Generator for NPR

Dual LXI arbitrary

waveform generator

(N8241A)

PSG Vector

Signal Generator

(E8267D)





Using a Vector Signal Generator and

an AWG for NPR

Repeatable, flat noise power, square notch….

Deeper notch with easily adjusted center frequency

Up to 80 MHz bandwidth in PSG & 1 GHz with the N8241A!

Conventional Analog NPR Stimulus Digitally Synthesized NPR Stimulus





Signal Studio for Multitone Distortion

Option 204

LAN/GPIB

PSG

Digitally Synthesized NPR

Stimulus

PXA

Synthesize NPR stimulus

Vary notch depth

Vary notch bandwidth

Vary notch frequency

Distortion correction

Vary stimulus’ CCDF



NPR Measurement

30

Amplifier Measurement

Notch Creation

38 GHz!

Using Noise and Band Power Markers





Arbitrary Waveform Generator Requirements

31

• Wide bandwidth (1 GHz)

• High dynamic range

• Low distortion products

• Flatness

• Repeatability

• IF and IQ generation

capabilities

N6030A (PXI) N8241A (LXI)





New ARB – M8190A

16 QAM Example with Analog IQ

Modulation using Vector Signal Generator





High-Precision AWG Example: Analog IQ

Modulation, Fc=10GHz

Wideband digital

modulation:

QAM16, 1.76G Sym/s

Fs = 7.2 GHz

with amplitude

correction

EVM=1.17%





High-Precision AWG Example: Digital

Upconversion, Fc= 1GHz (without PSG RF

Sig Gen)

Wideband digital

modulation:

QAM16, 1G Sym/s

Fs = 7.2 GHz

with amplitude

correction

EVM=0.89%



Vector Modulation Analysis - 89600 VSA SoftwareUse the SAME measurement tool at ALL stages of your block diagram!!!

DSP

Digital (SSI) IF/RF BB (I-Q)

Logic Analyzer Oscilloscope Signal Analyzer

DUT



Wideband High-Frequency Vector Measurement Options

Bandwidth

Dyn

am

ic R

an

ge

PXA 160MHz

@78 dB

Wide Band VSA

(PXA + Infiniium)

900 MHz @ 40 dB*

X93204A Infiniium scope

32 GHz @ 40 dB

MXA 25MHz

@78 dB

http://cp.home.agilent.com/agilent7/s7viewers/flash/genericzoom.swf?logo2=false&serverUrl=/agilent7/is/image/&contentRoot=/agilent7/skins/&locale=en&config=Agilent/AGILENT-IMGSET&image=Agilent/PROD-1820590-IS



Instrument and System Calibration

Calibration

Amplitude Flatness Phase linearity

Minimum Error Vector MagnitudeEVM

I

Q

Ideal Signal

Measured

signal

θ

Amplitude error

Phase linearity error

The goal is to measure the EVM of the DUT not the

EVM introduced by the measuring system



0-3.6 GHz low band

3 Hz-26.5 GHz

Input

Cal input

2 dB-step mech atten

μW converters

8.3-14 GHz LO

10.9M

.3M

4.8 GHz LO

RF converter

3.8-8.73 GHz LO

2nd converter

FPGA

300 MHz LO

200 MHz CK

100 MHzCK

ADC

ADC

Switched filters,

F0=22.5 MHz

X1 3.6-13.6 GHz

X2 13.6-26.5 GHz

140 MHz

3.5-26.5 GHz high band

FPGA

140 MHzFront End

Swept IF & 10 MHz BW

& 25 MHz BW (option B25)

25 MHz

966K

303K

79K

9K

Switched filters,

F0=322.5 MHz

140 MHz BW (option B1X)

2 2 6 10 20 30

RF preamp

40 MHz

400 MHz CK

40 MHz BW (option B40)

ADC

F0=250 MHz

F0=300 MHz

F0=322.5 MHz

Linearity

Corrections

Low noise path

μWpreamp

YIG filter with

bypass relay

1 dB-step electronic atten

F0=5.1225 GHz

4 GHz

ASIC

2Gbyte

SDRAM

ASIC

2Gbyte

SDRAM

PXA Simplified Block Diagram



Example of Quality of Magnitude and Phase Corrections on

140 MHz BW

EVM Results for 138 MHz OBW QPSK Signal vs. Center Frequency in Band 0



PXA Simplified Block Diagram (900 MHz IF Path)

3 Hz-26.5 GHz

Input

8.3-14 GHz

3.6-13.6 GHz Path

3.5-26.5 GHz high band

Front EndLow noise path

μWpreamp

YIG filter with

bypass relay

13.6 - 26.5 GHz Path

Aux IF out

Option CR3Option MPB

Rear Panel

900 MHz IF BW centered at 600 MHz



Setting up the Vector Signal Analyzer

1. Connect a source to

the PXA

2. Connect the VSA the

scope over the LAN, and

the PXA’s wideband IF

output to channel 1 of

the scope’s input



Setting up the Vector Signal Analyzer

3. Configure the VSA for use

with a downconverter.



Correcting Downconverter Frequency Response

1. Configure your

source for a 0 dBm

CW at the desired

carrier

2. Set VSA averaging

to “continuous peak

hold”



Correcting Downconverter Frequency Response

1. Program your source to

slowly sweep across the

desired band

2. Be sure that the ADC is not

being overdriven during the

sweep.

3. Use an external leveling

loop if necessary



Applying Frequency Response Corrections

1.

2.



Applying Frequency Response Corrections



Amplitude Corrections



Vector Analysis with Wideband QPSK, APSK, and SOQPSKAgilent 89601B VSA

10 GHz QPSK signal

BW = 900 MHz and

EVM = 1.4%

APSK and SOQPSK





Digital Waveform CreationAgilent’s SystemVue

Modulation models

• BPSK, SBPSK, QPSK, OQPSK, 8PSK, pi/4 DQPSK, SOQPSK, Pi/4 CQPSK, MSK, GMFSK, CPM, CPFSK

• FHSS, DSSS

FEC – forward error correction

• Convolutional Coding

• Turbo Coding

• LDPC

Multiple Access Scheme

• FDMA

• TDMA

• CDMA

Satellite Channel Model

Transponder





Summary

Nonlinear behavior must be characterized and addressed

Common test signals include two-tone, multitone, and NPR signals

Digital signal generation approach provides repeatability and cost advantages over analog

generation approaches including notch depth, bandwidth, and adjustability

Multiple techniques and measurements non-linear behaviors in the PXA

Using the PXA as a down-converter is a technique for lower cost high-frequency

wideband measurement

VSA software enabled is a flexible platform that allows for easy input of corrections as

well as a platform to make a large amount of vector/modulation quality measurements

53



Thank You!

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