HP-AN1287-4_Network Analyzer Measurements Filter and Amplifier Examples

8/14/2019 HP-AN1287-4_Network Analyzer Measurements Filter and Amplifier Examples

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Netw ork AnalyzerMea su rem en ts: Filter andAmp lifier Exam ples

Applica t ion Note 1287-4Table of Conten ts

Page

Introduction 2

Measuring a F ilter 2

Er ror Correction for Accurat e

Passband Measurements 3

Swept-Power Amplifier

Measurements 4

Evalua ting AM-to-PM

Conversion 5


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2

Introduct ion The net work ana lyzer is u sed for a variety of device an d componentcharacterization tasks in both laboratory and production environments.

This highly accurate instrument can evaluate both active and passive

component s, as will be demonstra ted in t his a pplicat ion n ote formeas urem ents of a filter a nd a mplifier. With t he addit ion of time-domain

capability, a network an alyzer can a lso gate out un want ed responses

dur ing measur ement s, leaving only the desired inform ation.

Hewlett-Packar d Compa ny offers a wide ra nge of RF a nd m icrowave

network an alyzers for m easur ement s from DC to 110 GHz. These

instrum ents ar e available with a wide range of options a nd test sets to

simplify measurements in stand-alone and automatic-test-equipment

(ATE) setups.

Often, both th e magnitu de and ph ase beha vior of a component can be

critical t o the per form ance of a comm unications system . A vector network

ana lyzer can pr ovide inform ation on a wide ran ge of these devices, from

active devices such as am plifiers an d tr ans istors, to passive devices such ascapacitors a nd filters. This app lication note illustr ates swept-frequen cy

measurem ents on an RF filter, and swept-power measur ements on a

comm unications-band a mplifier. The a mplifier is typical of th ose used in

Global Syst em for Mobile Commu nicat ions (GSM) service.

Complete cha ra cterization of filters is typically achieved with swept-

frequency meas ur ement s. Shown in Figure 1 ar e the frequency responses

of a filter. On th e left a nd bottom we see the t ra nsm ission r esponse in

log magnitu de form at , and on th e right we see th e reflection response

(retur n loss).

The most comm only meas ur ed filter char acteristics are inser tion loss an d

bandwidth , shown on t he lower plot with an expanded vertical scale.Anoth er comm on measu red pa ra meter is out -of-band r ejection. This is a

meas ure of how well a filter pass es signals within its ba ndwidth wh ile

simulta neously rejecting signals well outside tha t sa me ban dwidth. A test

systems dynam ic ra nge generally determ ines how well it can evaluat e

this characteristic.

Measu ring a Fi l ter

CH1S11 log MAG 5 dB/ REF 0 dB

CENTER 200.000 MHz SPAN 50.000 MHz

log MAG 10 dB/ REF 0 dBCH1S21

START .300 000 MHz

STOP 400.000 000 MHz

Cor

69.1 dBStopbandrejection

Return lossSCH1 21 log MAG 1 dB/ REF 0 dB

Cor

Cor

START 2 000.000 MHz STOP 6 000.000 MHzx21 2

Insertion loss

m1: 4.000 000 GHz -0.16 dB

m2-ref:2.145 234 GHz 0.00 dB

1

2ref

Figure 1.

Test ing Filters

w it h Frequency

S w e e p s


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3

Error Correction forAccurate Passban dMeasurements

The return loss plot is typical of passive reflective filters, showing high

reflection (near 0 dB) in t he st opbands, and good impedance mat ching in

the passba nd. A different t ype of filter, known a s an absorptive filter, tends

to be well matched in both t he st opban d and passba nd, providing a goodma tch over a br oad frequen cy ra nge.

Var iation from a const ant am plitude response within t he filter s bandwidth

resu lts in signal distortion. Err or corr ection is often essen tial for a ccur ate

meas ur ement s of filter passba nds. When a filters passban d is meas ur ed

with a net work a nalyzer without calibra tion, the r esponse may vary

considera bly, depending on th e network a nalyzer an d test cables used

(Figure 2).

When th e same filter is evaluated a fter doing a response calibration

(norm alization), the t est system s tr ans mission-tr acking frequency-

response error is removed from th e measu red response, resulting in a

much n ar rower am plitude-distortion window. After norm alization, thefilter s displayed frequen cy response still shows some a mplitude ripple

caused by intera ction between t he test systems source and load ma tch.

This ripple even goes above the 0 dB reference line, indicating gain (which

is impossible since pass ive devices can not a mplify signals). This a ppar ent

anoma ly is due to misma tch meas ur ement err or. By perform ing a two-port

calibration prior to the filter measurement, these errors are removed.

Following vector-error correction (two-port calibration), it is apparent

tha t t he filter s pass band a mplitude r esponse varies by only 0.1 dB

ar ound the center frequency. The 1 dB amplitude var iations measu red

previously with t he un corr ected test system ar e not represent at ive of

the filter s actu al pass band response. By perform ing error corr ection

with a vector net work a nalyzer, the t ru e nat ure of the filter is revealed

as ha ving minima l amplitude variat ion ar ound the center frequency,meeting a relat ively tight a mplitude per form ance window for low

distortion a pplicat ions (seeApplying E rror Correction to N etwork

Analyzers Measurements , Hewlett-Packard Application Note 1287-3).

CH1 S21&M log MAG 1 dB/ REF 0 dB

Cor

CH2 MEM log MAG REF 0 dB1 dB/

Cor

Uncorrected

After two-port calibration

START 2 000.000 MHz STOP 6 000.000 MHzx2 1 2

After response calibration

Measuring filter insertion loss

Figure 2.

R esponse versus

Two-Port

Calibrat ion


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The most comm on measu rem ent of amplifier compr ession is th e

1-dB-compression point (Figure 4). This is defined as the input power (or

sometimes th e corresponding outpu t power) that r esults in a 1-dB decreas e

in am plifier gain r eferenced to th e am plifiers sm all-signal or linear gain.

An am plifiers 1-dB-compression point can be mea sur ed by displaying

norma lized gain from th e power sweep.

4

Sw ept-Pow er AmplifierMeasurements

In addition to perform ing the swept-frequen cy meas ur ement s used to

evaluat e a filter, many n etwork an alyzers can a lso execute swept-power

measurements, which are useful in characterizing the nonlinear behavior

of a device. The example in Figure 3 shows an am plifiers outpu t powerversus inpu t power measu red at a single frequency. The amplifier ha s a

linear r egion of opera tion at which gain is const ant regar dless of power

level. The gain in this region is called sma ll-signal gain, an d is pr oport ional

to th e slope of the power r esponse.

As the input power contin ues to increase, the point on th e curve at wh ich

am plifier gain begins to decrease defines where th e compr ession r egion

begins. The am plifiers outpu t is no longer sinusoidal in t his r egion, an d

some of the output a ppears in har monics rather t han only in th e

fundam enta l frequency of the signa l. As input power is increa sed even

more, the a mplifier becomes sat ura ted, and output power r emains

const ant . At t his point, the a mplifier s gain dr ops to zero, and increa ses

in inpu t power will not produce increased outpu t power. While this is

tr ue for m ost types of amplifiers, th e out put power of tr aveling-wavetube a mplifiers an d a few other types actua lly decreases beyond th e

satur ation point.

To measur e an am plifiers sat ur at ed output power over a power sweep,

a net work a nalyzer mus t ha ve sufficient output power to drive th e

am plifier into sat ur ation. A booster am plifier is often n eeded at t he inpu t

of high-power a mplifiers to achieve satu ra ted conditions because of th e

relat ively low levels of test port power a vailable from a network a na lyzer

at h igher frequencies.

Saturated

output power

OutputPower(dBm)

Input Power (dBm)

Compressionregion

Linear region

(slope = small-signal gain)

Figure 3.

P o w e r S w e e p s

Characterize the

Compress ionR egion


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5

CH1 S21 1og MAG 1 dB/ REF 32 dB 30.991 dB12.3 dBm

C2

IF BW 3 kHz SWP 420 msec

START -10 dBm CW 902.7 MHz STOP 15 dBm

1-dB compression: input power

resulting in 1-dB dropin gain

Ratioed measurement

Output power available (non-ratioed

measurement)

In t his display, the flat pa rt of the t ra ce is th e linear, small-signal region

of operation, while the par t of the t ra ce with a n egative slope corr esponds

to compr ession a t h igher inpu t power levels. For t he exam ple amplifier

under test, 1-dB compr ession occur s a t +12.3 dBm of input power, when

measu red a t a fixed CW frequen cy of 902.7 MHz.

Since it is often us eful to know the output power corresponding to the

1-dB-compression point, t he dual-channel capability of most network

ana lyzers can be em ployed to simu ltan eously show normalized gain and

absolute power. Display marker s can rea d out both t he input power an d

output power where 1-dB compression occurs. Alternatively, the gain of

the a mplifier at th e 1-dB-compression point can s imply be added to the

1-dB-compression input power to comput e th e corr esponding out put power.

In F igure 4, the outpu t power at the 1-dB-compression point is12.3 dBm + 31.0 dB = 43.3 dBm .

In t hese types of compression tests, th e power-sweep range mu st be lar ge

enough to drive the am plifier un der t est from its linear region of operat ion

to its region of compression. Modern network ana lyzers typically provide

power sweeps with 15 to 20 dB of ran ge, which is grea t en ough to drive

most a mplifiers into compr ession. It is also very importa nt to sufficiently

att enua te th e outpu t of high-power am plifiers not only to prevent dam age

to the n etwork a na lyzers receiver, but als o to keep power levels low

enough to avoid receiver compression.

Evalu ating AM-to-PMConvers ion

Measur ement s of am plitude-modulation-to-phas e-modulat ion (AM-PM)

conversion ar e also useful in chara cterizing the nonlinear behavior of

high-frequency amplifiers. These mea sur ement s requir e a vector network

ana lyzer. AM-to-PM conversion is a meas ur e of th e undesir ed pha se shifts

that occur as a result of any amplitude variations in a system.

In comm unications systems, unwa nted ph ase modulation can be cau sed by

unintentional amplitude variations such as power-supply ripple, thermal

drift, or m ultipat h fading. Var iations can also result from t he type of

modulation used in the system, as is the case with Qua drat ure Amplitude

Modulat ion (QAM) or bur st modulation.

Figure 4.

1-dB Compress ion


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Sugges ted Reading

AM-to-PM conversion is critical in systems based on pha se m odulation,

such as qua dra tur e phase sh ift keying (QPSK), since phase distortion can

cause signal degradation in analog systems and increased bit-error rate

(BER) in digital systems. AM-to-PM conversion is directly related to BER,

and measu rem ents of AM-to-PM conversion can h elp provide insight int o

the cause of increased BER in a given system. The mea sur ement

complement s BER meas ur ement s, which in th emselves do not provide any

real insight into the phen omenon cau sing the bit errors.

AM-to-PM conversion is usu ally defined as the change in outpu t ph ase

for a 1-dB incremen t in t he input power t o an am plifier, expressed in

degrees/dB. An ideal a mplifier would have n o interaction between its ph aseresponse a nd t he level of the input signal. AM-to-PM conversion can be

meas ured with a power sweep on a vector n etwork ana lyzer (Figure 5).

The test da ta is displayed as the pha se of forwar d tra nsm ission (S21)

versu s power. The AM-to-PM conver sion for a DUT can be comput ed by

using a sm all increm ent of the amplitu de (such as 1 dB) center ed at a

par ticular power level, and n oting the change in phase. The changes in

amplitude and phase can be easily measured with tr ace markers.

Dividing the ph ase chan ge by the am plitude chan ge yields the AM-to-PM

conversion. In Figure 5, AM-to-PM conversion is 0.86 degrees/dB, centered

at an input power of 4.5 dBm and a n outpu t power of 16.0 dBm

Und erstand ing the Fu ndam ental Principles of Vector Network An alysis,

Hewlett-Packar d Application Note 1287-1.

Exploring the Architectures of Network Analyzers , Hewlett-Packard

Applicat ion Note 1287-2.

Applying Error Correction to Network Analyzer Measurements,

Hewlett-Packar d Application Note 1287-3.

6

1:Transmission Log Mag 1.0 dB/ Ref 2 1.50 dB

Use transmission setupwith a power sweepDisplay phase of S21AM - PM = 0.86 deg/dBat 4.5 dBm input power

Start -10.00 dBm Stop 0.00 dBmCW 900.000 MHz

Start -10.00 dBm Stop 0.00 dBmCW 900.000 MHz

2:Transmission /M Phase 5.0 deg/ Ref -115.7 deg

1

2

1

1

2

Ch1:Mkr1 -4.50 dBm 20.48 dB

Ch2:Mkr2 1.00 dB 0.86 deg

Figure 5.

AM-to-PM

Conversion


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F o r m o r e i n f o r m a t i o n a b o u t

Hewlett -Packard test and measure-m e n t p r o d u c t s , a p p l i c a t i o n s ,s e r v i c e s , a n d f o r a c u r r e n t s a l e s

o f fi c e l i s t i n g , v i s i t o u r w e b s i t e ,ht tp :/ /w ww.hp .com /go/tmdi r. Youcan also contac t one of the following

c e n t e r s a n d a s k f o r a t e s t a n dmeasurement sa les represent ative .

United States:Hewlett-Packard CompanyTest and Measu rement Call CenterP.O. Box 4026En glewood, CO 80155-40261 800 452 4844

Canada:Hewlett-Packard Canada Ltd.5150 Spectru m WayMississauga, OntarioL4W 5G1(905) 206 4725

Europe:

Hewlett-PackardEuropean Marketing CentreP.O. Box 9991180 AZ AmstelveenThe Netherlands(31 20) 547 9900

Japan:

Hewlett-Packard J apan Ltd.Measurement Assistance Center9-1, Takakura-Cho, Hachioji-Shi,Tokyo 192, J apa nTel: (81-426) 56-7832Fax: (81-426) 56-7840

Latin America:

Hewlett-PackardLatin American Region Headquar ters5200 Blue La goon Dr ive, 9th F loorMiam i, Florida 33126, U.S.A.(305) 267 4245/4220

Australia/New Zealand:

Hewlett-Packard Australia Ltd.31-41 Joseph Str eetBlackburn, Victoria 3130, Australia1 800 629 485

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Data Subject to Change

Copyright 1997Hewlett -Packard CompanyPrin ted i n U.S.A. 5/97

5965-7710E

HP-AN1287-4_Network Analyzer Measurements Filter and Amplifier Examples

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