HP-AN1287-6_Using a Network Analyzer To Characterize High-Power Components

8/14/2019 HP-AN1287-6_Using a Network Analyzer To Characterize High-Power Components

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Using a Netw ork Analyzerto Cha racterizeHigh-P ow er Compon en ts

Applica t ion Note 1287-6

fi

DUT


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2

Table o f Conten ts Page

Introduction 3

Defining High Powe r 3

Why High-Powe r Measu remen ts Can be Challenging 4

Netw ork Analyzer Configurations for Measuring

High-Pow er Devices 5

Configura t ion 1 6

Configura t ion 2 7

Configura t ion 3 9

Configur at ion 4 11

Configura t ion 5 12

Configura t ion 6 14

Addit iona l Configura t ions 14

Source Level ing 15

Source Leveling using Power-meter Calibra t ion 15Source Leveling using External Leveling 16

Calibration Purpose and Types 17

Calibrating Tips for Best Re sults 17

Dynamic Accuracy 17

Choosing Calibra t ion Power Levels 18

Calibrat ing at One Power Level versus Two Power Levels 18

Common P roblems of High-Pow er Measurements 19

Amplifier s with AGC Loops 19

On-wafer Devices (Pulsed Measurements) 19

Appendix 23

Network AnalyzersDefin it ion and Capabilit ies 23

Suggested Reading 27


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3

This application note describes linear a nd nonlinear m easur ement s

of high-power components an d h ow to use a network an alyzer for

mak ing them . It covers th e power limitat ions of a network a na lyzer,

an d special net work-ana lyzer equipmen t configurat ions for high-power m easur ements . How to improve th e accuracy of high-power

measu rement s a nd solve common problems when mak ing high-power

measurements are also described.

To get t he m ost from th is note, you should have a basic understa nding

of network a nalyzers and the m easurements you can make with a

network a na lyzer. For a basic review, please see the a ppendix at th e

end of th is note. Additional network an alysis literat ur e and st udy

mat erials can be ordered t hr ough H ewlett-Pa ckard. A reference list

is included at th e end of th e note.

Wha t m ight be considered high-power device out put (e.g. 30 dBm

or 1 Wat t) in one applicat ion, can be insignifican t in an other a ppli-

cat ion like radar test tha t u ses devices with power levels in the 60 dBm

(1,000 Watt) range. In this note, high power refers to a power level

above th e compr ession level and certa inly above th e da mage level

of a st an dar d net work an alyzer. Therefore, a power a mplifier with

an output beyond th e measurement capabili ty of a standa rd net-

work analyzer would be classified as a high-power device. We extend

our definition to also include devices th at require a drive level tha t

is higher t han a st anda rd n etwork ana lyzer can provide. So a high-

power device is one th at delivers more power th an a sta nda rd

network an alyzer can mea sur e, or requ ires more inpu t powerthan the analyzer can provide.

Defining High-Pow er

Introduction


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Two main challenges exist when measuring high-power devices:

1. The mea surem ents performed on high-power devices can be

different th an th ose required t o cha ra cterize lower-power d evices.Measur ement s of high-power devices a lso can be performed different ly

tha n t hose made at lower power levels.

Pu lsed measurem ents a re a good example. Measur ement s typically

are not pu lsed at lower power levels since device overheat ing tend s

not to be a problem. High power can heat up a device, affecting its

measu red char acteristics. Many on-wafer measu rement s, for exam ple,

require pulsed RF a nd pu lsed DC bias, which r educes the average

power dissipation and keeps the tempera tur e of the device consta nt.

2. High-power m easur ement s require special network-analyzer

configur at ions. This can mean adding at tenu at ion or a coupler

between th e outp ut of the device under test (DUT) an d th e input of

the test instru ment to protect the receiver. It can a lso mean addingam plificat ion t o the stim ulus signa l if more power is required.

Calibration and accura te mea surem ents become significan tly more

complex as additional equipment is added to the t est setu p. In some

configura tions the additional ha rdwar e can ma ke some types of cal-

ibration impossible, or limit the n umber of measu rable par amet ers.

For example, reverse S-par am eters cannot be measur ed in some

configurations. The inability to perform certain calibrations can

limit th e accura cy of th e measur ement s.

This application note will show configurations ranging from those

tha t a re easy to assemble but may ha ve limited accur acy or mea -

sur ement capability, to more complex configur at ions tha t ar e very

accura te and can ma ke the same measurements as a standar d

network an alyzer.

Why High-Pow erMeasurements

Can be Chal lenging


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The simplest high-power devices to measur e are th ose th at h ave

high ga in but dont r equir e high levels of drive power, and t ypically

ar e tested in t heir linear r egion. Since these devices ha ve high gain,

the receiver mu st be pr otected from the h igh output power. Anatt enua tor or coupler added between the output of th e DUT and th e

an alyzers test port pr otects th e receiver. If using a coupler, termi-

nat e the t hrough arm of the coupler with a chara cteristic impedance

load. The coupled ar m of the coupler sen ds a sma ll portion of th e

inpu t signa l to port 2. For a 20-dB coupler, the signa l at t he coupling

arm is 20 dB less tha n t he strength of the signal at th e input.

Determine the ma ximum power out of the DUT, subtra ct th e power

level required at th e test port, and th en choose th e appropriate

attenu ator or

coupler value. Choose components th at ar e specified to h an dle

your chosen power level.

Configur at ion 1 m akes both forwar d an d reverse, reflection an dtra nsmission measu rements (S11, S12, S21, and S22) if using an a na lyzer

with a S-parameter test set.

This particular setup a llows both forwar d and r everse measu rement s

so full two-port er ror-correction, th e most a ccur at e calibrat ion, can

be performed. Calibration is performed a t t est-port 1 (or at t he end

of the cable atta ched to the port) and a t test -port 2 (with the a tten -

ua tor an d an y test cable). Include th e att enu at or or coupler on port 2

when per forming t he calibrat ion t o remove any misma tch between

th e att enu at or and th e ana lyzer s test port. Since calibrat ion is per-

formed with a ll ha rdwa re in place an d all error terms corr ected, the

measurements can be as accura te as th e standar d ana lyzer.

The at tenu ator on port 2 degrades t he un corrected directivity ofport 2 by twice the a tten ua tion value. This limits th e stability of

the calibra tion an d can m ake S 22 measu remen ts very noisy. If a S22measu remen t is needed, calibrate a t a higher power level, lower the

power level when making forward mea surements, and t hen ra ise the

power level for the r everse measu remen ts. There is more inform a-

tion on calibrating a nd power levels at th e end of th is note.

A common assu mption is th at th e DUTs displayed gain sh ould

be increased by the am ount of the attenu ation, but r emember t hat

the calibration includes the a tten ua tor so th e values as displayed

are corr ect.

Network analyzer

DUT Attenuator

Calibration points

1 2

Configuration 1

Calibration

Configuration 1Setup and Features

Configuration Summary

Low complexity

No boosted source

Forward and reversemeasurements

Full two-port orresponse calibration


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The simplest way to increase t he power of the stimulus signal is to

add a booster a mplifier at th e test port of a sta nda rd net work a na -

lyzer. The inpu t of th e booster am plifier conn ects to th e an alyzer s

source port an d th e output conn ects t o the DUT (see Configur at ion2). This configura tion boosts t he signa l level going int o the DUT

without m odifying the a na lyzers built-in t est set or ha ving to add

add itional coupler s. The an alyzer s source power plus t he gain of

the booster am plifier gives t he power a vailable for testing t he DU T.

Pr otect th e receiver by adding an a tt enua tor or coupler between th e

output of the DU T an d port 2 of th e ana lyzer, as sh own in

Configuration 1.

Network analyzer

DUTAttenuator

Calibration points

DUTBoosteramplifier

Configurat ion 2Setup and Features

Configuration 2

Configurat ion 2 is convenient becau se it consists simply of a sta nda rd

network a na lyzer. However, it ha s nu merous limitat ions. When

boosting th e signal at port 1, only high-power tr an smission m ea-

sur ement s in th e forward direction (S21) and nonboosted r eflection

measu rement s in th e reverse direction (S22) are possible. The position

of the booster amplifier, with its high reverse isolation, makes for-war d re flection (S11) an d reverse tra nsm ission (S12) measurement s

impossible. Nonboosted reverse r eflection mea sur ement s can be

made, but the limitat ions a s described in Configura tion 1.

The a ccura cy of Configura tion 2 is limited because t he n etwork

an alyzer m easur es th e reference signa l before the booster am plifier

(coupling ta kes place inside the an alyzer). As a r esult, an y mismat ch

between th e booster amp lifier an d th e DUT is not rat ioed out , and as

a r esult, ripple will appear in t he mea surem ent (Figure 1). This ripple

can be significan t due to the poor source mat ch of the booster am plifier.

Even the uncorrected source match of the network analyzer is likely to

be much better th an th e sour ce ma tch of th e booster amp lifier.


Low complexity

Boosted source

Forward transmissionmeasurement only

Response calibration only


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The un certainty a ssociated with t he m easurement will depend upon

th e quality of the mat ch between th e booster am plifier an d th e DUT.In a ddition, an y drift or power fluctuat ions a ssociated with th e

booster am plifier will appea r t o be from th e DUT since th e reference

signal is not m easur ed after th e booster amp lifier. This configur at ion

is best for ana lyzers tha t do not allow direct access to the R (reference)

chan nel and when the a ccura cy of the measuremen ts is not importa nt.

If the an alyzer a llows access to the R chann el and t he r eference signal

is coupled after the amplifier, a configuration that allows more

accura cy is possible. (See Configura tion 3).

Only tra nsmission response calibrat ion is possible with t his setu p.

The location of the booster amplifier does not allow the analyzer to

make tra nsmission measurements in th e reverse direction, ma king

a full two-port calibrat ion impossible. Perform a response calibrationby conn ecting th e output of th e booster am plifier to the a tt enua tor

or coup ler on port 2. A basic response calibra tion does not rem ove

mismat ch errors due t o the DUT only the frequency response

errors an d an y mismat ch associated with t he booster a mplifier, the

att enua tor, an d th e an alyzer. Since mismat ch is n ot corr ected for

during calibration, even measurements made with a response

calibrat ion ar e limited in t heir a ccur acy.

In t his setu p you can impr ove the source mat ch by adding an isolat or

between the booster a mplifier and the DUT, or by adding at tenu at ion.

Adding an isolator between t he outpu t of the booster amp lifier a nd

the input of the DU T will remove the effects of any m ismatch. If th e

booster am plifier has en ough gain, a 3-dB or 6-dB at tenu at or at its

output can be added to improve its outpu t ma tch. Be sure to

include t he isolat or or a ny at tenu at ion in th e calibrat ion if you

include it in t he measur ement setup. Also make sur e that the isolator

or attenuator can ha ndle th e power level being tested. Instea d of

usin g a high-power isolator, a circulator with a high-power load on

its third port can be used.

Calibration

CH1 S21 &M log MAG 2 dB/ REF 11 dB

CENTER 2.500 000 000 GHz SPAN 4.000 000 000 GHz

Cor

PRm

LINE TYPE

1

18 Dec 1997 16: 16:33

1

1_: 13.704 dB

2.512 960 000 GH z

True response

True responsewith

mismatch ripple

Figure 1.Ripple i s caused

by a mismatchbetween the DUT

and the boosteramplifier.


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Normally, the ana lyzer at tempt s to pha se-lock near th e star t

frequen cy of the sweep. When th e an alyzer cann ot ph ase-lock at t he

star t frequency though, the a nalyzer performs a pre-tun e calibrat ion

routine. During pre-tu ne calibration th e ana lyzer att empts tophase-lock at a set frequency (100 MHz for the HP 8753D family),

which could be below th e sta rt of th e an alyzer's sweep. The pre-tun e

calibration routine would be performed, for example, if the analyzer

is switched to external reference mode without a signal att ached to

the R chan nel. When th e externa l R cha nn el signal is conn ected

the a na lyzer is attem pting to phase-lock at t he pretu ne frequency

(100 MHz). With a band-limited device in the R channel path (e.g.,

a booster a mplifier) there ma y not be sufficient signal at this pr etun e

frequen cy and p ha se-locking err ors can occur on some an alyzers.

On t he H P 8753E n etwork a na lyzer, tur ning PLL Aut o off (locat ed

in the S ervice Modes menu ) will stop the a na lyzer from performing

th e pr e-tun e calibrat ion routine. Tur nin g PLL Aut o off rar elyaffects th e accur acy of inst rum ent m easur ement s. Another wa y to

prevent this phase-locking error is to use frequency-offset mode in

the H P 8753E. In frequency-offset mode the a na lyzer assu mes th at

a ban d-limited device is being measu red, and as a result t he an alyzer

never goes out side the sweep ra nge to pha se-lock. Setting LO = 0 H z

when in frequency-offset mode allows the analyzer to be used normally.

Ban d-limited devices in th e R cha nn el path ma y cau se pha se-locking

problems in th e HP 8720D family of network a na lyzers un less the

an alyzer h as t he h igh-power option (Option 085) or frequ ency-offset

option (Option 089). With eith er of th ese options, ph as e-locking is

not a problem. This is becau se th e intern al reference switch switches

to the int erna l signa l when ph ase-locking. After th e an alyzer is

pha se-locked, the r eference switch ret urn s to the extern al signal

and t he an alyzer is ready to make measurements.

Calibratin g this configurat ion is similar to the calibrat ion p erformed

in Configuration 2. Only a response calibration is possible since

only forward mea sur ement s can be made. Perform t he response

calibrat ion by conn ecting t he out put of th e coupler s t hr ough ar m

to the at tenu at or on test -port 2 of th e ana lyzer.

Calibration


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HP 8753E with option 011

DUT

Reversecoupler

Forwardcoupler

Calibration points

Boosteramplifier

R A BRF out Attenuator

AttenuatorAttenuator

Configuration 4

Calibration

High-power reflection and tr an smission measu rement s in the

forward direction can be made on a na lyzers t ha t pr ovide access to

the A cha nn el and the R chann el. Forwar d reflection mea sur ement s

ar e possible by adding a r everse coupler between th e coupler u sed forth e R chan nel an d th e DUT. This reverse coupler, which is connected

to the A chann el, allows th e forwar d reflection measu remen t.Configuration Summary

Medium complexity

Boosted source

Forward transmission andreflection measurements only


Again only a response calibration is possible since a reverse

measu remen t cannot be made. When performing th e calibration,

conn ect th e output of the second coupler to th e att enua tor or out put

coupler so tha t t he calibrat ion includes all hard ware.


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DUTDUT

IsolatorIsolator

Coupler

SwitchSwitch

Coupler

1 2

R in

RF inRF out

Calibrationpoints

AttenuatorHP 8720Dwith option 085

Coupler

Boosteramplifier

The configur at ions pr esented u p to now ha ve made u se of an

an alyzer s build-in test set, or include a dditiona l externa l har dware

to mak e high-power mea surem ents. Another a pproach is to modify

the st an dar d test set to make it more suitable for high-power mea-sur ement s. Typically modifications to th e intern al test set a re

options th at H P provides at t ime of pur chase.

Configuration 5


High-power Option


High complexity

Boosted source

Forward and reversemeasurements

Full two-port orresponse calibration

An example of a modified test set is t he h igh-power option, Opt ion 085,

for the HP 8720D family of vector network analyzers. This option

provides four featur es tha t a llow for the m easur ement of higher

power levels. These are: 1) access to the RF pat h between th e source

an d t he t ra nsfer switch; 2) direct a ccess to th e R chan nel; 3) directaccess to the RF pat h between the t ran sfer switch a nd th e test ports;

and 4) step attenuators between the couplers and the samplers on

th e A and B chann els. The HP 8720D with Option 085 allows input

power an d output power up t o +43 dBm at th e test ports of

the analyzer.


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Access to the RF pat h between th e sour ce an d th e tra nsfer switch

allows th e source signa l to be amplified and th en be switched

between port 1 or port 2, a llowing forward a nd reverse high-power

measu remen ts. The inpu t of th e booster amplifier connects to theRF out conn ector on t he a na lyzer, and t he outpu t of the booster

am plifier conn ects to th e coupler. The coupled ar m of th e coupler

conn ects t o the R-cha nn el input t o provide the reference signal u sed

for ratioing. To insure an optimum power level at the R receiver,

add any needed attenuation between the reference coupler and the

an alyzer. The optimum power range for t he r eceiver is pr ovided by HP.

The through arm of the coupler connects to the RF in connector

on the ana lyzer. The a mplified signa l goes th rough th e tr an sfer

switch, which directs it to either t est port. Ju mpers between t he

tr an sfer switch and t he test port s give access to the RF signal path .

They allow th e user to add high-power isolators t o protect the tra nsfer

switch. Without isolators, signals with too much power can dam age

the tra nsfer switch. Isolators on both sides of th e tr an sfer switch

ensure th at after the signal has been measur ed by the coupler any

signal will be termina ted, th ereby protecting th e switch.

The final feat ur e of this h igh-power option is the intern ally

controlled step at tenu ators t ha t pr otect t he r eceivers. Located after

the couplers a nd before th e receivers, th ese 55-dB step a tten ua tors

(with 5-dB increment s) reduce th e signal t o an optimum level for

the r eceiver. These at tenu ators a re cont rolled from th e front pa nel

of the analyzer.

A major benefit of th is configura tion, besides th e ability to do both

forward a nd r everse measu remen ts, is th e ability to do full two-portvector error corr ection. Calibra ting in th is man ner t akes int o account

the effects of the h ar dware th at h as been added t o the setup (isola-

tors, am plifiers, couplers, etc.) an d all err ors associated with t he

ana lyzer and the measu rement setup up to the point of calibration.

Perform the calibration at the point wh ere th e DUT will be con-

nected.

When u sing this configur at ion, it is extremely importa nt not to

dam age an y of the inter na l component s because high power levels

are inside the analyzer itself. Since amplification takes place before

the tra nsfer switch, the power-handling capabilities of th e switch

must be kn own. Analyzers th at allow signal a mplification before

the t ra nsfer switch m ust specify how much power th e switch canha ndle. HP specifies maximum power for t he t ra nsfer switch for

two conditions, when the switch is switching, and wh en t he switch

is not switching. Typically the nonswitching power-rating is higher

tha n t he switching power r atin g. The power-ha ndling capa bility of

all component s in the RF pat h mu st be considered when m aking

high-power mea sur ement s. When u sing this configur at ion, it is

importan t t o understand how the ana lyzer works and the power-

Calibration

Forward and ReverseMeasurements by Ampl i fyingthe Source Signal

Protect ing the Rece ivers


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ar e possible only in t he forward direction.

Two-port vector err or calibra tion cannot be used in t his configura tion.

Since the extern al R chan nel is being used an d is conn ected only to

port 1, the r eference signa l is not a ccura te when doing a r everse

sweep. This mean s only a r esponse calibration can be done between

test-port 1 an d t est-port 2. Since full two-port calibration cannot be

done, th is configur at ion sh ould only be used if more power is

needed th an can be provided by Configura tion 5.

Often HP can design t est sets to specifically match your needs.

Contact your local HP sales representative if you would like to

investigate a specially configured test set. For example, solid-stateswitches may be subst itut ed for m echa nical switches in a high-power

test set . High-power t est sets u sua lly use mechan ical switches to

handle higher power levels. If continuous switching is required,

solid-state switches need to be used. Special configurations can

include solid-sta te switches in high-power t est set s if needed.

Oth er special configurat ions include h igh-power t est set s th at allow

th rough-reflect-line (TRL) calibra tions for n oncoaxial mea sur emen ts .

Special configur at ions for applications t ha t increase th e power

ra nge over wh ich a network a na lyzer sweeps su ch a s compr ession

measurements, are also available.

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ha ndling capa bility of each component in th e signal path .

Configura tion 6 uses th e sam e high-power test set a s Configura tion 5,

but th e hardware is re-arra nged so that measurements can be made

at even higher power levels. Configuration 5 is desirable because itallows high-power measurements in both directions, but the maximum

power level does not rea ch the t est ports. Due t o losses in t he t est set

an d the power limitat ions of th e tra nsfer switch, th e power at t he

test ports is less tha n t he couplers can ha ndle. To test at power levels

up t o the level th at the couplers can safely han dle (+50 dBm), the

HP 8720 with Option 085 is used in an a lterna te setup.

Amplification is done after t he t ran sfer switch a nd before the test port

to get the m aximum power possible at th e test ports. This configu-

ration allows higher power levels than possible using Configuration 5,

but h igh-power mea sur ement s, both t ran smission an d reflection,

DUT

HP 8720D with option 085

RF out RF in

Attenuator

Switch

CouplerSwitch

Coupler

R in

DUT

1 2

CalibrationpointsCoupler

Boosteramplifier

Addit ionalConfigurat ions

Configuration 6

Calibration



High complexity

Boosted source

Forward transmission andreflection measurements only



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Source Level ing When am plifiers operat e in their nonlinear r egion, the measu redresponse ma y differ from th e tr ue r esponse of the a mplifier. If the

amp lifier is operating in its nonlinear r egion, variations in th e

stimu lus signal might n ot be duplicat ed at t he outpu t of th e amplifier.In cont rast , the r esponse of an amp lifier tested in its linear ra nge is

not a ffected by t his level var iation. By ratioing the r esponse t o the

stimulu s, the n etwork an alyzer r emoves the effects of th e stimulus

variat ion a nd d isplays th e tr ue performan ce of the am plifier.

However, if the a mplifier is operating in its n onlinear ra nge, at or

near sat ura tion, th e amplifier does not produce an output signal

with a variat ion pr oportional to the var iation of the signal present

at t he input . The ratioing process in the network an alyzer th en

creates a n erroneous display (Figure 2).

Figure 2.When a device

operate s l inea rly, itstrue response can be

measured. However, adevice that operates

nonlinearly is affected bynonlevel input signals.

To optimize measu rement s in t he n onlinear region, a leveling

scheme must be u sed to produce a stimulus signal with a frequency

response tha t is as flat as possible.

Using power-meter calibration in cont inuous sa mpling mode is one

way to level the source (Figure 3). The power level at each frequency

point in t he sweep is measured with a power meter. The network

an alyzer, connected to the power meter via HP-IB, adjusts its source

power un til the power meter measu res th e desired power level.

Then t he mea sur ement is made. Since the accura cy of th e power

meter is very high (uncertaint y in the t enth s of a dB r an ge), you

can h ave confidence tha t t he power level is a ccur at e as well as flat .


DUT

Power meter

HP-IB

R A BRF out

Power sensor

Power

Figure 3.

Power-meter

calibration levels

the source signal

and removes the

nonl inear r ipple .

Source Level ing Us ingPow er-mete r Calibration


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When u sing either power-meter calibra tion or externa l leveling,

keep the source-leveling process active, just as it will be during the

measu remen t. Then p erform a response calibration as you would if

source leveling was not used.


Attenuator

DUT

R A BRF out

Power sensor

Powersplitter

Ext AM+

+

REF

50

Loop gain

RF filterand load

Figure 4.This c ircuit

shows a commonway to implementexternal level ing.

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Usin g power-met er calibra tion for sour ce leveling work s well, but

can slow down th e measu rement s too much for some applicat ions.

A faster method of source leveling is external leveling. External

leveling ma kes u se of an externa l AM (am plitude modulation)input , available on some n etwork an alyzers. Adjust ing the voltage

at th e AM input adjust s th e ana lyzers source power. By creat ing a

leveling circuit tha t is external to the n etwork a na lyzer, and a djusting

the voltage to th e externa l AM inpu t, source power can be leveled

on a r eal-time basis.

The extern al leveling circuit can be implemented in several ways.

One common implementation is to create a circuit consisting of a

detector diode and an opera tional am plifier. Changes in th e power

measu red by t he diode are det ected by t he operat iona l amplifier circuit.

The circuit conn ected t o the AM input of the an alyzer adjusts t he

source power by adjusting t he volta ge at t he AM input (Figure 4).

Source Level ing Us ingExternal Level ing


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17

Calibrating an analyzer eliminates systematic errors and makes

measu remen ts more accur at e. This note ha s shown calibrat ions for

various configurat ions a nd wher e in the test setup t o perform t hese

calibrat ions. Of th e two types of calibrat ion discus sed, full two-portcalibrat ion is t he m ost accura te because it r emoves all systemat ic

errors in t he measur ement setup, ideally up to where the DUT

conn ects t o the a na lyzer. Response calibration is less accur at e an d

removes only the frequen cy-tracking err ors. Fr equency-tr acking

errors a re a ssociated with th e differences in frequency response

between one chann el and a nother. In a full two-port calibrat ion, th e

an alyzer sweeps in both forward a nd r everse directions, while in a

response calibration, the a na lyzer sweeps only in t he forwa rd direc-

tion.

In a ddition t o underst an ding wha t t ype of calibration is possible foreach configur ation an d wher e to perform the calibrat ion, it is importan t

to un dersta nd h ow to get t he m ost out of a calibrat ion. The following

topics discuss t echn iques an d considera tions for achieving th e best

calibrat ion r esults.

By examining th e dynam ic accur acy response of an an alyzer, you can

optimize th e power levels for calibration an d mea sur ement . Dynamic

accura cy refers t o the u ncertaint y associated with calibrat ing at

one signal level and measu ring at another. The dynam ic accura cy

plot for the HP 8753E shows extremely low uncertainty (0.02 to

0.06 dB) if receiver power is between 10 an d 50 dBm in dependent

of the calibration power level (Figure 5). Perform calibration and

measu remen ts so that t he signal level at t he receiver is in this

high-accura cy ra nge. At lower signa l levels, noise is a factor; at

higher levels, receiver compr ession is a factor.

Calibration Pu rpose and Types

Calibration Tipsfor Bes t Results

Dynamic Accuracy

10 dBm

20 dBm

30 dBm

Reference power Level

Accuracy(dB) 1

0.1

0.01

10 0 10 20 30 40 5060 70 8090 100

Test Port Power (dBm)

Figure 5.

HP 8753E

dynamic

accuracy.


18/28

Calibrating at OnePow er Level versusTwo P ow er Leve ls

18

If you are m easur ing an a mplifier and the outpu t power level is

higher th an th e input power level, at wh at power level should you

calibrate: at the higher power level, the lower power level, some-

where in between t he t wo levels, or a t a different level for each port?Calibrating and measuring with th e test port powers in th e ran ge

of lowest u ncertaint y, 10 to 50 dBm for t he H P 8753E, r esults in

accur at e measur ement s as shown above. Calibrating at t he input

(lower) power level can r esult in a power level at th e out put receiver

tha t is t oo low, because att enua tion is r equired to protect t he a na lyzer

dur ing measur ement of the DUT. Calibrating at t he output (higher)

power level might n ot be possible since th e gain of th e DUT can

result in a n outpu t power level th at t he net work a na lyzer s source

together with t he booster amp lifier cann ot generat e. Therefore, you

might n eed to perform some calibrations a t a power level between

the inpu t power level and t he outpu t power level. In genera l, to

reduce noise calibrate at th e highest possible power below th e

onset of receiver compr ession.

A response calibra tion is done at only one power level since only

one forward sweep is mad e. Full two-port calibrat ion, on th e other

han d, can be done at different power levels since both forwar d an d

reverse sweeps are made. The an alyzer can calibrate one port using

one power level an d th e other port using a nother power level to

more accura tely match th e power levels present dur ing the

measu remen t of a DUT.

As an example, the open, short, an d load st an dar ds for port one,

and t he reverse through measu rements ma y be made at th e lower

power level. The open, short, an d load st an dar ds for port two andthe forward thr ough measu rements could then be made at the h igher

power level. It is also possible to perform a full two-port calibrat ion

at one power level as in a response calibrat ion. Th e power level

used would be determined from the dynamic accuracy specification

of the analyzer.

A fina l note on calibrat ion: power-ha ndlin g capa bilities of th e

calibrat ion st an dar ds can become an issue when calibrating at high

power levels. The comm on stan dar ds used for calibrat ion a re th e open,

short, load, and thr ough. The open, short, and through sta ndards

are not a problem since they do not dissipate an y energy. The load,

however, does dissipate ener gy so when you are calibrating m ake

sure th e standa rds can ha ndle the power level.

Choosing Calibrat ionPow er Level s


19/28

19

Common Problemsof High-Pow er

MeasurementsAmplifiers withAGC Loop s

Some amplifiers contain an (automatic-gain control) AGC loop. AGC

loops attemp t to keep th e outp ut power of th e amplifier constan t by

adjust ing gain to account for var iations at th e input of th e amp lifier.

Amplifiers with AGC loops can pose a problem when measured onsome network an alyzers, especially at high power levels.

Network a na lyzers sweep across th e selected frequency ran ge while

holding th e power a t t he desired consta nt power level. At t he end

of th e sweep, some net work an alyzers might blan k, or t ur n off

th e source, during the time it tak es the an alyzer to reset itself an d

set u p for a nother sweep the retr ace time. Tur ning off the source

is a pr oblem for a mplifiers conta ining a n AGC loop.

When an amp lifier with a n AGC loop is measu red an d the inpu t

signal is turned off at the end of the sweep, the AGC loop of the

am plifier compensat es for t he t ur ned-off signal by increasing its gain

to keep the output power level constan t. When th e sweep begins

again, th e network an alyzer rest ores its signal, and t here is poweragain at the inpu t of th e amp lifier, which h as r am ped-up its gain.

The m omentar y high outpu t power can cause dama ge or destr oy

th e a mplifier or t he a na lyzer s receiver if th e AGC loop cann ot

respond quickly enough .

Using a network analyzer th at keeps its power const an t dur ing retrace

will reduce the possibility of destroying the device or damaging the

ana lyzer. The HP 8753E keeps power const ant except when switching

frequen cy bands a t 300 kH z and 3 GH z. The HP 8720D fam ily of

network an alyzers a llows th e user to keep the power constan t or to

blank du ring retr ace (th e defau lt is to ha ve the power rema in con-

stant). The HP 8720D family will briefly blank during band changes

at 2.55 GHz. The HP 8722D blan ks a t 20.05 GHz as well. Be aware

tha t th is blan king occurs if the an alyzer sweeps across these bands.

A problem commonly encoun tered when measu ring h igh-power

on-wafer devices is th e hea ting u p of th e DUT. Devices on-wafer t end

to heat up quickly becau se th ey lack su fficient h eatsink ing. This

heat ing up requires tha t th e tempera tu re of the DUT be controlled

in some way since the r esponse of a DUT m ay change as the t em-

perature of the DUT increases. Two common ways to control heating

up is to pau se the n etwork ana lyzer between measur ements, or

pulse the RF a nd/or DC bias signa ls so th at a constan t DUT

temperatur e is maintained.

On-Wafer De vic es(Pulsed Measurements )


20/28

t t

IF BW

PRF

Pulsed RF IF Filtering Modulated IF

f

PRF

PRF

Figure 6.When the PRF is

less than the IFbandwidth, pulse

sidebands aremeasured

in addi t ionto the centerspectral l ine .

20

Pr esuming th at a device will not overheat dur ing a single sweep,

pausing between sweeps can keep the average temperature within

boun ds. A test-sequencing progra m t ha t cont ains a user-defined

pau se between each m easur ement will do this au tomatically. Moreinform at ion on t est sequencing is available in your n etwork ana lyzer s

user s guide. Pa using between sweeps dra ma tically slows down

testing time an d might not be pra ctical in some situat ions. Pu lsed

measu remen ts can be a better solut ion to the heat ing problem.

In some cases pulsed RF an d DC-bias measu remen ts might be needed

rath er than the usua l CW measurements made with a network

an alyzer. Pu lsed measur ement s ar e used for several rea sons. As

noted above, pulses can be configured so that an isoth erma l mea-

sur ement is achieved. A pulsed signal might also be used becau se it

is representa tive of th e signa ls that th e DUT encoun ters in actua l

use (rada r is a good examp le of this), or if there is int erest in the

tra nsient response of a device stimu lated with a pulse, etc.

Pu lsed measur ement s typically requires a test set designed for this

pur pose. The HP 85108 is an example of a network an alyzer system

designed for pulsed measu remen ts.

While not common, it is possible to make pu lsed RF mea sur ements

with a standard network analyzer under certain conditions. The

ability to make pulsed measu rement s depends on the pulse repetition

frequen cy (PRF ) of the pulse used, relative to the IF bandwidth an d

the sa mpling rat e of the network a na lyzer an d th e dut y cycle of the

signal. There ar e thr ee cases to consider wh en deter mining if pulsed

measu remen ts ar e possible using a network an alyzer. (Note th at we

are referring to finding a DUTs st eady-sta te frequency response

un der pu lse conditions, not a DU Ts tra nsient t ime-varian t response

to a pulse. A sta nda rd net work a na lyzer can not be used to measu re

the transient time-variant response to a pulse.)

1. The first case is when t he PRF of the pu lse is less than th e

IF BW of th e an alyzer. When th e PRF is less tha n t he r eceiver s

IF BW, the pulse sidebands pa ss thr ough t he IF filter a nd th e modu-

lated RF (the pulse) can not be m easur ed (Figure 6). So for pulsed

measurements with a P RF less than the IF BW of the ana lyzer, a

sta nda rd net work a na lyzer will give inaccur ate resu lts.


21/28

21

3. The final case is when th e PRF is greater t han the sam pling rat e

of the an alyzer s ADC. In t his case, the IF filter m easur es only the

cent er spectra l line (car rier) of the pu lsed-RF spectru m a s if it wer e a

CW signa l in a n onpulsed network a na lyzer configur at ion (Figure 8).

Since this type of measur ement actua lly filters off the m odulation

sidebands, the P RF of the pu lse does not affect th e measur ement .

Only cha nges to th e dut y cycle affect t he m easur ement .

Decreasing t he du ty cycle of the RF spr eads t he en ergy of this spectral

line t o the pu lsed-RF sideband s. The ma gnitude of this center spec-

tr al line, will be proport iona l to the m agnitu de response of th e DUT,

but will also contain a ma gnitude decrease caused by th e loss of th e

energy in the sidebands. The am ount of th e decrease caused by the

energy loss to th e sidebands is calculat ed using t he formu la:

Pu lse Desens itizat ion = 20*log(Dut y Cycle)

This desensitization factor decreases th e dyna mic range of

the measurement.

2. The next case is for a PRF grea ter t ha n t he IF BW, but less than

th e sam pling ra te of th e ana lyzers ADC (an alog-to-digita l convert er).

Pu lsed measur ement s are possible as long as t he PRF does not

generat e a sideband at t he sam pling ra te of the a na lyzer. For th eHP 8753E and HP 8720D fam ily, the ADC sampling rat e is 16 kHz.

If any PRF sideband occur s at 16 kHz, it will be downconverted a nd

be sampled by the receiver (Figure 7). Therefore, signals with a

PRF of 4 kH z, 8 kHz or 16 kH z would n ot be allowed.

LPF 4 kHz

4 kHz*

1 MHz(from 1st LO)

996 kHz(2nd LO)

984 kHz(from first lower sideband of

a signal with a PRF= 16 kHz)

4 kHz

16 kHz

IF

To IFFilter12 kHz

Sample and hold

*Mixing product from12 kHz IF and 16 kHz

sampling rate

Figure 7.Pulse sidebands

are downconvertedand sampled by

the receiver.

t t

IF BW

PRF PRF

f

Figure 8.When P RF is

greater than theIF bandwidth,

pulse sidebands

are rejected.


22/28

4 kHz

4 kHz

199 MHz(1st LO)

1 MHz

200 MHz(RF in)

198 MHz(1st lower sideband

from signalwith PRF = 2 MHz)

1 MHz

996 kHz(2nd LO)

To IFFilter

Figure 9.Any PRF which

generates a side-band at the ADC

sampling rate wi l lbe downconverted

and sampled by

the receiver.

22

When th e PRF of th e signa l is greater t ha n th e ADC sampling rate,

signals with a certa in PRF mu st be a voided since image frequencies

ma y mix with t he r eceiver s LO to produ ce the IF frequen cy. For

exam ple, the H P 8753s first LO is tun ed to 1 MHz below th e RFtest signal. For a 200-MHz RF test signal t he LO is 199 MHz.

Under pulsed conditions, say for a P RF equa l to 2 MHz, the first

lower sideba nd w ill fall at 198 MHz. Th is 198-MHz signal will mix

with t he 199-MHz LO to also produce a 1-MHz IF. Therefore, it is

importan t t o avoid signa ls with a PRF of 2 MHz/N, where

N = 1,2,3, ... (see F igure 9).

The har dware needed to make m easurements under pulse conditions

includes t wo splitter s, a p ulse generat or, an d a modulator. Conn ect

a power splitter to th e out put of th e ana lyzer s RF source out put .

One a rm of th e splitter connects to the reference (phaselock) cha n-

nel since the ph ase reference cha nn el can not be pulsed. The other

arm of th e splitter goes to a pulse modulator. The pulsed RF is sent

to a second splitter to make ratioed transmission measurements.

Ratioing is necessar y to remove the pu lse tran sient r esponse. The

tra nsm ission measu remen t of inter est is B/A (Figure 10).


Pulse generator

Modulator

R A BRF out

DUTDUT

Figure 10.Using a pulse

generator and amodu lator, pulse

measurementscan be made

with a standardnetw ork analyzer.


23/28

23

Before discussing the measurements made with a network analyzer,

it is importan t to have an u nderst an ding of a net work a na lyzer

block diagram and how analyzer ma kes measurement s.

Network a na lyzers pr ovide a wea lth of informa tion about a device,including its ma gnitude, phase, a nd group-delay response to a signal.

The ha rdwa re inside a network an alyzer includes a source for stim-

ulus, signal-separation devices for measuring a portion of the incident

signal an d for sepa ra ting signals t ra veling in opposite directions on

the same tra nsm ission line, receivers for signa l detection, a nd

display/processing circuitr y for reviewing resu lts (Figur e 11).

Network an alyzers measur e a portion of the source power to use

as a reference signa l. The remainder of the signal r eaches the DUTwhere pa rt of th e signal reflects ba ck from th e device and pa rt of

th e signa l tran smits th rough th e device. The reflected signal creat es

a st an ding wave consisting of both forward a nd r everse tra veling

waves. The signal-separat ion equipment allows for the detection of

each of these waves sepa ra tely. After measu ring t he r eflected or

tra nsm itted signal, th e ana lyzer ra tios it with the reference signal

to measu re t he char acteristics of th e DUT.

RECEIVER / DETECTOR

PROCESSOR / DISPLAY

REFLECTED(A)

TRANSMITTED(B)

INCIDENT(R)

SIGNALSEPARATION

SOURCE

Incident

Reflected

Transmitted

DUTFigure 11.

General networkanalyzer block diagram.

Appendix

Network Analyzers

Definit ions andCapabil it ies


24/28

24

A network an alyzer consists of either th ree or four cha nn els

(Figure 12). In n etwork a na lyzer t erminology, a cha nn el refers t o

the h ardwa re used to detect a signal. There ar e one or two R chann els

in a network analyzer. A portion of the stimulus signal is coupledout an d goes to the R chann el for rat ioing. The A cha nn el denotes

the chan nel associated with t est port 1. The A chann el measur es

the reflected signal when m aking a forward measur ement (stimulus

signal is at port 1), and m easures t he tra nsmitted signal when making

reverse measu remen ts (stimulus signal is at port 2). The B chann el,

associated with test port 2, measures th e tran smitted signal dur ing

forward measurements and the reflected signal during

reverse measur ements.

Port 1 Port 2

Transfer switch

Source

B

R

A

3 Receivers

Port 1

Transfer switch

Port 2

Source

B

R1

A

R2

4 Receivers

Figure 12.Three versus

four channels ina test set .


25/28

25

There ar e man y variations of network a na lyzers, but one of the most

distinguishing factors of a n etwork an alyzer is its t est set. A test

set is a collection of switches and couplers that directs the source

power an d separa tes forwar d an d reverse tra veling signa ls. Somenetwork a na lyzers, like the HP 8720D fam ily of vector n etwork

ana lyzers, have an S-parameter t est set with a tr ansfer switch th at

directs t he source power t o eith er port 1 or port 2 allowing th e

ana lyzer to mak e forwar d and reverse measur ements. Other n etwork

an alyzers, like th e HP 8711C family of network a na lyzers, ha ve a

tra nsm ission/reflection (T/R) test set t ha t does not ha ve a tr an sfer

switch and source power only goes to port 1 these test sets allow

only forward m easur ement s to be made (Figur e 13). In th e past,

test sets were sometimes not included in the network analyzer, but

almost all of todays net work an alyzers h ave a bu ilt-in t est set .

Port 1 Port 2

Source

B

R

A

DUTFwd

Figure 13.Transmission/

ref lect ion versusS-parameter

test set .

Test S ets

RF power always comes out

of port 1

Port 2 is always receiver

Response, one-port cal

available

Transmission/Reflection Test Set

ort 1 Port 2

Transfer switch

Source

B

R

A

DUTFwd Rev

RF power comes out of

port 1 or port 2

Forward and reverse

measurements

Two-port calibration

possible

Transmission/Reflection Test Set


26/28

26

Key to network ana lyzer measu rement s is calibrat ion. Calibration

does two things. First it esta blishes a r eference amplitude an d

reference phase a t a point in th e system. Second, it deter mines th e

accur acy of the measu remen t. Network a na lyzer calibrat ion correctsfor systematic errors (t ime invariant instrument and test setup

errors) in t he measu remen t setup. Ideally, calibrat ion corrects for

all errors up to the point wh ere th e DUT will be conn ected.

Calibration


27/28

27

Understanding the Fundamental Principles of Vector Network Analysis,

Hewlett-Packard Application Note 1287-1,

literature number 5965-7707E.

Exploring the Architectures of Network A nalyz ers,



Applying E rror Correction to N etwork An alyzer Measurement s,



Network An alyzer Measurem ents: Filters and Am plifier Examples,



Improving Th roughpu t in N etwork An alyzer Applications,Hewlett-Packard Application Note 1287-5,


8 Hin ts for Making B etter Network A nalyzer Measurements,



Suggested Reading


28/28

fi

For more informationabout Hew lett-Packard testand measu re-ment products,applications, services, and for

a current sales off ice l i st ing,visi t our web si te ,http://www.hp.com/go/tmdir.You can also conta ct one of thefol lowing cen ters and ask fora test and measurement salesrepresentative .

United States:Hewlett-Packard CompanyTest and Measu rement Ca ll 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 n

Tel: (81) 426 56 7832Fa x: (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.Tel: (305) 267-4245

(305) 267-4220Fax: (305) 267-4288

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 ChangeCopyrigh t 1998Hewlett-Packard CompanyP i t d i U S A 5/98

Documents

HP-AN1287-6_Using a Network Analyzer To Characterize High-Power Components