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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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4
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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7
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.
Configuration Summary
Low complexity
Boosted source
Forward transmissionmeasurement only
Response calibration only
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8
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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10
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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11
Configurat ion 4Setup and Features
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
Response calibration 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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12
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
Configurat ion 5Setup and Features
High-power Option
Configuration Summary
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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13
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.
14
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
Configurat ion 6Setup and Features
Configuration Summary
High complexity
Boosted source
Forward transmission andreflection measurements only
Response calibration only
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15
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 .
HP 8753E with option 011
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.
HP 8753E with option 011
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.
16
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.
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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
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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 )
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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.
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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.
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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).
HP 8753E with option 011
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.
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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
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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 .
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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
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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
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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,
Hewlett-Packard Application Note 1287-2,
literature number 5965-7708E.
Applying E rror Correction to N etwork An alyzer Measurement s,
Hewlett-Packard Application Note 1287-3,
literature number 5965-7709E.
Network An alyzer Measurem ents: Filters and Am plifier Examples,
Hewlett-Packard Application Note 1287-4,
literature number 5965-7710E.
Improving Th roughpu t in N etwork An alyzer Applications,Hewlett-Packard Application Note 1287-5,
literature number 5966-3317E.
8 Hin ts for Making B etter Network A nalyzer Measurements,
Hewlett-Packard Application Note 1291-1,
literature number 5965-8166E.
Suggested Reading
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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 .
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