Agilent Technologies AN 1287-6 Instructions d'installation et d'utilisation

Agilent AN 1287-6

Using a Network Analyzer

to Characterize High-Power

Components

Application Note

DUT

Introduction

Defining high power

Why high-power measurements can be challenging

Network analyzer configurations for measuring

high-power devices

Configuration 1

Configuration 2

Configuration 3

Configuration 4

Configuration 5

Configuration 6

Additional configurations

Source leveling

Source leveling using power-meter calibration

Source leveling using external leveling

Calibration — purpose and types

Calibrating tips for best results

Dynamic accuracy

Choosing calibration power levels

Calibrating at one power level versus two power levels

Common problems of high-power measurements

Amplifiers with AGC loops

On-wafer devices (pulsed measurements)

Appendix

Network analyzers—definition and capabilities

Test sets

Calibration

Suggested reading

Table of Contents

Introduction

This application note describes linear and nonlinear

measurements of high-power components and how

to use a network analyzer for making them. It covers

the power limitations of a network analyzer, and

special network-analyzer equipment configurations

for high-power measurements. How to improve

the accuracy of high-power measurements and

solve common problems when making high-power

measurements are also described.

To get the most from this note, you should have a

basic understanding of network analyzers and the

measurements you can make with a network ana-

lyzer. or a basic review, please see the Appendix

at the end of this note. Additional network analysis

literature and study materials can be ordered

through Agilent Technologies. A reference list is

included at the end of the note.

Defining high power

What might be considered “high-power device

output” (e.g., 30 dBm or 1 Watt) in one application

can be insignificant in another application, such as

a radar test that uses devices with power levels in

the 60 dBm (1,000 Watt) range. In this note, “high

power” refers to a power level above the compres-

sion level and certainly above the damage level of

a standard network analyzer. Therefore, a power

amplifier with an output beyond the measurement

capability of a standard network analyzer would be

classified as a high-power device. We extend our

definition to also include devices that require a

drive level that is higher than a standard network

analyzer can provide. So a high-power device is

one that delivers more power than a standard

network analyzer can measure, or requires more

input power than the analyzer can provide.

Why high-power measurements can be

challenging

Two main challenges exist when measuring high-

power devices:

1. The measurements performed on high-power

devices can be different than those required to

characterize lower-power devices. Measurements

of high-power devices also can be performed dif-

ferently than those made at lower power levels.

Pulsed measurements are a good example.

Measurements typically are not pulsed at lower

power levels since device overheating tends not to

be a problem. High power can heat up a device,

affecting its measured characteristics. Many on-

wafer measurements, for example, require pulsed

R and pulsed DC bias, which reduces the average

power dissipation and keeps the temperature of

the device constant.

2. High-power measurements require special

network-analyzer configurations. This can mean

adding attenuation or a coupler between the out-

put of the device under test (DUT) and the input

of the test instrument to protect the receiver. It

can also mean adding amplification to the stimulus

signal if more power is required. Calibration and

accurate measurements become significantly more

complex as additional equipment is added to the

test setup. In some configurations the additional

hardware can make some types of calibration

impossible, or limit the number of measurable

parameters. or example, reverse S-parameters

cannot be measured in some configurations. The

inability to perform certain calibrations can limit

the accuracy of the measurements.

This application note will show configurations

ranging from those that are easy to assemble but

may have limited accuracy or measurement capa-

bility, to more complex configurations that are

very accurate and can make the same measure-

ments as a standard network analyzer.

Network analyzer configurations for measuring

high-power devices

The high-power test configurations described in

this note are designed to boost the power coming

from the network analyzer’s source as necessary,

and also to protect components such as the

receivers, couplers, and switches inside the

network analyzer from excessive power levels.

To select the correct network analyzer configura-

tion, you will need to consider the DUT and the

required measurements and accuracy. This section

shows high-power test configurations including the

necessary hardware, how to set up and calibrate

these configurations, and the advantages and

limitations of each.

The configurations are ordered by the degree of

measurement capability. In general, the increased

capability results in increased complexity in the

configuration. The first configuration is simple —

it uses a standard network analyzer and doesn’t

require high drive power, which means the network

analyzer’s source does not need to be boosted. The

other configurations measure devices that require

high-power inputs and also have high-power out-

puts. Measuring these devices requires amplifying

the network analyzer’s source signal somewhere

along the R path before it reaches the DUT. The

high output power also requires protection for the

couplers, receivers, and switches inside the analyzer.

Available high-power measurements Available calibrations

Forward

Forward transmission

Boosted transmission and reflection Forward/ Full

Configuration Complexity source only only reverse two-port Response

1 low X X X

2 low X X X

3 medium X X X

4 medium X X X

5 high X X X X

6 high X X X

Configuration 1

Configuration summary

• Low complexity

• No boosted source

• orward and reverse measurements

• ull two-port or response calibration

Setup and features

The simplest high-power devices to measure are

those that have high gain but don’t require high

levels of drive power, and typically are tested in

their linear region. Since these devices have high

gain, the receiver must be protected from the high

output power. An attenuator or coupler added

between the output of the DUT and the analyzer’s

test port protects the receiver. If using a coupler,

terminate the through arm of the coupler with a

characteristic impedance load. The coupled arm of

the coupler sends a small portion of the input sig-

nal to port 2. or a 20-dB coupler, the signal at the

coupling arm is 20 dB less than the strength of the

signal at the input. Determine the maximum power

out of the DUT, subtract the power level required

at the test port, and then choose the appropriate

attenuator or coupler value. Choose components

that are specified to handle your chosen power level.

Configuration 1 makes both forward and reverse,

reflection and transmission measurements (S11,

S12, S21, and S22) if using an analyzer with an

S-parameter test set.

Calibration

This particular setup allows both forward and

reverse measurements so full two-port error-

correction, the most accurate calibration, can be

performed. Calibration is performed at test-port 1

(or at the end of the cable attached to the port)

and at test-port 2 (with the attenuator and any test

cable). Include the attenuator or coupler on port 2

when performing the calibration to remove any

mismatch between the attenuator and the analyzer’s

test port. Since calibration is performed with all

hardware in place and all error terms corrected,

the measurements can be as accurate as the

standard analyzer.

Configuration 1

The attenuator on port 2 degrades the uncorrected

directivity of port 2 by twice the attenuation value.

This limits the stability of the calibration and

can make S22 measurements very noisy. If an S22

measurement is needed, calibrate at a higher

power level, lower the power level when making

forward measurements, and then raise the power

level for the reverse measurements. There is more

information on calibrating and power levels at

the end of this note.

A common assumption is that the DUT’s displayed

gain should be increased by the amount of the

attenuation, but remember that the calibration

includes the attenuator so the values as displayed

are correct.

Network analyzer

DUT Attenuator

Calibration points

Configuration 2

Configuration summary

• Low complexity

• Boosted source

• orward transmission measurement only

• Response calibration only

Setup and features

The simplest way to increase the power of the

stimulus signal is to add a booster amplifier at the

test port of a standard network analyzer. The input

of the booster amplifier connects to the analyzer’s

source port and the output connects to the DUT

(see Configuration 2). This configuration boosts

the signal level going into the DUT without modify-

ing the analyzer’s built-in test set or having to add

additional couplers. The analyzer’s source power

plus the gain of the booster amplifier gives the

power available for testing the DUT. Protect the

receiver by adding an attenuator or coupler

between the output of the DUT and port 2 of

the analyzer, as shown in Configuration 1.

Configuration 2

Configuration 2 is convenient because it consists

simply of a standard network analyzer. However,

it has numerous limitations. When boosting the

signal at port 1, only high-power transmission

measurements in the forward direction (S21) and

nonboosted reflection measurements in the reverse

direction (S22) are possible. The position of the

booster amplifier, with its high reverse isolation,

makes forward reflection (S11) and reverse transmis-

sion (S12) measurements impossible. Nonboosted

reverse reflection measurements can be made, but

with limitations as described in Configuration 1.

The accuracy of Configuration 2 is limited because

the network analyzer measures the reference signal

before the booster amplifier (coupling takes place

inside the analyzer). As a result, any mismatch

between the booster amplifier and the DUT is not

ratioed out, and therefore ripple will appear in the

measurement ( igure 1). This ripple can be signifi-

cant due to the poor source match of the booster

amplifier. Even the uncorrected source match of

the network analyzer is likely to be much better

than the source match of the booster amplifier.

Network analyzer

DUT Attenuator

Calibration points

DUT

Booster

amplifier

The uncertainty associated with the measurement

will depend upon the quality of the match between

the booster amplifier and the DUT. In addition, any

drift or power fluctuations associated with the

booster amplifier will appear to be from the DUT

since the reference signal is not measured after the

booster amplifier. This configuration is best for

analyzers that do not allow direct access to the R

(reference) channel and when the accuracy of the

measurements is not important. If the analyzer

allows access to the R channel and the reference

signal is coupled after the amplifier, a configura-

tion that allows more accuracy is possible (see

Configuration 3).

Figure 1. Ripple is caused by a mismatch between the

DUT and the booster amplifier.

Calibration

Only transmission response calibration is possible

with this setup. The location of the booster

amplifier does not allow the analyzer to make

transmission measurements in the reverse direction,

making a full two-port calibration impossible.

Perform a response calibration by connecting the

output of the booster amplifier to the attenuator

or coupler on port 2. A basic response calibration

does not remove mismatch errors due to the DUT

— only the frequency response errors and any

mismatch associated with the booster amplifier,

the attenuator, and the analyzer. Since mismatch

is not corrected for during calibration, even

measurements made with a response calibration

are limited in their accuracy.

In this setup you can improve the source match by

adding an isolator between the booster amplifier

and the DUT, or by adding attenuation. Adding an

isolator between the output of the booster amplifier

and the input of the DUT will remove the effects of

any mismatch. If the booster amplifier has enough

gain, a 3-dB or 6-dB attenuator at its output can

be added to improve its output match. Be sure

to include the isolator or any attenuation in the

calibration if you include it in the measurement

setup. Also make sure that the isolator or attenuator

can handle the power level being tested. Instead of

using a high-power isolator, a circulator with a

high-power load on its third port can be used.

CH1 S

21 &M log MAG 2 dB/ REF 11 dB

CENTER 2.500 000 000 GHz SPAN 4.000 000 000 GHz

Cor

PRm

LINE TYPE

18 Dec 1997 16:16:33

1_: 13.704 dB

2.512 960 000 GHz

True response

with

mismatch ripple

Configuration 3

Configuration summary

• Medium complexity

• Boosted source

• orward transmission measurement only

• Response calibration only

Setup and features

Compared to Configuration 2, Configuration 3

improves measurement uncertainty and flexibility

by measuring the incident signal after the booster

amplifier rather than before. Measuring the inci-

dent signal after the booster amplifier ratios out

the effects of any source mismatch.

Boost the analyzer’s source power by connecting

the booster amplifier to the “R out” port on the

analyzer. An external coupler connected to the

output of the booster amplifier directs a portion of

the boosted source signal to the R channel of the

analyzer. This signal becomes the reference signal

used in ratioing (Configuration 3).

Configuration 3

Measuring the reference signal after the amplifier

ratios out any mismatch ripple between the booster

amplifier and the DUT and any drift or power

fluctuations of the booster amplifier. The reverse

isolation of the booster amplifier makes forward

reflection measurements and reverse transmission

measurements impossible.

In configurations where the user provides an

R channel signal, determining the proper level is

important. Too low a signal level can be affected by

noise and can cause phase-locking problems and

inaccurate measurement results. Too high a signal

level can be affected by receiver compression,

causing inaccurate results. The receiver can also

be damaged if the signal is high enough. To find

the proper signal level for the R channel (a typical

range might be 0 to –30 dBm) look at the analyzer’s

specifications. Then determine the signal level at

the coupled arm of the coupler, and add attenua-

tion to achieve the proper signal level at the

R channel.

DUT

Calibration points

Attenuator

R in

Booster

amplifier

Coupler

Network analyzer

Attenuator

Normally, the analyzer attempts to phase-lock

near the start frequency of the sweep. When the

analyzer cannot phase-lock at the start frequency

though, the analyzer performs a “pre-tune cali-

bration” routine. During pre-tune calibration the

analyzer attempts to phase-lock at a set frequency

(100 MHz for the Agilent 8753D family), which

could be below the start of the analyzer’s sweep.

The pre-tune calibration routine would be per-

formed, for example, if the analyzer is switched to

external reference mode without a signal attached

to the R channel. When the external R channel

signal is connected the analyzer is attempting to

phase-lock at the pretune frequency (100 MHz).

With a band-limited device in the R channel

path (e.g., a booster amplifier) there may not be

sufficient signal at this pretune frequency and

phase-locking errors can occur on some analyzers.

On the 8753E network analyzer, turning “PLL

Auto” off (located in the Service Modes menu) will

stop the analyzer from performing the “pre-tune

calibration” routine. Turning “PLL Auto” off rarely

affects the accuracy of instrument measurements.

Another way to prevent this phase-locking error

is to use frequency-offset mode in the 8753E. In

frequency-offset mode the analyzer assumes that

a band-limited device is being measured, and as a

result the analyzer never goes outside the sweep

range to phase-lock. Setting LO = 0 Hz when in

frequency-offset mode allows the analyzer to be

used normally.

Band-limited devices in the R channel path may

cause phase-locking problems in the Agilent 8720E

family of network analyzers unless the analyzer

has the high-power option (Option 085) or frequency-

offset option (Option 089). With either of these

options, phase-locking is not a problem. This is

because the internal reference switch switches to

the internal signal when phase-locking. After the

analyzer is phase-locked, the reference switch

returns to the external signal and the analyzer

is ready to make measurements.

Calibration

Calibrating this configuration is similar to the

calibration performed in Configuration 2. Only a

response calibration is possible since only forward

measurements can be made. Perform the response

calibration by connecting the output of the coupler’s

through arm to the attenuator on test-port 2 of

the analyzer.

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