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Hints
for making
Better
Network
Analyzer
Measurements
Application Note 1291-1
This brochure contains a variety of Overview of network Scalar network analyzers
hints to help you understand and analyzers A scalar network analyzer (SNA)
improve your use of network measures only the amplitude
analyzers, and a quick summary Network analyzers characterize portion of the S-parameters,
of network analyzers and their the impedance or S-parameters of resulting in measurements such as
capabilities. active and passive networks, such transmission gain and loss, return
as amplifiers, mixers, duplexers, loss, and SWR. Once a passive or
filters, couplers, attenuators. These active component has been designed
components are used in systems as
Contents using the total measurement
common and low cost as a pager, capability of a VNA, an SNA may be
HINT 1. Measuring high-power or in systems as complex and a more cost-effective measurement
amplifiers expensive as a communications tool for the production line to reveal
HINT 2. Compensating for time or radar system. Components can out-of-specification components.
delay in cable have one port (input or output) or While SNAs require an external
measurements many ports. The ability to measure or internal sweeping signal source
HINT 3. Improving reflection the input characteristics of each and a signal separation test set,
measurements port, as well as the transfer char- they only need simple amplitude-
HINT 4. Using frequency-offset acteristics from one port to another, only detectors, rather than complex
for mixer, converter gives designers the knowledge to (and more expensive) phase-
and tuner configure a component as part of a coherent detectors.
measurements larger system.
HINT 5. Noninsertible device
Network/spectrum analyzers
measurements Types of network A network/spectrum analyzer
HINT 6. Aliasing in phase or analyzers eliminates the circuit duplication in
delay format
Vector network analyzers a benchtest setup of a network and
HINT 7. Quick VNA calibration
(VNAs) are the most powerful kind spectrum analyzer. Frequency
verification
of network analyzer and can coverage ranges from 10 Hz to
HINT 8. Make your
measure from as low as 5 Hz to up 1.8 GHz. These combination
measurements real-
to 110 GHz. Designers, and final instruments can be an economical
time, accurate and
test in manufacturing, use VNAs alternative in design and test of
automated
because they measure and display active components like amplifiers
the complete amplitude and phase and mixers where analysis of signal
characteristics of a network. These performance is also needed.
characteristics include S-parameters,
transfer functions, magnitude and
phase, standing wave ratios (SWR),
insertion loss or gain, attenuation,
Incident Transmitted group delay, return loss, and
DUT reflection coefficient.
SOURCE Reflected
VNA hardware consists of a sweep-
SIGNAL
SEPARATION
ing signal source (usually internal),
INCIDENT REFLECTED TRANSMITTED
a test set to separate forward and
(R) (A) (B)
reverse test signals, and a multi-
channel, phase-coherent, highly
RECEIVER / DETECTOR
sensitive receiver. In the RF and
microwave bands, typical measured
PROCESSOR / DISPLAY
parameters are referred to as
S-parameters, and are also com-
monly used in computer-aided
Network Analyzer Block Diagram
design models.
2
How to Boost and Attenuate Signal HINT
Levels when Measuring High-power
1
Amplifiers
Testing high-power amplifiers can The frequency-response effects of
sometimes be challenging since the the attenuators and couplers can
signal levels needed for test may be be removed or minimized by using
beyond the stimulus/response range the appropriate type of error-
of the network analyzer. High- correction. One concern when cali-
power amplifiers often require high brating with extra attenuation is
input levels to characterize them that the input levels to the receiver
under conditions similar to actual may be low during the calibration
operation. Often these realistic cycle. The power levels must be
operating conditions also mean significantly above the noise floor
the output power of the amplifier of the receiver for accurate mea- HP 8753D
exceeds the compression or burn- surements. For this reason, network
analyzers that have a narrowband,
ACTIVE CHANNEL ENTRY
out level of the analyzer's receiver. RESPONSE
tuned-receiver are typically used STIMULUS INSTRUMENT STATE R CHANNEL
Ref In
When you need an input level for high-power applications since R
PROBE POWER
FUSED
L T
HP-IB STATUS
S
their noise floor is typically 90
H 8753D 30 KHz-3GHz
NETWORK ANALYZER
higher than the network analyzer's PORT 1 PORT 2
source can provide, a preamplifier dBm, and they exhibit excellent
can be used to boost the power level receiver linearity over a wide range
prior to the amplifier under test of power levels.
Coupler
(AUT). By using a coupler on the AUT
output of the preamplifier, a portion Some network analyzers with full Preamp High-power
of the boosted input signal can be two-port S-parameter capability load
used for the analyzer's reference enable measuring of the reverse
channel. This configuration removes characteristics of the AUT to allow
the preamplifier's frequency re- full two-port error correction. If
sponse and drift errors (by ratioing), attenuation is added to the output
which yields an accurate measure- port of the analyzer, it is best to
ment of the AUT alone. use a higher power in the reverse
direction to reduce noise effects in
When the output power of the AUT the measurement of S22 and S12.
exceeds the input compression level Many VNAs allow uncoupling of
of the analyzer's receiver, some type the test-port power to accommodate
of attenuation is needed to reduce different levels in the forward and
the output level. This can be reverse directions.
accomplished by using couplers,
attenuators, or a combination of
both. Care must be taken to choose
components that can absorb the
high power from the AUT without
sustaining damage. Most loads
designed for small-signal use can
only handle up to about one watt of
power. Beyond that, special loads
that can dissipate more power must
be used.
3
HINT Compensate for Time
Delay for Better Cable
2
Measurements
A network analyzer sweeps its Figure 1 shows this effect when
source frequency and tuned receiver measuring the transmission
at the same time to make stimulus- response of a twelve-foot long cable
response measurements. Since the on a HP 8714C network analyzer.
frequency of a signal coming from a The upper trace shows the true
device under test (DUT) may not be response of the cable, using a
exactly the same as the network 1-second sweep time. The lower
analyzer frequency at a given trace uses the default sweep time
instant of time, this can sometimes of 129 msec, and the data is in
lead to confusing measurement error by about