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Improving Network Analyzer
Measurements of
Frequency-translating Devices
Application Note 1287-7
LO
RF
LO-RF LO+RF
RF IF
LO
IF IF
LO
RF
LO-RF LO+RF
RF IF
LO
IF IF
LO
RF
LO-RF LO+RF
RF IF
LO
IF IF
LO
RF
LO-RF LO+RF
RF IF
LO
IF IF
Table of Contents
Page
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Network Analyzer Mixer Measurement Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Scalar Network Analyzer Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Vector Network Analyzer in Frequency Offset Mode Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
The Upconversion/Downconversion Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Conversion Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Definition and Importance of Conversion Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Measurement Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Mismatch Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Considerations Unique to the Scalar Network Analyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Importance of Proper Filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Frequency Response Error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Considerations Unique to the Vector Network Analyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Importance of Proper Filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Sampling Architecture and Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
R-Channel Phase-Locking Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
LO Accuracy and Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Power-Meter Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Accuracy Comparison of the 8757D and a Vector Network Analyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Fixed IF Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Relative Phase Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Relative Phase and Magnitude Tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Group Delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Important Parameters when Specifying Group Delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Absolute Group Delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Upconversion/downconversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Modulation delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Time domain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Measuring delay linearity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Reflection Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Isolation Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Feedthrough Measurement of Converters and Tuners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Absolute Group Delay -- A More Accurate, Lower Ripple Technique . . . . . . . . . . . . . . . . . . . . 27
Measurement Configuration Using the Mixer Pair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Labeling Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Proper Filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
LO Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
System Calibration and Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Calibrating the Test System with the Calibration Mixer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Calibration Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Calibration Error Terms and Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Test Procedure for Calibrating the Test System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
First-Order Error Correction: Frequency Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Second-Order Error Correction: Frequency Response and Input Match . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Third-Order Error Correction: Frequency Response, Input and Output Match . . . . . . . . . . . . . . . . . . . . . . . . 33
Appendix A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Calibration mixer attitudes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Appendix B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Program for Fixed IF Measurements with One External LO Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Appendix C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Uncertainty in Mixer Group Delay Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Appendix D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Related Application and Product Notes/Other Suggested Reading/Third Party Companies . . . . . . . . . . . . . . . . . 39
2
Introduction Network analyzer mixer
measurement configurations
Frequency-translation devices (FTDs)
such as mixers, converters, and tuners Network analyzers used for testing
are critical components in most RF frequency-translation devices include
and microwave communication scalar network analyzers, vector
systems. As communication systems network analyzers with frequency
adopt more advanced types of offset capability, and vector network
modulation, FTD designs are increas- analyzers using an upconversion/down
ingly complex, tests are more conversion configuration. Each
stringent with tighter specifications, solution has its own advantages and
and the need to reduce costs is more disadvantages. This section provides a
important than ever. synopsis of the three configurations so
you can quickly evaluate which is the
The measurement trade-offs for best fit for your measurement needs.
frequency-translating devices vary Detailed information about each
widely among different industries. solution is discussed in later sections.
Measurement accuracy, speed, cost
and ease of setup are among the
considerations for determining the
best test equipment. This application
note explores current test equipment
solutions and techniques that can be
used to accurately characterize and
test frequency-translating devices.
Frequency-translating devices present
unique measurement challenges since
their input and output frequencies
differ. These require different
measurement techniques than those
used for a linear device such as a
filter. This note covers linear
frequecy-translation measurements,
such as magnitude, relative phase,
reflection and isolation. Corresponding
accuracy issues are also discussed.
To get the most from this note, you
should have a basic under-standing of
frequency translation terminology,
such as "RF port," "IF port" and "LO
port." Understanding of fundamental
RF and network analyzer terms such
as S-parameters, VSWR, group delay,
match, port, full two-port calibration,
and test set is also expected. For a
better understanding of such terms, a
list of reference material appear in the
Appendix section.
3
Scalar network analyzer
configuration
The most economical instrument for
FTD tests is a scalar network analyzer.
A scalar network analyzer uses diode 8711C
detectors that can detect a very wide RF network analyzer
band of frequencies. This capability
enables a scalar network analyzer
to detect signals when the receiver
frequency is different from the source
10 dB Lowpass filter
frequency. Magnitude-only measure- External LO source
ments such as conversion loss,
absolute output power, return loss and 10 dB
isolation can be made, as well as
nonlinear magnitude measurements
such as gain compression. Group delay
information is available in some scalar Figure 1. 8711C scalar network analyzer configuration
network analyzers using an AM-delay
technique, which employs amplitude
modulation.
AM-delay measurements are less
accurate than group delay measure-
ments obtained with a vector network
analyzer. AM delay typically has an 8757D
uncertainty of around 10 to 20 ns, Sweeper scalar network analyzer
whereas group delay with a vector
network analyzer has an uncertainty
as good as 150 ps. Advantages of the
scalar solution include low cost and Directional bridge
good magnitude accuracy. As shown in
Figure 1, fully integrated scalar Lowpass filter
External LO source
network analyzers such as the 8711C
or 8713C provide economical RF Precision detector
measurements up to 3 GHz, and
include AM delay capability. The
8757D scalar network analyzer, shown
in Figure 2, measures up to 110 GHz,
and provides very good absolute Figure 2. 8757D scalar network analyzer configuration
power measurements, particularly
when installed with an internal power
calibrator and used with precision
detectors. In certain cases, such as
measuring FTDs with an internal filter,
the 8757D with internal power
calibrator and precision detector can
typically make more accurate magni-
tude measurements than a vector
network analyzer.
4
Vector network analyzer in frequency The upconversion/ As shown in Figure 5, two mixers are
offset mode configuration downconversion technique used to upconvert and downconvert
the signals, ensuring the same
A more versatile solution for FTD test A vector network analyzer in normal frequencies at the network analyzer's
is a vector network analyzer. A vector operating mode can also be configured source and receiver ports. Second, this
network analyzer uses a tuned-receiv- for frequency-translation measure- configuration provides a potentially
er narrowband detector, which allows ments. This configuration has two more accurate method for measuring
measurements of both magnitude and main advantages. First, the instrument absolute group delay. You can simply
relative phase. The vector network can be used to measure a FTD's measure two mixers and halve the
analyzer's frequency offset mode off- magnitude and relative phase response response, accepting the resulting
sets the analyzer's receiver from it's without the need for frequency offset. uncertainty.
source by a given LO frequency, and
makes frequency-translation measure- FREQ OFFS
Vector Network Analyzer ON off
ments possible.
LO
MENU
There are two common vector net-
Lowpass DOWN
work analyzer configurations for FTD RF in filter CONVERTER
measurements. The simplest configu- UP
ration is shown in Figure 3, and is CONVERTER
10 dB 10 dB
practical for testing upconverters and RF > LO
downconverters. This configuration Start: 900 MHz Start: 100 MHz RF < LO
allows magnitude-only measurements Stop: 650 MHz Stop: 350 MHz
VIEW
with a limited dynamic range. For Fixed LO: 1 GHz MEASURE
LO power: 13 dBm
example, if you are interested in the RETURN
magnitude response of the FTD's
Figure 3. Vector network analyzer in frequency offset mode
passband, the 8753E vector network
analyzer has 35 dB of dynamic range
in the R channel and provides a quick CH1 CONV MEAS log MAG 10 dB/ REF 10 dB
and easy solution
To also measure the FTD's relative
phase and out-of-band response,
Figure 4 illustrates a high-dynamic Vector Network Analyzer
range configuration. An alter-native START 640.000 000 MHz STOP 660.000 000 MHz
high-dynamic range configuration can RF out
be achieved by splitting the analyzer's Lowpass filter Reference mixer
RF in
RF output power between the device
under test (DUT) and the reference
10 dB 10 dB
mixer (This configuration is similar to
the one shown in Figure 24). In both
configurations the vector network
analyzer has around 100 dB of dynam- External LO source
Power
ic range. A signal into the reference R splitter
channel is always necessary for proper
phase-locking of the vector network
Figure 4. Vector network analyzer, high dynamic range configuration
analyzer. In addition, the R channel
provides a reference for ratioed mea-
surements such as relative phase or
Vector network analyzer
magnitude and phase tracking. Vector
network analyzers such as the 8720D
series and the 8753E have frequency
offset capability to 40 GHz and 6 GHz,
respectively.
Bandpass Bandpass
DUT filter Mixer filter
RF IF IF RF
6 dB LO 6 dB 6 dB LO 6 dB 6 dB
Power
External LO source splitter
Figure 5. Upconversion/downconversion configuration
5
Conversion loss
Definition and importance of RF IF Conversion loss =
conversion loss mag(f IF )
20*log [ ]
mag(f RF)
Conversion loss, as shown in Figure 6, LO
measures how efficiently a mixer
converts energy from one frequency
to another. It is defined as the ratio of
Power level
the output power to the input power Conversion loss
at a given LO (local oscillator) power.
A specified LO power is necessary
because while the conversion loss of a
mixer is usually very flat within the
frequency span of its intended
operation, the average loss will vary Frequency
with the level of the LO, as the diode Figure 6. Conversion loss
impedance changes. As shown in
Figure 7, conversion loss is usually
measured versus frequency, either the
IF frequency (with a fixed LO) or the
Conv loss vs IF freq Conv loss vs RF freq
RF frequency (with a fixed IF). The (fixed LO freq) (fixed IF freq)
configuration for a fixed IF measure-
ment is different from those described
up to this point. (See the Fixed IF
RF IF RF IF
Measurement section.) Figure 8
illustrates the importance of a flat
conversion-loss response. The DUT is
LO LO
a standard television-channel convert-
er. The input signal consists of a visual
carrier, audio carrier and a color
subcarrier. Since the frequency
response of the converter has a notch
in the passband, the color subcarrier
Loss
is suppressed and the resulting output Loss
signal no longer carries a valid color-
information signal. 0 0
IF freq RF freq
Figure 7. Two types of conversion loss measurements
Converter response
Visual Audio
carrier Color carrier Color sub-carrier
sub-carrier attenuated
LO
Input Signal DUT Output signal
Figure 8. TV tuner conversion loss example
6
Measurement considerations Mismatch errors Once the DUT is connected, interac-
Mismatch errors result when there is tion between the DUT's ports and the
Conversion-loss measurements can be a connection between two ports that network analyzer's ports cause mis-
made with either a scalar network have different impedances. Commonly, match errors. As shown in Figure 9,
analyzer or a vector network analyzer, a device's behavior is characterized mismatch effects generate three first-
using the configurations shown in within a Z0 environment, typically order error signals. The first is
Figures 1 through 5. The measure- having an impedance of 50 or 75 ohms. interaction between the network
ment uncertainties are different for Although the test ports of a network analyzer's source port and the DUT's
each type of analyzer. For both types analyzer are designed to be perfect Z0 input port. The second is between the
of analyzers, the two main systematic impedances, they are not. The imper- network analyzer's receiver port and
errors are port mismatch and frequen- fect source and receiver ports of the the DUT's output port.
cy response. The scalar network ana- network analyzer create errors in the
lyzer approach requires additional calibration stage. Therefore, even The third is between the network
care to minimize errors due to the before a device under test (DUT) is analyzer's source port and receiver
analyzer's broadband detector. For connected, some errors have already port. For an FTD measurement, this
some vector network analyzers, an been created in the calibration stage third interaction is usually negligible
internal process, called sampling, and (see Figure 9). Once the DUT is because the conversion loss and
phase-lock requirements can also cre- connected, the total measurement isolation of the FTD will attenuate the
ate errors. Next we will examine each uncertainty is equal to the sum of the reflected signals. As frequency transla-
of these error terms and explore tech- calibration error plus the measurement tion precludes conventional two-port
niques to minimize their effects. error. error correction, attenuators can be
used to improve port match.
Calibration:
Source Calibration plane
Receiver
Measurement:
Source DUT source receiver
RF IF
Receiver
LO
( )
source DUT input
( )
receiver DUT output
( )
source receiver
Total Uncertainty = Calibration Error + Measurement Error
Figure 9. Mismatch effects
7
By adding a high-quality attenuator to
a port, the effective port match is
improved by up to approximately source E ff source match
twice the value of the attenuation.
A high-quality attenuator has around
32 dB of port match. The effective
match is a function of the quality of Source Attenuator
the attenuator as well as its attenua- ( )
attenuator
tion, as shown in Figure 10.
(source )( attenuation ) 2
As shown in Figure 11 and Figure 12, a
well-matched attenuator can
significantly improve the effective port
2
match. For example, a 10-dB attenua- E ff source match = (attenuator ) + (source )( attenuation )
tor, with a port match of 32 dB, can
transform an original port match of 10 Figure 10. Effective match as a function of attenuator's match.
dB into an effective match of 25 dB.
However, as the match of the attenua-
tor approaches the match of the
original source, the improvement
diminishes. As shown in Figure 12, the
larger the attenuation, the more nearly
the resulting match approaches that of 35
the attenuator. However, excessive 32 dB attenuator match
Effective match (dB)
attenuation is not desired since this 30
will decrease the dynamic range of the 26 dB attenuator match
25
measurement system. The port match 21 dB attenuator match
of an FTD can be poor, typically 20
around 14 dB. Therefore, it is 18 dB attenuator match
recommended that attenuators be 15
Region when attenuator
placed at the FTD's input and output 10 no longer results in improved match
ports. Scalar network analyzers use 0 5 10 15 20 25 30 35
different detection methods than Original match (dB)
vector network analyzers that should
be considered when testing FTDs.
Figure 11. Effective match as a function of attenuator's match (fixed 10 dB attenuator)
35
30 20 dB attenuation
Effective match (dB)
25
10 dB attenuation
20
15 6 dB attenuation
10 3 dB attenuation
5
Region when attenuator
0 no longer results in improved match
0 5 10 15 20 25 30 35
Original match (dB)
Figure 12. Effective match as a function of attenuation (attenuator match = 32 dB)
8
Considerations unique to the Frequency response error For analyzers that do not precisely
scalar network analyzer Without performing any sort of measure absolute power, corrections
calibration on a scalar or vector for the frequency response error are
Scalar network analyzers use broad- network analyzer, the frequency less accurate. The input and output of
band diode detectors. Although response of the test system cannot be the DUT are at different frequencies,
capable of both narrowband and separated from the FTD's response. but the normalization can only be
broadband detection, the 8711 series, One way to correct these errors is to performed over one frequency range.
which includes the 8712C and 8714C perform a frequency-response normal- The result is that part of the test
vector network analyzers, uses ization or calibration, using a through system is characterized over a differ-
broadband detection for FTD measure- connection in place of the DUT. ent frequency range than that which is
ments. Therefore, if you use an 8712 used during the actual measurement.
or 8714, use the same FTD test For scalar network analyzers such as
considerations as you would for a the 8757D, which very accurately There are two choices for the frequen-
scalar network analyzer. measures absolute power, the normal- cy range used for the normalization:
ization calibration can be performed either the DUT's input (RF source)
Importance of proper filtering in two steps. See Figure 21. First, the range, or the DUT's output (receiver)
A scalar network analyzer's broadband absolute RF power is measured and range. The normalization should be
diode detector will detect any signal stored in memory. Second, the DUT done to correct the portion of the test
that falls within its passband. Although is inserted and the absolute IF power system that contributes the largest
a broadband diode detector is an is measured. Conversion loss is uncertainty; for example, this would
economical way to measure FTDs, it displayed using the Data/Memory be the portion with the most loss or
also can allow certain detection errors. format. The conversion loss value is frequency roll-off. Systems and
The diode detector will detect the very accurate since the measurements components tend to have poorer
desired IF signal, as well as other of the two absolute power levels, RF performance at the higher frequen-
mixing products or spurious signals. and IF, are very accurate. Ratioing two cies, therefore the calibration should
To minimize the detection of undesired very accurate absolute power levels normally be performed at the higher
signals, a filter should be placed at the removes the frequency response error. frequencies. In general, high-quality,
detector port to pass the desired IF In some cases, a scalar 8757D with an low-loss cables and connectors should
signal but reject all other signals. internal power calibrator and precision be used to minimize frequency-
Figure 13 shows an example of the detector can make more accurate response errors.
incorrect measurements that might conversion loss measurements than a
result when improper IF filtering is vector network analyzer. In the For higher accuracy, combine a nor-
used in a scalar network analyzer Accuracy Comparison of the 8757D malization calibration with external
configuration. and a Vector Network Analyzer error-term correction. During the
section, error terms are used to normalization, only one section of the
In Figure 13, the conversion loss illustrate how a scalar analyzer with test configuration should be connect-
measurement without the IF filter internal power calibrator can be more ed, either the DUT's input range or
appears to be better than it really is. accurate than a vector network the DUT's output range. For highest
The lack of an IF filter generates analyzer. accuracy, the removed section can be
erroneous results. The broadband characterized separately. An external
diode detector cannot discriminate the computer is used to extract the
frequency of the received signal(s) -- removed section's S-parameters from
it measures the composite response. the network analyzer. This data is
If the source is set at 1 GHz, it is then used to modify the network ana-
"assumed" that this is the frequency lyzer's error terms to account for the
of the detected signal. Any signal that effects of the removed section.
falls within the passband of the diode
1:Conv Loss /M Log Mag 1.0 dB/ Ref 0.00 dB
detector will be detected. If the 2:Conv Loss /M Log Mag 1.0 dB/ Ref 0.00 dB
output of a DUT is composed of the dB
Swept Conversion Loss Ch1:Mkr1 1000.000 MHz
-1 -6.38 dB
desired IF signal plus the image Ch2:Mkr1 1000.000 MHz
-2 -4.84 dB
frequency, LO and RF feedthrough
-3
and other spurious signals, the diode No IF filter
-4
detector will detect the composite of 2
-5 2
all the signals within its passband.
1
This composite signal will be incor- -6
1
rectly displayed as a response that -7
IF filter
occurs at 1 GHz. -8
-9
Abs
Start 900.000 MHz Stop 1 000.000 MHz
Figure 13. Conversion loss response with and without an IF filter
9
Considerations unique to the The IF of the 8753E vector network Figure 14 illustrates an example of
vector network analyzer analyzer is 1 MHz. Errors might result this sampler effect where the desired
because the incoming signal is not IF output signal of the mixer is
Now that we have covered the filtered until after it is downconverted 110 MHz. In order to correctly detect
important measurement considera- to the IF. If there is only one signal at this signal, the 8753E will use a VTO
tions of the scalar network analyzer, the receiver, this signal will mix with of 54.5 MHz, where its second
let's continue with a discussion of the one LO harmonic and is properly harmonic (109 MHz) will properly
vector network analyzer. The impor- downconvert to 1 MHz. However, if downconvert 110 MHz to the desired
tant considerations include: the need there are multiple signals that are 1 MHz IF signal. In the illustration, we
for proper filtering, an accurate and 1 MHz away from any of the LO show two mixer products (6 LO-2RF
stable LO, and power meter calibra- harmonics, these signals will be down- and 9 LO-RF) that would also produce
tion for the most accurate measure- converted to 1 MHz, which creates IFs at 1 MHz. Notice that these two
ments. erroneous responses. spurs occur on either side of the LO
harmonics (18 VTO and 42 VTO,
Importance of proper filtering respectively), but as long as they are
A vector network analyzer has a 1 MHz away, they will be downcon-
narrowband tuned receiver. Since the verted to 1 MHz. Aside from the
received signal is heavily filtered by signals which downconvert to 1 MHz,
an internal narrowband IF filter, signals that will directly pass through
broadband detection issues encoun- the finite passband of the 1 MHz band-
tered by the scalar network analyzer pass filter can cause problems. In the
are not present. However, proper 8753E, IF BWs from 10 Hz up to 6 kHz
filtering is still very important for are available.
vector network analyzers with
sampler-based receivers, such as the
Given RF = 410 MHz IF = RF