PRACTICAL DESIGN CONSIDERATIONS TO INCREASE
MEASUREMENT ACCURACY IN A LOW COST VECTOR
NETWORK ANALYZER
WEI LI, JAN VANDEWEGE
Department of Information Technology (INTEC)
University of GENT
St.Pietersnieuwstraat 41, B-9000 Gent
BELGIUM
Abstract: Precision design work and tight manufacturing tolerances demand highly accurate measurements. A
Vector Network Analyzer (VNA) is one of the most important aids for linear RF analog designs. It can give
accurate information about the passive network performance in both the frequency domain and the time
domain. Designers heavily rely on these measurement results to locate the design problems and to evaluate the
network performance. But the measurement errors always exist. In any high frequency measurement, there are
measurement errors associated with the system that contribute uncertainty to the results. Therefore,
measurement accuracy of a VNA is an important concern to the instrument designers and suppliers. This paper
describes the main issues of determining measurement accuracy associated with VNA measurement. Based on
the low cost Vector Network Analyzer developed by INTEC, University of Gent, Belgium, the practical design
considerations have been discussed and employed in this instrument. The measurement results are present to
illustrate the improvement of the measurement accuracy after taking into account these measurement issues in
the design procedure.
Key-Words: Vector Network Analyzer, Measurement, Analog circuit design.
1 Introduction
1.1
Importance of measurement accuracy of
Vector Network Analyzer
Precision design work and tight manufacturing
tolerances demand highly accurate measurements. A
Vector Network Analyzer (VNA) is one of the most
important aids for linear RF analog designs. It can give
accurate information about the passive network
performance in both the frequency domain and the
time domain, such as insertion loss, return loss, phase
response, group delay, impulse response and so on.
Designers heavily rely on these measurement results to
locate the design problems and to evaluate the network
performance. But the measurement errors always exist.
In any high frequency measurement , there are
measurement errors associated with the system that
contribute uncertainty to the results. Therefore,
measurement accuracy of a Vector Network Analyzer
is an important concern to the instrument designers
and suppliers. As well as the VNA measurements
themselves, most errors in VNA measurements are
complex quantities that vary as a function of
frequency. By characterizing and virtually removing
the systematic errors, measurement accuracy can be
improved by several orders of magnitude. A plenty of
commercial network analyzers are now offered with
built-in, high-speed computational hardware that can
perform the complex mathematics required for
sophisticated error correction.
1.2
Main factors
measurement accuracy
of
determining
Commercial network analyzers, such as the series of
network analyzers produced by Hewlett-Packard,
Rohde & Schwarz, etc., are instruments that measure
transfer and/or impedance functions of linear networks
through sine-wave testing. A network analyzer system
accomplishes these measurements by configuring its
various components around the device-under-test
(DUT). The first part of a VNA instrument is the
VNA transmitter, a broadly tunable sine-wave signal
source to stimulate the DUT. Since transfer and
impedance functions are ratios of various voltages and
currents, a means of separating the appropriate signals
from the measurement ports of the DUT is required.
Finally, the network analyzer receiver must detect the
separated signals, form the desired signal ratios, and
display the results in the format requested by the user.
A perfect measurement system would have infinite
dynamic
range,
isolation,
and
directivity
characteristics, no impedance mismatches in any part
of the test set up, and a flat frequency response. It is
obviously impossible to make perfect hardware which
apparently would not need any form of error
correction. Even making the hardware good enough
not to require any error correction for most devices
would be extremely expensive. The best balance is to
make the hardware as good as practically and
economically possible, balancing performance and
cost. Error correction is then very useful to improve
measurement accuracy.
Generally, there are two basic types of test sets that
are used with network analyzers. It is illustrated in
Fig.1.
Source
Source
Transfer Switch
R
R
A
A
B
Port 1
Port 2
Fwd
DUT
Transmitter/Receiver-based analyzers (T/R test set),
however, it offers only one-path, two-port calibration.
Some errors still exist after this procedure.
2 The low cost Vector
Analyzer (VNA99)
Network
A low cost VNA(VNA99) has been developed by
INTEC-design team, University of Gent, Belgium. It
comprises a T/R test set with one directional coupler,
two measurement channels and one reference channel
for measuring the magnitude and phase responses of
the forward S-parameters S11 and S21 of a linear two
port. The analyzer ‘s built-in synthesized source
produces a swept RF signal in the range of 50kHz to
200MHz with a resolution of 100Hz. It can deliver the
output power from –25dBm to +6dBm with a flatness
of less than 1dB. VNA99 works in 50-ohm
environment. The system block diagram is illustrated
in Figure 2.
B
Port 1
Port 2
Fwd
DUT
Rev
Coupler
“Transmission/Reflection Test Set”
“S-Parameter Test Set”
Fig. 1 The two basic types of test sets that are used
with network analyzer
A real life VNA measurement is the vector sum of
the DUT response plus all error terms. The precise
effect of each error term depends upon its magnitude
and phase relationship to the actual test device
response. Network analysis measurement errors can be
separated into systematic, random, and drift errors. In
a well conceived high frequency measurement, the
systematic errors are the most significant source of
measurement uncertainty. Since each of the systematic
errors can be characterized, and the DUT is operated
in its linear range, their effects can be effectively
removed to obtain a corrected value for the DUT
response. These errors are qualified as directivity,
source match, load match, cross-talk, and frequency
tracking errors. Directivity and cross-talk errors are
related to signal leakage, while source and load match
errors are related to signal reflections. The final class
of errors is related to imperfect frequency response of
the receivers. Meanwhile random and drift errors can
not be precisely quantified, so they must be treated as
producing a cumulative uncertainty in the
measurement results.
In general, calibration can be applied to remove
the system errors before measurement. For the
RF
Transmitter
Transmitter
LO
DUT
DUT
source
load
Receiver
Receiver
Data
Data
Processing
Processing
&
&
Display
Display
Fig.2 The system diagram of the VNA99
3 Practical design considerations to
increase measurement accuracy of
VNA99
Practically, the system characteristics is affected by
magnitude at the device input, frequency response
accuracy, directivity, crosstalk, source match and load
match. A more detail discussions are presented in the
following sections about how to take into account the
practical design issues to increase the measurement
accuracy.
3.1 Directivity of the Directional coupler
An ideal directional coupler has low loss and high
reverse isolation. However, due to the difficulty of
making a truly broadband multi-decade coupler,
bridges are often used. The bridge configuration of
the VNA99 directional coupler is illustrated in Fig.3.
50 ohm
Source
50 ohm
Detector
50 ohm
Test Port
Fig. 3 The configuration of the directional bridge
Bridges are very useful for measuring reflection
because they can be made to work over a very wide
range of frequencies. The main trade-off is that they
exhibit substantial loss to the transmitted signal,
resulting in less power delivered to the DUT for a
given source power. A bridge ‘s equivalent directivity
is the ratio of the bridge detector signal level measured
at maximum balance (measuring the perfect Zo load)
and at minimum balance (measuring a short or open)
respectively. It is known that the reflection
measurements are limited by directivity only. The
following factors should be taken into account.
3.1.1 Coupled arm power loss
Directional couplers separate signals based on the
direction of signal propagation. If a directional
coupler is used to sample power in the measurement
configuration, the coupled arm power loss should be
taken into account, and compensation may be required
for the directional coupler response.
3.1.2 HF components
It should be noted that control of the behavior of the
passive bridge components at higher frequencies is
essential to make the directional bridge coupler truly
balanced. Parasitic components from the board
material as well as the 50-ohm resistor HF behavior
have impact on the coupler performance, as the
frequency for the network analyzer is sweeping from
50KHz to 200MHz. The passive component behavior
is practically more critical here in the bridge than in
other places of the VNA99. At least 3dB ~ 5dB
improvement of directivity could be obtained in this
Vector Network Analyzer by using HF resistors. Some
flexibility should be designed in, to compensate for
both the cable effect and the unbalance from the bridge
arms. Very careful HF layout is required.
3.1.3 Interaction of Directivity with the DUT
(without error correction)
Directivity error is the main reason why in some cases,
a ripple pattern occurs in the measurement of the
return loss. At the peak of the ripple, the directivity
error signal is adding in phase with the reflection from
the DUT. In some cases, the directivity error signal
may cancel the DUT’s reflection, resulting in a sharp
dip in the response. So it is essential to obtain good
directivity over the whole frequency range. The
solution to this issue is the combination of the design
considerations above and in the following section.
3.2 High dynamic range in the transmission
channel
Dynamic range is very important for measurement
accuracy. To get low measurement uncertainty, more
dynamic range is needed than the DUT exhibits. For
example, to get less than 0.1dB magnitude error and
less than 1 degree phase error, the noise floor should
be more than 35dB below the measured power levels.
System dynamic range in the transmission channel is
limited by the noise level relative to a “ through “. It is
calculated as the difference in dB between the
maximum receiver input level and the receiver noise
floor. The system dynamic range is applied to
transmission measurements only.
The dynamic range is affected by several factors,
such as the test port input power, the test port noise
floor and the receiver crosstalk. An efficient way to
remove the effect of random noise is to apply weighted
successive measurement traces, called “averaging”.
But its consequence is that the speed of measurement
will decrease when more averaging is applied, as more
measurement samples are needed for each frequency
point to be measured. Another way to reduce the noise
floor is to reduce system bandwidth, i.e., the
bandwidth of the heterodyne receiver intermediate
frequency (IF). In the VNA99, the RX IF bandwidth
is fixed to 3 kHz, as a compromise between
measurement speed and RX dynamic range, and
cannot be changed by the user.
In the transmission channel, the dynamic range of
the mixer has been measured to approach 90dB,
defining a 0.1dB compression point, and in the case of
a 30MHz working frequency. The A/D converter used
is a high linearity sigma-delta converter with a 18-bit
serial output. The effective number of bits (ENOB) of
such a A/D converter is 15bit, which means it can
provide maximum 92.06dB dynamic range. This ADC
can perform direct IF sampling as the RX is downconverting towards a low IF frequency in the upper
audio range. The analog input drive circuitry of the
ADC must amplify the IF output signal of the RX
mixer to make it swing the full ADC input scale, in
order to get the maximum dynamic performance.
It is noted that the input impedance of the
integrated RX mixer has a frequency-dependent
characteristic, i.e., the input impedance is not constant
in the whole 200MHz frequency range. The
characteristic of the amplitude of the input impedance
of the mixer is plotted in Fig.4. The unit of frequency
is MHz.
broadband RX matching network, which has to take
peak level into account in order to avoid RX input
overload, and the RX dynamic range, which is reduced
by an amount roughly equal to the peak to peak RF
signal variation. Moreover, a small change of the
reference signal, which is splitted by the injected RF
signal, can result in a considerable change of the
transmission and reflection signals, adding to the
overall measurement uncertainty. In the VNA99, the
RF signal variation is limited to less than 1 dB, to
guarantee a low frequency tracking error contribution.
Actually, an ideal directional coupler can never be
implemented. A variation of the directivity over the
200MHz frequency range will also cause frequency
tracking errors. In the design of the directional
coupler, a capacitor is used in order to compensate the
impact from the coaxial cable. This compensation
includes both the parasitic capacitance and the parallel
capacitance in the equivalent circuit of a cable.
4 Measurement results
Fig.4 The characteristics of load impedance of
matching network
In order to get the maximum dynamic range for
the receiver, while realizing a proper receiver input
match to 50 ohms, broadband RX input matching
networks have to be designed. To not exceed the
0.1dB compression point of the mixer with the loopedthrough TX output signal, a 6dB attenuator should be
added in front of the RX mixer connected to the
transmission channel, and a similar attenuator
lowering the reflection power level has to be added in
the RX mixer connected to the reflection channel.
Meanwhile we have to keep a high return loss at Port 1
and Port 2. Actually, the broadband matching
networks for both the transmission channel and
reflection channel have been designed to meet the
demands above.
The practical design considerations discussed above
have been employed in the design procedure. The
measurements of the Transmission Characteristics,
Reflection
Characteristics,
Time
Domain
Reflectometry (TDR), as well as the directivity of the
direct coupler have been done to evaluate the system
performance.
Figure 5, 6 show the transmission characteristics
of MCL-PBP-21.4 band-pass filter from MiniCircuits and the reflection characteristics of a Tlinear network respectively. The final measurement
errors of S21(dB) and S11 (volt) are calculated as
0.58dB and 0.001volt respectively. And the directivity
of VNA99 illustrated in Fig.7 is at least 62dB.
3.3 Reducing the frequency tracking error
When estimating the frequency tracking errors, the
following factors should be taken into account.
First aim is to reduce the variation of the injected
RF signal, and the reference signal from Transmitter,
with frequency. This will significantly increase the
measurement accuracy. RF signal variation will cause
variation of directivity, and consequently cause
directivity error. It will also penalize the design of the
Fig. 5 The transmission characteristics of MCL-PBP21.4 measured by VNA99
.
Fig. 6 S11 behavior of the T-linear network measured
by VNA99
Fig. 9 The TDR measurement of the cables with
different characteristics impedance
5 Conclusion
Fig.7 Directivity measurement of VNA99
The measurement configuration of TDR is
illustrated in Fig.8. The measured coaxial cable
consists of two sections, one is 5 meter 50 ohm
characteristic impedance cable and the other is 20
meter 75 ohm characteristic impedance cable. This
cable is terminated by a 50 ohm load.
The
measurement result illustrated in Fig.9 shows the
location of the mismatch of the different characteristic
impedance of the cable.
5 m 50 ohm cable
20 m 75 ohm cable
Port 1
50 ohm load
VNA
VNA
Port 2
Fig. 8 The TDR measurement configuration
The main issues of determining the measurement
accuracy of Vector Network Analyzer are discussed.
The practical design considerations to obtain the
accurate measurement of VNA99 are applied in the
design procedure, particularly in the optimization of
the whole system. To illustrate the performance
obtained after optimization, several measurement
results are shown of transmission and reflection
characteristics, and of TDR (Time Domain
Reflectometry) measurements. It can be seen that the
measurement errors of S21(dB) and S11 (volt) have
been reduced to 0.58dB and 0.001volt respectively.
Meanwhile the directivity of VNA99 can reach at least
62dB. It indicates that in the limited frequency domain
up to 200 MHz, the measurement accuracy of the low
budget VNA99 compares very well with results
obtained by expensive laboratory equipment.
References:
[1] D. Ballo , Network Analyzer Basis , Application
note from Hewlett-Packard, 1997.
[2] Vizmuller, Peter, RF Design Guide – System,
Circuit and Equations, ARTECH HOUSE,INC,
1995.
[3] Herbert L.Krauss, Charles W.Bostian, Solid State
Radio Engineering, New York, 1983.