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Comparison of Mixer
Characterization using
New Vector Characterization
Techniques
White Paper
Abstract
This paper presents a novel method for characterizing RF mixers, yielding
magnitude, phase, and group delay response of the conversion loss, as well
as the input match and output match. This technique works for mixers
which have reciprocal conversion loss and for which the image response
can be filtered out. This technique is compared with a new vector corrected
power technique, and with traditional power meter measurements. An
error analysis on the new technique is presented for the first time.
This paper was originally presented at the European Microwave Conference,
September 24, 2002 [1].
Introduction
Many wireless and RF systems require frequency converters or mixers
with specified and well-controlled amplitude and group delay response.
The amplitude response measurements of these devices have always had
large uncertainties. Further, there have been few techniques that can
characterize the phase or group delay response of mixers.
A previous method described making group delay measurements by making
three measurements on three pairs of mixers [2]. From these measurements,
one can calculate the amplitude and phase response by solving the three
linear equations for the overall response. This method makes use of
up/down conversion but requires an IF filter between the pairs of mixers
to avoid re-converting the unwanted side band. A second key aspect of
the technique is that it assumes at least one of the mixers is reciprocal in
its response, that is, it has the same conversion loss and group delay in the
up-conversion mode as in the down-conversion mode. For some mixers,
this holds approximately true.
A difficulty with the previous method is that it requires three sets of meas-
urements, with reconnections between the mixer pairs and the filter. With
each connection, there is room for random error (connector repeatability)
and systematic error, for example, mis-match effects between the filter and
mixer pairs, and between the mixers and test equipment. Mixers typically
have poor return loss, so these mis-matches can be quite severe. It has been
suggested that adding padding (attenuation) between the mixers and filters
can reduce this effect, but it introduces further sources of mismatch
between attenuators and the filter, and it reduces the levels of signal
measured creating a noisy measurement.
2
Overview of the new method
One of the novel methods presented here can characterize the amplitude,
phase, and delay response of a mixer, without the need to employ any other
mixers. The input and output matches of the mixer are also determined.
A second method can characterize the amplitude response of a mixer,
correcting for input and output match through proper calibration steps
of a vector network analyzer. These new methods are compared with
traditional measurements using power meters.
The first method, which was described for the first time at the 2002
Microwave Theory and Techniques Symposium [3], does not require the LO
signal of the test mixer to be accessible. The only requirement is a filter that
can separate the response of the desired IF output signal from the unwanted
side band. This method assumes a reciprocal mixer response, as do the
previous methods, and any errors in this assumption will be errors in the
final result. This method provides additional information on the input and
output match of the mixer, which can be used in a later calibration step to
remove instrumentation mismatch errors at the input and output of the test
equipment [4]. Because this does not require multiple mixer connections,
connector repeatability does not add into errors of the measurement.
A second method, using a power meter to calibrate a vector network analyzer,
is novel in that it corrects for all the mismatches in the system, including
the mis-match between the source and the power meter, the source and the
receiver, and the mixer-under-test and the source and receiver of the network
analyzer. The authors believe this provides the state-of-the-art in amplitude
measurements of frequency converting devices. Further, this method is
less sensitive to LO leakage and does not require filters to remove the
image signals.
Characterization method details
Reciprocal Cal Method
The first new method requires a vector network analyzer (VNA). The analyzer
is first calibrated for return loss measurement (S11) over the RF frequency
range. This provides a "perfect" network analyzer (with only small residual
errors). The mixer to be characterized is connected to this port, followed
by the IF filter, as shown in Figure 1. Measurements of a metrology grade
open/short/load are made (though any other one-port calibration method
will do) and these are saved. Note that the signals measured at the test
port are a composite of the reflection from the mixer input port, and a pair
of converted signals, one undesired which is reflected of the IF filter, and re-
converted to the input signal, and another which is the desired IF signal that
passes through the IF filter, and is reflected off the various standards used
for a one-port calibration.
The three measurements, GO, GS, GL, are sufficient to calculate the "one-port"
error model of the mixer/filter combination. That is, the values of GO, GS, GL
can be used as inputs to the formulations for calculating the error terms of a
one-port calibration.
3
From
Vector Network Analyzer
IF- G
O,S,L
O
S
RF IF+ L
LO IF- = RF - LO
Figure 1: Mixer characterization diagram. Three measurements are made: open, short, and load
response.
The one-port error model generates three terms: Directivity, Source Match
and Reflection Tracking. The Reflection Tracking term represents the two-
way transmission (magnitude and phase) through the mixer, T1*T2. If the
mixer is reciprocal in nature, taking the square root of this term gives the
one-way insertion loss of the mixer, that is, T1=T2. The one-port error model
already extracts the effects of the source match, and the calibrated VNA
with which the data was taken, and includes effects of the input match.
Thus, this term can be used as the actual two-way conversion loss of the
mixer. Since this data is taken with a calibrated network analyzer, the
effective mismatch, tracking, and directivity of the VNA are eliminated.
In practice, the one-port calibration function of the VNA can be used to
extract the one-port error model of the mixer by downloading the previously
measured GO, GS, GL responses into the VNA as the calibration standards of a
one-port cal. However, remember that the standard data was taken over the
RF range of the mixer, and while the standards are applied at the IF range,
so it is necessary to reset the analyzer frequency before the download to
ensure that the proper frequencies are used for the models of the calibration
standard. Further, for cases where the RF is less than the LO frequency, it is
necessary to reverse the data before downloading, so the response matches
the "backward" sweep of the IF. However, the terms of the mixer can be
found without using the VNA functions. Formulations for determining the
extraction of Directivity, Source Match, and Tracking can be found in many
references [5].
Figure 2: Conversion loss using the first new method, up and down conversion.
Figure 2 shows the result of performing this characterization on an RF
mixer. A filter-mixer-filter combination was created, and characterized two
times, once as an up-converter, and once as a down-converter. This mixer
can also be used as a calibration mixer in a test system as described by
Dunsmore in [3].
4
Power Meter Calibrated VNA
An alternate method for characterizing the amplitude response of mixers
consists of using a power meter to characterize the vector network analyzer,
then using the vector network analyzer to characterize the mixer. Using a
power meter is not new [6], but previous methods have been sensitive to
mismatch, and to power level fluctuations. The new method uses a one-port
error correction to characterize the source match of the VNA, and to measure
the return loss of the power meter. The source power is measured by the
power meter and corrected for source match interactions. This calibration is
transferred to the reference channel of the VNA to allow continuous meas-
urement of the source output and correct for source level fluctuations.
Next, a one-port calibration is performed at the output frequency of the
mixer under test, and port 1 of the VNA is connected to port 2. The load
match of the VNA port 2 is measured, as well as the receiver response. This
response is also referenced to the reference channel. An additional step of
performing a one-port calibration on port 2 can be done if it is desired to
correct for mixer output match. Finally, a mixer is connected between ports
1 and 2. Figure 3 shows the measurements using a vector corrected power-
meter calibrated VNA. In this measurement method, which can only measure
the amplitude response, it is apparent that there is some non-reciprocal
nature over a portion of the mixer's frequency response. However, the
average of the up and down conversion is nearly identical to the response
in Figure 2, thus validating that the first method does indeed characterize
the "round-trip" conversion loss of the mixer.
Average of up and down conversion Up conversion
Down conversion
Figure 3: Conversion loss using a power corrected VNA with match correction.
5
Evaluating the test method errors
The test method errors can be evaluated to understand their interactions.
The errors come from a variety of sources, including power meter linearity,
image signals, calibration kit residuals, and dynamic accuracy limitations in
the VNA. Analysis of the error of the first method gives the error in the
"round-trip" conversion loss as
r1