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PALO ALTO, Calif., Sept. 24, 2002
As a fundamental component of every superheterodyne
receiver, the mixer plays an important role in the overall performance
of the system. A mixer's behavior has wide-ranging effects and
must be well characterized. However, unlike measurement of other
RF and microwave components, mixer characterization presents
significant challenges, principally because the devices are
inherently nonlinear. Both desired and undesired frequency conversion
takes place within a mixer, and the device must be excited by
two sources operating at different frequencies, with analysis
performed at a third port.
In addition, most systems require mixers with
well-controlled amplitude response, phase and group delay, including
systems for voice or data communications, radar, and various
defense applications such as electronic warfare and electronic
countermeasures. In the past, instruments designed to measure
these characteristics accommodated conversion loss reasonably
well. However, characterizing conversion phase and group delay
continues to require cumbersome connection and reconnection
and multiple external components, with an ever-present chance
of operator error in both the measurement process and the interpretation
of results.
When Agilent developed the PNA Series of network
analyzers, it addressed both conversion phase and group delay
measurement challenges, and produced a far more accurate and
less complicated technique that requires fewer external components.
This can be shown by comparing the new Agilent technique with
two other methods.
In the first method, engineers make three
measurements on three pairs of mixers, then the amplitude and
phase responses are calculated by solving the three linear equations
for the overall response. The technique uses upconversion and
downconversion and employs an IF (intermediate frequency) filter
between the pairs of mixers to avoid reconverting the unwanted
sideband. This method assumes that at least one of the mixers
is reciprocal (has the same conversion loss and group delay
in upconversion as downconversion). The most obvious drawback
is that engineers must perform three sets of measurements, and
reconnect the mixer pairs with the filter. In addition, both
connector repeatability and the mismatch effects between the
filter and mixer pairs and between the mixers and test equipment
produce errors.
Another method for characterizing mixer group
delay measures mixer return loss along with an air line that
is terminated in a short, and takes the time domain transform
of the response. The time delay to the response of the short
is subtracted from the length of the airline, which yields the
two-way delay of the mixer. This technique has proven useful
only for broadband mixers, and delay resolution is limited by
the time domain resolution. In addition, the delay response
is a combination of the response from both the sum image and
image, and the measurement requires a reciprocal mixer. On the
positive side, the technique does not require additional mixers
or locking the local oscillator (LO) to any of the signals.
In contrast to these two methods, the PNA
Series' vector-corrected mixer calibration technique uses reflection
measurements to fully characterize a reciprocal calibration
mixer without any additional mixers. This calibration mixer
can be used to calibrate a test system, which can then measure
the conversion loss, conversion phase, and absolute group delay
of any mixer under test. The method can characterize non-reciprocal
mixers and is highly automated, with all of the external equipment
controlled by instrument firmware, such as signal sources for
the LOs of the device under test and power meters for system
calibration.
The instrument's interface to the measurement
environment presents a clear picture without requiring the user
to enter obscure and confusing values. All values are set up
on a single screen. Figure 1a shows the dialog box for single
conversion devices, and Figure 1b the dialog box for dual-conversion
devices. The firmware ensures that all the values entered are
within acceptable ranges and provides help when requested.
The vector mixer characterization technique
is a two-step process. A mixer with reciprocal properties is
characterized first, and this mixer becomes a through standard
with which to calibrate the test system. The system can then
be used to characterize nearly any reciprocal or non-reciprocal
mixer without the need for reconnection of the calibration mixer.
The technique also provides information about the input and
output match of the calibration mixer, which can be used to
remove instrument mismatch errors at the input and output of
the test equipment. Since there is no need for multiple mixer
connections during the mixer-calibration process, connector
repeatability is eliminated as a source of measurement error.
The use of a mixer as a device for calibrating
a vector mixer measurement system is not new. However, calibrating
the phase response of the system has always been extremely difficult.
In addition, measurement systems that use this technique are
not well matched and no prior method has been proposed to correct
for the calibration mixer's input and output effects, which
have been impossible to determine. Agilent's vector calibration
technique is currently one of the only commercially available
methods that corrects for both input and output mismatch effects
for the calibration mixer and the mixer under test.
A classic network analyzer procedure can be
used to demonstrate the viability of this technique. First,
a mixer can be measured by itself, and then with an air line
(which is a low-loss, well-matched delay line). In an ideal
measurement, the test system should show the conversion loss
of the mixer reduced by exactly the loss of the air line, and
mismatch effects will cause ripple on the measurements. Figure
4 shows the results of the measurement just described, performed
with scalar calibration and vector calibration. The ripple in
the scalar measurement is nearly an order of magnitude greater
than that of the vector-calibrated measurement.
Figures and photos of the product are available
at www.get.agilent.com/press/index.cgi?PSP_NEXT=ShowPR&Release:release_id=59.
# # #
Contact:
Janet Smith, Agilent
+1 970 679 5397
janet_smith@agilent.com
Sara Gaugl
Weber Shandwick, for Agilent
+1 425 452 5430
sgaugl@webershandwick.com
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