Hello. My name is Mark Lombardi. I'm an engineer here at Keysight Technologies.
This video focuses on component testing and how making non-linear distortion measurements
on amplifiers can impact measurement analysis bandwidth.
Keysight Technologies provides a wide range of test solutions for aerospace engineers
deploying satellite systems. As aerospace engineers strive to update satellite systems
to address the ever-growing needs of data, there is a greater focus on high data rate,
modulation schemes
These typically require bandwidth that can expose amplifier effects that may not impact
narrow band signals. For example, non-linear behaviors over a satellite channel might have
little impact on a five megabit signal, but render the signal unrecoverable at 500 megabits per second.
This can be especially true in the case of space base platforms when engineers maybe
focus to trade off amplifier linearity for efficiency. Issue for designers or integrators
is to maintain the correct test procedures to be able to address the new system requirements.
Many of these new wideband systems employ a digital pre-distortion technique in order
to optimize the system throughput. For engineers evaluating amplifiers that will be utilized
in these schemes, wideband distortion measurements on perspective amplifiers can be very informative.
If the amplifiers used in the design had been selected before the full details of the modulation
and signal processing algorithms are finalized, it can be helpful to have a wide range of
measurements that help engineers predict the outcome of the design decisions.
The better the measurements represent the future operational conditions the closer the
system will match expectations. When the distortion properties of an amplifier are measured and
understood under the various operational conditions, including supply voltage and temperature,
more accurate predictions can be made as to the final system performance.
Let's now look at a few of these amplifier measurements using the PXA with the 510 MHz
wideband analysis option and the VSA software. To measure the distortion, we measure the
input of stimulus and the output of the response of the power amplifier. We then compare the
two in order to determine the amount of distortion that has been contributed to the signal by
the power amplifier.
One way to accomplish this on a single channel, vector signal analyzer is by storing the stimulus
wave form in the VSA software trace memory and then running the distortion measurements
relative to the output response of the power amplifier.
This approach allows for an uncorrected test stimulus to be used with very good results.
This also makes it possible to make the distortion measurements with signals that more closely
represent the modulation type ultimately used in the satellite payload.
In order to make these measurements, it's important to not only have enough analysis
bandwidth to capture the signal interest, but also consider capturing the multiple orders
of spectral regrowth, especially if that information will be utilized in the systems signal processing.
One example would be digital pre-distortion. If the modulated signal is 144 MHz and the
third order non-linear impacts will be used in the signal conditioning, the PXA would
provide 510 MHz of analysis bandwidth. That's enough bandwidth to capture the 144 MHz wide
modulated signal along with the third order spectral regrowth components. Typical distortion
measurements include AM/AM, AM/PM, gain compression, and differential error vector magnitude.
Many times, these measurements would have been done utilizing two tones as the stimulus
and increasing the power to sweep out a curve. But once more of the design decisions have
been made, a complex modulated signal similar to what the transponder would use in operation
can be also used in a test stimulus. This lets us test with realistic signal dynamics
like peak to average power ratio and channel loading of the composite signal.
AM/AM shows the stimulus magnitude versus response magnitude, or input versus output.
The X value of the points are the magnitude of the stimulus and the Y values are the magnitude
in DBM of the response voltage. AM/PM shows phase difference versus the stimulus magnitude.
The X value is the magnitude of the stimulus voltage and the Y value represents the phase
difference between the stimulus and the response shown in degrees.
Positive phase difference for a point indicates that the response phase is greater than the
stimulus phase for the same corresponding time point. Having an understanding of an
amplifiers AM/AM and AM/PM characteristics are important, because directly translates
into errors in the data recovered from a modulated signal.
Having measurements that cover the non-linear behaviors in an amplifier can help engineers
during the design phase of a communication system either by matching the modulation to
the amplifier behavior or selecting an amplifier whose non-linear behavior best matches the
modulation, a system design can be optimized.
Please contact your local Keysight representative to learn more about distortion measurements
utilizing the PXA with the wide band option, as well as a full line of test solutions for
satellite new space. Thank you.