Version 15 of the 89600 VSA Software brings the ability to run multiple measurements concurrently
and view their results together.
Each measurement is performed on data acquired from measurement hardware.
The VSA provides three basic acquisition styles: shared acquisition; independent concurrent
acquisition; and independent non-concurrent acquisition.
This video will cover these acquisition styles in more detail, as well as give an in-depth
example for the Shared Acquisition style.
The Shared Acquisition style is where multiple measurements analyze the same time record
acquired from a single instrument. In this example, the radio is transmitting three separate
carriers, which are located relatively close in frequency.
This signal is fed into an instrument and all carriers of interest are captured in the
instrument’s analysis bandwidth. The time record is then sent to the 89600 VSA software
where it is made available to four measurements. First is the overall measurement, which looks
at the combination of the three carriers. Then there are the GSM, W-CDMA, and LTE measurements
that shift and resample the time record to place the carrier of interest at the center.
One advantage of this acquisition style is that measurements are exactly time synchronized
since they all share the same time record.
Lets dig a little deeper into shared acquisition with an example. As described in the previous
page, we’ll have four measurements: an overall vector measurement, a GSM measurement at 1850.8
MHz, a W-CDMA measurement at 1855 MHz, and an LTE measurement at 1862.4 MHz.
The combined bandwidth of these three signals is about 15 MHz, which fits into the 25 MHz
analysis bandwidth of the MXA I am using.
Ok, let’s look at this signal using the VSA software. In the measurements dialog,
you can see the four measurements listed, as well as a visual representation of their
relative center frequencies and spans….
I’m going to close the measurements dialog so we can see behind it.
The lower three traces show a spectrum and constellation trace for each signal format
that is present, giving a visual indication of quality. On the top-right are the Error
summary traces with the EVM metric visible, giving a numerical indication of signal quality.
The trace in the upper left shows the overall spectrum of the GSM, W-CDMA, and LTE carriers.
Overlaid on the spectrum is a CCDF plot, which shows the peak-to-average statistics for all
three signals combined, which can be helpful in designing a multi-carrier power amplifier.
Each of the carriers is set to the same power relative to the others. Notice the average
EVM for LTE is about 1% and the EVM for WCDMA is about 4%. Now I will increase all carriers
by 3 dBm and rerun the measurements. Notice that the LTE signal EVM has increased by 1
%points but the WCDMA signal has not increased significantly. I will increase the power again
by 3 dBm and run the measurements. Notice that the LTE EVM is now 4 points different
than the initial value of 1% and WCDMA is only different by about 2.5 points.
Since each carrier’s EVM degrades at different rates, we can deduce that the relative carrier
powers are probably not set optimally for this multi-carrier power amplifier.
The 89600 VSA software’s multi-measurement capability allows you to perform a combined
analysis of multiple signals and in this scenario, enables you to find the optimal operating
point more easily and quickly because you can perform all these measurements simultaneously.
The Independent Concurrent Acquisition style is where multiple measurements acquire their
data concurrently from different instruments.
Here we have a radio that transmits a combination of GSM and Bluetooth. However, the frequency
separation between the two carriers can be greater than the analysis bandwidth of a single
instrument.
So, we can split the signal and use two instruments to capture time data at separate center frequencies.
The time data would then be sent to the VSA software and demodulated by separate measurements.
Acquisition styles can also be combined. For example, if the radio also transmitted a W-CDMA
signal that was close in frequency to the GSM signal, the time capture from the first
analyzer could be shared between a GSM and a W-CDMA measurement.
Note that if you want stricter time alignment between time records, you could configure
the measurements to use external triggering and then send a trigger signal to all instruments
after they are waiting for trigger.
For a more in-depth explanation of this scenario, see the video named “Troubleshooting Interactions
in a Multi-standard Radio”.
Independent concurrent acquisition can also be used for any case where there are signals
at physically separate test points. For example, if you have two separate antennas, or if you
want to compare results between different points in the RF chain, such as baseband vs
RF or other combinations. This way allows you to pinpoint the source of the problem
by measuring EVM at each point to see where it increases significantly or by measuring
CCDF to find the source of clipping.
Lastly we have the Independent, Non-concurrent Acquisition style. This style is different
from the previous two styles in that the measurements do not run concurrently, but are sequenced
instead. The sequencing can be done through the 89600 VSA software’s SCPI interface
or the .NET API, or you could do it manually by selecting a measurement to run.
One scenario where this acquisition style would be useful is when you have two signals
that are separated in frequency by more than the analysis bandwidth of the instrument,
but you only want to use a single instrument.
Another scenario is where you have two measurements: one measurement which requires many measurement
sweeps with a short time record and the other measurement which requires only a few measurement
sweeps, but a long time record. Running these two measurements concurrently would merge
their requirements causing many long time captures to be taken, increasing the overall
test time. Instead, you could write a macro to run the measurements in sequence. Since
both measurements can reside in memory simultaneously, you won’t incur the time of setting up a
measurement when switching.
In summary, we went over the three basic acquisition styles.
Shared Acquisition where you have multiple measurements that share the time record from
a single instrument. Independent concurrent acquisition where you
have one instrument per measurement. And Independent non-concurrent acquisition
where you sequence the measurements’ access to a single instrument.
For more information about the VSA software, including how to get a trial license, see
the product home page at www.agilent.com/find/89600_VSA