One of the features added in version 15 of the 89600 VSA software is the ability to define
and run multiple measurements concurrently. This video will cover a multi-standard radio
scenario to show you one of the many uses for the 89600 VSA software’s multi-measurement
capability.
In the traditional approach, you have a single instrument setup which acquires data from
the signal of interest and an analysis tool which analyzes the data and visually presents
the results.
In this case, we have a 5MHz LTE FDD downlink signal that is located in a cell band and
we want to make sure that the signal is being properly modulated and transmitted.
Here are the demodulation results for a complete LTE frame. Notice that we have a problem.
Although the average EVM is fine at 4.2%, there appear to be periodic EVM spikes on
the order of 30-40% that we can see in the Error Vector Time trace shown below.
A possible cause is that simultaneous transmissions within the multi-standard radio are interacting,
causing the EVM problem.
So, why not capture both signals with a single analyzer and use the VSA software’s multi-measurement
Shared Acquisition feature?
Well…the radio is capable of LTE and WiFi transmissions which means there is likely
a large frequency separation between the two carriers.
Here we have an 89600 VSA advantage. The ability to use multiple analyzers to capture data
at different center frequencies and demodulate the results in separate measurements.
In this approach, we will use two analyzers to demodulate the following signals. The first
is the LTE signal described previously.
The second is a burst 802.11g OFDM signal, transmitting at 54 Mbps throughput. The center
frequency of this signal is set to 2.412 GHz.
To setup the hardware, the signal containing the LTE and WiFi carriers is split and fed
into two analyzers. In the VSA software, two measurements are created, one for LTE and
the other for the WiFi signal. Each of these measurements is linked to an analyzer and
the center frequencies are set to demodulate the carriers of interest.
An external trigger signal is used to provide loose time synchronization between the time
records captured by each analyzer.
So, as you can see, the VSA software is demodulating the LTE and 802.11g signals in freerun triggering
mode. On the right-hand side of the window are the constellation, spectrum, and Error
Summary traces which give you an overall indication of the quality of the signal.
On the left hand side of the window, the top trace shows LTE Error Vector time with the
EVM spikes, and below that are the 802.11g Raw Main Time and the LTE Search Time traces.
I’m going to turn on External Triggering by clicking Input > Triggering and setting
the trigger style to External. This sets the triggering for the selected measurement which
is shown in the toolbar and with a filled radio button in the Measurements dialog. I
also need to enable external triggering for the other measurement which I can do by locking
the Input dialog and selecting the other measurement. Now both analyzers are waiting for a trigger
signal.
Now I will go ahead and perform a single sweep by sending a single trigger signal.
There appears to be a relationship between the LTE and WiFi signals as shown by the LTE
Error Vector Time trace and 802.11g Raw Main Time trace. We could use delta markers to
measure the width and period of the WiFi bursts and compare to the EVM spikes in the LTE signal,
but there is a better way to correlate the two.
The high EVM spikes in the LTE signal appear to be caused by the Wifi bursts, but how can
we make sure?
If we set the x axis limits for the 802.11g Raw Main Time trace so that it shows the same
time interval as shown in the LTE Error Vector Time trace, we will be able to see whether
the WiFi bursts align with the high EVM spikes in the LTE signal.
Since both analyzers were triggered together, the 0 ms trigger time on both Raw Main Time
traces is aligned. LTE Search Time shows the time record starting from the trigger point
and its time axis is shifted so that 0 ms corresponds to the frame start.
The start time for Search Time is -t1 ms, which means that the frame starts t1 ms into
the Search Time trace. Since the beginning of the Search Time trace is aligned to the
0ms point in the Raw Main Time trace, we can cut off the part of Raw Main Time before the
LTE frame start by setting the Raw Main Time Left Reference to t1 ms. In this case, since
the LTE Measurement Interval is 10 ms, we can cut off the part of Raw Main Time after
the LTE frame by setting the right reference to 10ms + t1 ms = ___.
The LTE time parameters have been set so that the LTE Error Vector Time trace contains data
beginning at the start of the LTE frame.
Ok, back to the 89600 VSA software.
I’m going to change the X-axis scaling for the 802.11g Raw Main Time trace to match the
LTE Error Vector Time trace, by right-clicking the x-axis of the Raw Main Time trace, selecting
x scale, and clearing Full Scale to set the Left and Right references.
Notice that the Search Time trace start time is set to -____ ms. This means I need to set
the left boundary of the Raw Main Time trace to +_____ms and the right boundary to ___
+ 10 ms.
Now you can see the WiFi bursts align exactly with the LTE EVM spikes, providing a high
degree of confidence that there are interactions between the LTE and WiFi signal.
The 89600 VSA software’s ability to perform independent, concurrent acquisitions for multiple
measurements is one of the many ways the 89600 VSA software can help you troubleshoot your
RF system.
For more information about the 89600 VSA software, including how to purchase, see the product
home page at www.agilent.com/find/89600_VSA