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Keysight Technologies
Using RF Recording Techniques
to Resolve Interference Problems

Application Note
Introduction

System engineers characterizing interference in either commercial wireless or Electronic Warfare
(EW) applications today face a difficult task. Interference, defined as anything that is not the signal
expected, is a highly pervasive problem; one that can be quite difficult to address since its measure-
ment can be unpredictable. Despite the challenge, the task of finding, identifying and analyzing inter-
fering signals, intentional or not, in a crowded spectrum has become increasingly important in
a wide array of applications.

This application note introduces a method for using gapless recording to resolve RF interference
problems in complex RF environments. The method uses the Keysight Technologies dual-channel
M9392A PXI Vector Signal Analyzer with either a regular PC hard disk drive or external mass stor-
age. When recording at wide bandwidths for long durations a RAID storage system is required. Data
interface cards and modules are also used, as are Keysight's M9392A with the 89600 Vector Signal
Analyzer (VSA) software. With gapless recording, engineers can now measure data continuously over
long durations and ensure the capture of all RF events of interest when they occur.
Measurement Generally speaking, there are two primary goals of RF interference testing: to
ensure interoperability and compatibility. Interoperability testing focuses on de-
Challenges sign compliance to a published standard, as well as margin testing, which helps
engineers understand how well a system meets design criteria in the presence
of real-world signal levels and interference. Compatibility testing, on the other
hand, focuses on the "unintended interactions" between a system-under-test
and other RF systems. It's important for engineers to understand whether radi-
os from different vendors can interoperate with one another, as well as if all the
systems in an RF environment can play together nicely. Ascertaining a system's
susceptibility to impact from and on other RF assets may also be critical.

Regardless of the RF interference testing in question, a number of critical
measurement challenges exist. First and foremost, measuring RF target signals
like intentional or unintentional interference in complex RF environments can be
unpredictable. Additionally, intermittent failure modes make data capture par-
ticularly challenging. Consequently, when the root cause of a problem is not yet
known, it can be difficult for engineers to setup a measurement that captures
the failure.

How can engineers capture a target RF signal and/or the cause of interference
when they don't know what the signal or culprit interferer is, when or where it
will occur, or how long it will last? Unfortunately, using a typical signal analyzer
performing continuous long-duration recording offers little help.

Signalbetter understand why this approach falls short consider the high-level block
To Analyzer Block Diagram
diagram of a typical signal analyzer shown in Figure 1. The main limitation to
long-duration recording is that test equipment typically has limited on-board
memory. Signals-of-interest enter the analyzer's RF input and are processed by
the subsequent stages, resulting in the displayed waveform shown on the right.
Up until the blue vertical line, between the DSP and RAM blocks, all of the
signals-of-interest within the instrument's capture bandwidth are processed
in real-time, assuming a fixed local oscillator. However, once the samples fill
the memory buffer or RAM, the instrument no longer looks at incoming digital
samples. Instead, it must process previously recorded samples.

The signal analyzer does not capture any samples while it post-processes the
previously captured data, effectively creating a gap in its data acquisition.
Consequently, if events occur while the previous event is being processed or if
the new event lasts longer than the available memory, it falls into this gap and
may be missed. Moreover, the analyzer's trigger setup only captures signals for
one set of limited conditions. Once the analyzer fails to capture the event, it is
gone forever.

Display
Preselect Down - Digitize Store Process
Convert
ADC DSP RAM P
RF Acquire Read &
Input Data Store

Move Data from Digitizer to P
Figure 1. Shown here is a typical signal analyzer block diagram.

3
Introducing Gapless While resolving RF interference problems in complex RF environments can be
a tricky task, gapless recording offers a viable solution to the measurement
Recording challenges presented by the typical signal analyzer. The technique solves the
problem of not knowing when or where an interference event will occur, or how
long it will last, by enabling continuous acquisition of data over long durations.
Because there is no gap in the data recorded, the signal-of-interest, such as an
intermittent RF event, is easily captured.

For comparison purposes, consider the data acquisition from a typical signal
analyzer with limited on-board memory, as shown in Figure 2. Note the gaps in
data that occur once its memory is filled up.

Acquisition Read Acquisition Read Acquisition Read

GAP GAP GAP

FAILURE
MISSED

Figure 2. With a typical analyzer, once its memory is filled up, data is "read" from the digitizer to
the microprocessor for processing and display. During this "read," any new samples available at the
digitizer cannot be processed and are missed, creating a gap in the continuous acquisition of data
and resulting in failures being missed.

Now, consider an example of a signal analyzer modified for gapless record-
ing (Figure 3). It is the same signal analyzer shown in Figure 1; however, it now
includes a high-speed data link or bus that allows the engineer to move data
from memory as it is acquired. By bypassing processing and display updates,
and writing the acquired data directly to final storage using a circular RAM buf-
Signal Analyzer Blockit's possible to create high-bandwidth recordings without gaps in the data.
fer, Diagram Modified for Gapless Recording
With a circular RAM buffer, the engineer can simultaneously write to and read
from it. When recording at wide bandwidths for long durations, a RAID storage
system is required.

Preselect down-convert Digitize Store process

RAM
ADC DSP RAM Storage
buffer
RF Acquire
Input Digitizer PC RAID
data

Figure 3. A signal analyzer modified for gapless recording.

4
Introducing Gapless An image of a gapless acquisition taken using the modified analyzer is shown in
Figure 4. Note that unlike the gap-filled acquisition in Figure 2, this acquisition
Recording (continued) is continuous. Recording the acquisition of data does not stop during the "read"
because it happens in parallel with the acquisition. Without gaps in the data
record, the signal-of-interest--in this case a failure--is easily captured.

Continuous acquisition 500MB/s

Read Read Read
>500 MB/s >500 MB/s >500 MB/s
GAPLESS

FAILURE
CAPTURED

Figure 4. In this gapless acquisition example data is being moved at a sustained rate of 500 MB/s
(equivalent to a 100-MHz bandwidth recording).

5
A Viable Recording An example of a gapless recording solution is Keysight's dual-channel M9392A
PXI Vector Signal Analyzer, which can provide two independently tunable chan-
Solution (continued) nels--each capable of recording data at a rate of 100-MHz bandwidth over
many hours. The M9392A is a five-module solution. Each module in the system
exists as a discrete component in its own right, with its own driver and soft
front panel. The overarching control of the five modules is provided by a layer of
instrument software called the M9392A.

The M9392A is used with either a regular PC hard disk drive (HDD) or external
mass storage. The HDD is used in cases where the required capture dura-
tion is only a few tens of seconds. External mass storage can be used with the
M9392A for long duration captures requiring more throughput and capacity.
When recording at wide bandwidths for long durations, a RAID storage system
is required.

Keysight's gapless recording system is available in predefined packages that
have been tested to guarantee sustained data rates. The configured systems
include data interface cards and modules, and can be used with Keysight's
89600 VSA signal analysis software to speed the process of finding, analyzing
and fixing problems.

An example of a configured M9392A recording system with 32 TB of storage is
shown in Figure 5. This system can be connected to a PXI chassis or external
workstation computer using a very fast PCIe