Radio frequency interference is pervasive in the wireless age.
Cell phone customers undoubtedly have encountered some level of
RFI at one time or another. For military users of wireless systems,
failure to safeguard against unwanted RFI—particularly when
it comes to sensitive defense systems—can have serious consequences.
While some RFI is innocuous, it can become a real threat when it
interferes with systems used by military forces, such as remotely
Because virtually every wireless device is subject to RFI, it is
important that the source and strength of this interference be determined
before it affects a component or system. The goal is to prevent
RFI signals (both continuous emissions as well as short duration
transient events) from interfering with any component or system
that depends on radio frequency signals for operation or is susceptible
to damage when exposed to high RF power levels.
Consider, for example, a communications satellite during assembly,
testing or preparations for launch. Prior to launch, the satellite
is subjected to a variety of quality-control tests and procedures.
If unwanted RFI signals are allowed to interfere with any of the
processes, they could contribute to ultimate failure of the satellite.
Electronic warfare systems or components undergoing tests at a
range also could be affected by unwanted RFI. If this interference
doesn’t cause direct harm (sometimes rendering a component
or entire system useless), it could prevent accurate validation
and calibration during the testing and evaluation phases. When RFI
incidents such as these occur, it can be difficult or impossible
to determine the cause of the failure or the reason for otherwise
It should be noted that RFI is not a new phenomenon. There are
established technologies that are employed to deal with its detection.
Among them are spectrum-analysis instruments that typically analyze
a discrete, narrow band of frequencies that are usually present
concurrently with other frequencies. Tuning to a discrete frequency
implies prior knowledge of the RFI. Therefore, the spectrum analyzer
is usually programmed to cover a range of frequencies. By design,
the spectrum analyzer scans (using a fixed RF bandwidth) over the
frequency range of interest. The scanning reduces the probability
of intercept. As the frequency range increases, the degradation
can be significant, resulting in unreliable RFI detection.
To circumvent this problem, an alternative approach has been to
combine multiple spectrum analyzers in an attempt to cover a broader
range of simultaneous signals from one or more RFI sources. These
systems are not foolproof, but certainly improve the mean time to
However, the characterization of short-duration events (on the
order of 100 nanoseconds) diminishes the performance of these systems.
Digital signal-processing techniques are required to analyze the
collected spectral data to determine the pulse width, pulse repetition
interval and peak amplitude.
The addition of digital signal processing implies the integration
of peripherals to the system and still does not address the intercept
and characterization of transient RFI events.
Other real-world effects, such as scan patterns and multipath,
further degrade the performance of traditional spectrum analyzer-based
systems. Multipath is the occurrence of multiple images of an RF
signal due to reflections from objects in the environment. Consider
that a receiver located at a fixed site can receive an RF signal
from an emitter by many paths including both direct (line of sight)
and from reflections. The multipath condition can result in degrading
processing at the receiver with respect to pulse width, amplitude
and pulse repetition interval.
Certain RFI monitors have taken advantage of the latest in high-speed
digitizers to directly sample the RF environment. These approaches
typically rely on “down conversion” and “channelization”
to cover broad RF bandwidths. Down conversion refers to the translation
of an RF signal to an intermediate frequency (IF) signal. This process
involves a device called a mixer and converts the input RF to a
different frequency, usually to a lower frequency. For example,
a receiver that processes frequencies between 2 and 6 GHz can use
a down converter to translate signals from 14 to 18 GHz into the
lower frequency band where processing can occur.
The channelization technique divides a wide RF bandwidth into several
frequency bands—each with a narrower bandwidth. Each subset
is referred to as a channel. For example, a 2-18 GHz RF bandwidth
could be divided into four channels, each with 4 GHz bandwidth.
Further, a single receiver that operates on 2-6 GHz can then be
used in conjunction with a down converter to process each channel,
thereby covering the entire RF bandwidth.
Recently, a new signal detection methodology was developed, based
on the application of instantaneous frequency measurement (IFM),
integrated with commercial processors that permit simultaneous detection,
monitoring, and tracking of RF signals (continuous wave, pulsed,
and transient) at frequencies from 0.5 to 18 GHz.
This advanced RFI signal detection technology relies on instantaneous
frequency measurement (IFM) receiver systems originally developed
for electronic warfare applications in the defense market. A typical
signal detection system employs two independent IFM receivers, one
low band [0.5-2 GHz] and one high band [2-18 GHz]. Each IFM receiver
incorporates a digital frequency discriminator, which provides digital
encoding of wide band RF input signal frequency data for pulsed
or continuous-wave signals. The digital frequency discriminator
is an essential component of an IFM receiver system. In addition
to encoding RF frequency, the discriminator also provides a threshold
based on the instantaneous RF signal-to-noise ratio, error detection
and various flag functions, including pulse on pulse, pulse on continuous
wave, frequency modulation on pulse and phase modulation on pulse.
Each IFM receiver also incorporates a digital amplitude quantizer
that characterizes the RF amplitude and generates a standard video
signal for measuring RF envelope pulse width and time of arrival
The IFMs incorporated in these new signal detection systems help
to assure accurate and reliable detection, monitoring, and tracking
of unwanted RF signals. Unlike spectrum analyzer systems, the IFM
technology does not scan and provides 100 percent probability of
intercept for RF events as short as 100 nanoseconds.
By integrating the IFM receiver technology with antenna arrays,
precise bearing to an RFI source can also be determined. In most
cases an omni-directional antenna is employed to ensure a high probability
of intercept for all azimuth angles. Characterization of the antenna
response is used with the measured RF amplitude to calculate the
field strength of the RFI.
This RFI monitoring architecture also employs off-the-shelf processors
and peripherals. Microsoft Windows compatible software applications
are used to collect the output of the IFM receivers and present
RFI related data to the operator using a variety of graphical displays.
One feature of the software allows users to select threshold parameters
for unwanted RFI and set an alarm point at which the unwanted RF
signal—regardless of its source—would be considered
objectionable or harmful.
Complete log files of all RFI events are maintained by the system,
which can run unattended. Other features include local area network
access and automatic paging of remote users.
Many systems that require safeguarding from RFI are operated continuously
for days and weeks (often in remote locations). An RFI monitor that
can operate reliably, unattended, significantly reduces the human
resources and logistics required to support the operation.
Because wireless components and systems are employed on many different
platforms, RFI detection systems must perform in airborne, shipboard
and land mobile environments. Configuring systems such as these
has traditionally been cost-prohibitive, mainly because of the “militarized”
nature of the various components. By using many commercially available
products, the technology can become more affordable.
Fred Ilsemann is general manager of Wide Band Systems, Defense
Systems Division, in Neshanic Station, N.J.