300 MHz. This signal has audio modulation; a squelch tone at
about 100 Hz. We can see that the rise time of this tone is around
50 ms. In the case of counter-IED systems, we must detect and
jam the transmission before the tone rises. To do this we want
to selectively jam emitters that are deemed a threat. The jammer
must have a lot of sensitivity, dynamic range, and search speed to
deal with the emitters, and distinguish enemy transmissions from
civilian transmissions while preserving system resources, namely
In order to evaluate jamming systems, a robust test capability
for simulating multiple emitters is essential. A wide bandwidth
arbitrary waveform generator (AWG) with sufficient dynamic
range of about 12 to 14 bits can be used to simulate width swaths
of spectrum and multiple emitters. Because of these stringent
requirements and new types of threats, AWGs of this type have
been developed and are currently available with 8 to 12 Msam-ples/s providing an RF modulation bandwidth of 3. 2 GHz to 5 GHz
and a spur free dynamic range (SFDR) of over 75 dBc.
The technical trends that facilitate moving the defense industry toward higher performance and more capable systems also
affect the requirements of test and measurement equipment.
Recent advances in semiconductor materials and processing has
aided the development of solid state power amplifiers with high
power capability, efficiency and linearity in a much smaller form
factor. Gallium Nitride (GaN) amplifiers are replacing traveling
wave tube amplifiers (TWTA) in radar applications and have led
to the development of active electronically scanned array (AESA)
radars. The elements of the array are made up of transmit/receive
modules (TRM). Beams are formed and steered through the application of phase offsets between the various elements of the array.
The key test challenge is characterizing element-to-element
phase and amplitude errors or misalignment at the point of radiation. These errors can be contributed from various components in
the antenna array, but must be calibrated out so that the system
in use operates as intended. Conventionally, a network analyzer has been used as the measuring receiver for antenna testing.
With the number of array elements increasing and the waveforms
employed having wider bandwidths, new methods are needed to
reduce test times and characterize the system under operational
Digital array radars (DAR) currently in the research phase will
change the paradigm of radar antenna test even further. In to-
day’s active arrays, the TRM is a classical analog in and analog out
device. With DAR, the analog to digital conversion (ADC) takes
place within the TRM making it an analog-in and digital-out
type of device, making conventional network analysis difficult to
implement. New test methodologies will need to be developed to
keep pace with these new systems.
Signal processing and computing capabilities are another
technological trend that has had a profound effect on both the
commercial and defense markets where performance, power
consumption and size are concerned. Advances in devices such
as digital signal processors (DSP), graphics processors (GPU) and
field programmable gate arrays (FPGA) have enabled more sophisticated algorithms and signals within radar, electronic warfare,
satellites and terrestrial communication systems. These devices
allow greater performance and capability in systems where power
and size constraints previously required tradeoffs to be made.
With these advanced tools, systems have the flexibility to perform multiple functions as their operating mode is now software
defined. For example, an airborne radar can support functions
such as communications, target tracking, fire control, ground
mapping and then to adapt to multiple detected targets, electronic
counter measures (ECM) and channel conditions.
Verifying the operation and performance of these multi-function
systems requires a test capability that is equally or more flexible
than the system under test itself. The newest generation of instruments includes internal signal processing capabilities to enhance
data analysis. With FPGA-based data reduction techniques such
as digital down conversion (DDC), the measurement noise floor
is reduced and signal capture memory conserved. On the signal
generation side, digital up conversion (DUC) in modern AWGs
gives the user greater flexibility in creating long and unique signal scenarios that simulate numerous operational environments.
Current economic conditions in the defense industry have
been disruptive to the old ways of thinking. As budgets trend
downward, a creative approach to meeting mission requirements,
staying on top of technological trends, and finding ways to reduce the cost of test is paramount. While this priority is being
aggressively addressed by the test and measurement industry,
we cannot lose sight of the important role high performance and
broad capability in measuring equipment play in verifying and
characterizing military systems. ECN
Figure 3. A 100-Hz audio squelch tone of a two way FM radio (300 MHz
carrier) with a turn on time of 50 ms.