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Keysight
Radar Measurements

Application Note
Introduction

Today, different types of radar systems are used in a variety of applications: avionic, military, automotive, law
enforcement, astronomy, mapping, weather, and more. Within this broad range of uses, several radar
technologies have emerged to meet specific needs in terms of performance, cost, size and capability. For
example, many police radars use continuous-wave (CW) radar to simply assess Doppler shifts from moving
cars; range information is not needed. As a result, low cost and small size are more important than advanced
capabilities and features.

At the other extreme, complex phased-array radars may have thousands of transmit/receive (T/R) modules
operating in tandem. In addition, these may rely on a variety of sophisticated techniques to improve per-
formance: sidelobe nulling, staggered pulse repetition interval (PRI), frequency agility, real-time waveform
optimization, wideband chirps and target-recognition capability are a few examples.

To provide a foundation the discussion presented here, this application note starts with a brief review of radar
basics. After that, the remainder of the note focuses on the fundamentals of measuring basic pulsed radars,
which is the basis of most radar systems. Where appropriate, this note will discuss adaptations of certain
measurements for more complex or modulated pulsed-radar systems. This note will emphasize the mea-
surement of radar signals for transmitter testing. A separate Keysight application note titled Radar, EW &
ELINT Testing: Identifying Common Test Challenges (publication 5990-7036EN) discusses signal generation
and the characterization of radar receivers and electronic countermeasure (ECM) systems.
Table of contents

1.0 Current trends and technologies in radar..................................................................... 4
Current trends and technologies in radar................................................................................... 4

2.0 Radar basics and the radar range equation.................................................................. 5
The fundamentals of radar operation........................................................................................ 5
Pulse characteristics........................................................................................................... 5
Pulse compression.............................................................................................................. 6
Doppler frequency.............................................................................................................. 7
The radar range equation...................................................................................................... 8

3.0 Radar block diagram and the radar range equation...................................................14
Relating the range equation to the elements of the radar design................................................. 15

4.0 Radar measurements................................................................................................... 16
Power, spectrum and related measurements........................................................................... 16
Maximum instrument input level........................................................................................... 17
Measuring pulse power with a power meter............................................................................. 18
Pulse frequency and timing measurements with a counter............................................................ 25
Measuring pulse power and spectrum with a signal analyzer......................................................... 28
Measuring with a vector signal analyzer.................................................................................. 40
Component and subassembly test......................................................................................... 53
Direct measurement of power loss with a power meter................................................................. 53
Measuring with a network analyzer........................................................................................ 54
Antenna measurements....................................................................................................... 59
Far-field versus near-field antenna test................................................................................... 60
Far-field test configuration.................................................................................................. 61
Near-field test configuration................................................................................................ 63
Example antenna measurement results................................................................................... 65
Solution for multi-aperture and phased-array antennas................................................................ 66
Radar cross section............................................................................................................. 68
Noise figure....................................................................................................................... 70
Y-factor measurement technique........................................................................................... 71
Direct-noise or cold-source measurement technique................................................................... 72
Selecting the best noise-figure measurement solution................................................................. 72
Time sidelobe level............................................................................................................. 74
Understanding time sidelobes.............................................................................................. 74
Applying the time sidelobe method....................................................................................... 74
Weighting the output signal................................................................................................ 76
Making time sidelobe measurements..................................................................................... 76
Applying the results.......................................................................................................... 77
Phase noise, AM noise, and spurs.......................................................................................... 78
Making a direct spectrum measurement of phase
noise with a spectrum analyzer............................................................................................. 80
Measuring phase noise using phase detector............................................................................
81
Measuring phase noise using a signal-source analyzer.................................................................82
Measuring phase noise with a dedicated test system.................................................................. 84

5.0 Summary..................................................................................................................... 85

Related information......................................................................................................... 86

References........................................................................................................................ 87
04 | Keysight | Radar Measurements - Application Note

1.0 Current trends and technologies in radar

Current trends and technologies in radar

For engineers and scientists, the names behind the earliest experiments in electro-
magnetism are part of our everyday conversations: Heinrich Hertz, James Clerk Maxwell
and Nikola Tesla. Fast forward from their work in the late 19th and early 20st centuries
to the early 21st Century: the fundamental concept--metallic objects reflect radio waves
--has evolved into a host of technologies that are pushed to the extremes in military
applications: detecting, ranging, tracking, evading, jamming, and more.

As is the case in commercial electronics and communications, the evolution from purely
analog designs to hybrid analog/digital designs continues to drive advances in radar
system capability and performance. Frequencies keep reaching higher and signals are
becoming increasingly agile. Signal formats and modulation schemes--pulsed and other-
wise--continue to become more complex, and this demands wider bandwidth. Advanced
digital signal processing (DSP) techniques are being used to disguise system operation
and thereby avoid jamming. Architectures such as active electronically steered array
(AESA) rely on advanced materials such as gallium nitride (GaN) to implement phased-
array antennas that provide greater performance in beamforming and beamsteering.

Within the operating environment, the range of complexities may include ground clutter,
sea clutter, jamming, interference, wireless communication signals, and other forms of
electromagnetic noise. It may also include multiple targets, many of which utilize materials
and technologies that present a reduced radar cross section.

This updated edition of our Radar Measurements application note reflects these realities.
Because any document begins to lag behind current reality at the moment of publication,
the content included here is a mix of timeless fundamentals--the radar range equation--
and emerging ideas such as the time sidelobe level measurement technique. Many of the
sidebars highlight products--hardware and software--that include future-ready capabilities
that can evolve along with the continuing evolution of radar systems.

Whether you choose to read this note from cover to cover or selectively sample the
sections, we hope you find material--timeless or timely--that will be useful in your day-to-
day work, be it on new designs or system upgrades.
05 | Keysight | Radar Measurements - Application Note

2.0 Radar basics and the radar range equation

The fundamentals of radar operation

The essence of radar is the ability to gather information about a target -- location, speed,
direction, shape, identity or simply presence. This is done by processing reflected radio
frequency (RF) or microwave signals in the case of primary radars, or from a transmitted
response in the case of secondary radars.

In most implementations, a pulsed-RF or pulsed-microwave signal is generated by the
radar system, beamed toward the target in question and collected by the same antenna
that transmitted the signal. This basic process is described by the radar range equation
found on page 6. The signal power at the radar receiver is directly proportional to the
transmitted power, the antenna gain (or aperture size), and the radar cross section (RCS)
(i.e., the degree to which a target reflects the radar signal). Perhaps more significantly, it
is indirectly proportional to the fourth power of the distance to the target. Given the large
attenuation that occurs while the signal is traveling to and from the target, having high
power is very desirable; however, it is also difficult due to practical problems such as heat,
voltage breakdown, dynamic power requirements, system size and, of course, cost.

Pulse characteristics

The characteristics of a pulsed radar signal largely determine the performance and
capability of the radar. Pulse power, repetition rate, width and modulation are traded off
to obtain the optimum combination for a given application. Pulse power directly affects
the maximum distance, or range, of a target that can be detected by the radar.

Pulse repetition frequency (PRF) determines the maximum unambiguous range to the target.
The next (non-coded) pulse cannot be sent until the previous pulse has traveled to the
target and back. (Coded pulses can be sent more frequently because coding can be used to
associate responses with their corresponding transmitted pulse.)

Pulse width determines the spatial resolution of the radar: pulses must be shorter than
the time it takes for the signal to travel between the target details; otherwise, the pulses
overlap in the receiver.

The pulse width and the shape of the pulse also determine the spectrum of the radar signal.
Decreasing the pulse width increases signal bandwidth. A wider system bandwidth
results in higher receiver noise for a given amount of power, which reduces sensitivity.
Also, the pulse spectrum may exceed regulated frequency allotments if the pulse is too short.

The shape can be the familiar trapezoidal pulse with rapid but controlled rise and fall times,
or any of a number of alternative shapes such as Gaussian and raised-cosine. Because the
pulse shape can determine the signal bandwidth and also affect the detection and identifi-
cation of targets, it is chosen to suit the application.

Short pulses with a low repetition rate maximize resolution and unambiguous range and
high pulse power maximizes the radar's range in distance. However, there are practical
limitations in generating short, high-power pulses. For example, higher peak power will
shorten the life of tubes used in high-power amplifier design. This conundrum would be
the barrier to increasing radar performance if radar technology stopped here. However,
complex waveforms and pulse-compression techniques can be used to greatly mitigate
the power limitation on pulse width.
06 | Keysight | Radar Measurements - Application Note

Pulse compression

Pulse compression techniques allow relatively long RF pulses to be used without
sacrificing range resolution. The key to pulse compression is energy. Using a longer pulse,
one can reduce the peak power of the transmitted pulse while maintaining the same
pulse energy. Upon reception, the pulse is compressed with a match-correlation filter into
a shorter pulse, which increases the peak power of the pulse and reduces the pulse width.
Pulse-compressed radar thereby realizes many of the benefits of a short pulse: improved
resolution and accuracy; reduced clutter; better target classification; and greater tolerance
to some electronic warfare (EW) and jamming techniques. One area that does not realize
an improvement is minimum range performance. Here the long transmitter pulse may
obscure targets that are close to the radar.

The ability to compress the pulse with a match filter is achieved by modulating the RF pulse
in a manner that facilitates the compression process. The matching filter function can
be achieved digitally using the cross-correlation function to compare the received pulse with
the transmitted pulse. The sampled receive signal is repeatedly time shifted, Fourier
transformed and multiplied by the conjugate of the Fourier transform of the sampled trans-
mit signal (or a replica). The output of the cross-correlation function is proportional to the
time-shifted match of the two signals. A spike in the cross-correlation function or match-
ing-filter output occurs when the two signals are aligned. This spike is the radar return
signal, and it typically may be 1000 times shorter in time duration than the transmitted
pulse. Even if two or more of the long transmitted pulses overlap in the receiver, the sharp
rise in output only occurs when each of the pulses are aligned with the transmit pulse.
This restores the separation between the received pulses and, with it, the range resolution.
Note that the receive waveform is windowed using a Hamming or similar window to reduce
the time-domain sidelobes created during the cross-correlation process.

Ideally, the correlation between the received and transmitted signals would be high only
when the transmit and receive signals are exactly aligned. Many modulation techniques
can be used to achieve this goal: linear FM sweep, binary phase coding (e.g., Barker codes),
or polyphase codes (e.g., Costas codes). Graphs called ambiguity diagrams illustrate how
different pulse compression schemes perform as a function of pulse width and Doppler
frequency shift, as shown in Figure 1. Doppler shift can reduce detector sensitivity and
cause errors in time alignment.
07 | Keysight | Radar Measurements - Application Note

Pulse type Frequency domain Ambiguity diagram

0 dB Doppler
frequency

Short RF pulse Pulse
width*

16 MHz

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