Faster, higher, further

The underlying principle of operation of radar (short for ‘radio detection and ranging’) is based on electromagnetic wave propagation theory. In a radar system, a known wave is transmitted in a known direction. When it encounters a target, the wave is reflected back to a receiver at the source, where it is captured and analysed.

As long ago as the Second World War, this basic technology was already being used to acquire information such as the direction, height, distance, trajectory and speed of moving objects such as aircraft and ships (see Figure 1).

Fig. 1: radar is a proven method of measuring the direction and speed of movement of flying objects

Advances in the technology and performance of certain circuit elements in a radar system, such as faster, more precise analogue-to-digital converters (ADCs), and digital signal processors offering greater throughput, have now made new and more sophisticated applications possible in industries such as automotive and defence electronics.

Developers and users of radar components and systems are therefore having to test and characterise devices operating at new, higher frequencies and speeds, and this is placing new demands on the types and specifications of test instruments they use.

This article explains how the fundamental operation of radar determines the parameters of the tests that engineers need to run, and shows how test instrumentation is evolving to handle the new, higher performance radar equipment of tomorrow.

How to make radar calculations

Either received signal power or the distance (or range) between the transmitter and the target can be calculated by solving for specific parameters of the radar equation:

where
P_r = received signal power
R = range between the transmitter and the target
P_t = transmitted signal power
G = gain of the radar antenna
σ = radar cross-section of the target
A_e = effective antenna aperture
λ = wavelength of the transmitted signal

Radar is a microwave technology, and at the high frequencies used by radar transmitters, the signals are able to penetrate fog and clouds, and to receive reflections from many kilometres distant, provided the system has a sufficiently high value of Pt. This is why radar is so widely used in applications such as defence surveillance, meteorology, and surface, marine and air navigation.

These applications are decades old. In the present day, radar has found a new application in the automotive sector in collision avoidance systems, and other forms of Advanced Driver Assistance Systems (ADAS). For instance, radar systems operating at approximately 77GHz are able to detect and track objects in front of, to the side of and behind a vehicle, and to trigger warnings to the driver of an imminent collision.

In the field of defence electronics, radar has long been the mainstay of early warning systems that detect incoming hostile aircraft and missiles. A different application of radar in defence is the ground penetrating radar (GPR). The special property of GPR is that its signals are not completely absorbed by soil - it may therefore be used to detect landmines buried below the surface, a life saving technology both during and after military conflict. GPR is also deployed in oil exploration and other extractive industries.

Interestingly, today’s radar technology is also crucial to the accuracy and detail of weather forecasts. Radar can gauge the size, quantity, state (solid as hail or snow, liquid as rain) and shape of water particles in clouds. Scientists today can also accurately measure wind speed in tornados using radar.

New, higher frequencies of radar signals

While the basic technology of radar was first applied many decades ago, today’s new systems are using much higher operating frequencies in order to gain more bandwidth. These developments have occurred as design engineers extend radar systems’ capabilities to keep up with ever more complex signal formats and modulation schemes.

In these systems, new, faster digitisers provide for the quick and easy conversion of analogue signals to the digital domain, thereby enabling real time analysis of incoming data. At the same time, antenna arrays have grown more complex, and components such as filters are sharper than ever before.

And the higher operating speeds and frequencies of today’s radar systems make new demands of the instruments that engineers use to characterise radar components and to test radar systems. Microwave test instruments for radar applications therefore, need to support higher frequencies and wider bandwidths, and offer an improved noise floor, in order to provide the accuracy and precision of measurement results required by installers and users.

The types of radar measurement

Before a radar is deployed in the field, testing will have been carried out at many levels - the sub-system, prototype and final production unit are all subject to various kinds of performance tests.

In fact, there are broadly four types of test that an instrument must be able to perform for radar engineers:

1: Component characterisation
Thorough testing of all RF components in the radar assembly is essential. Accurate characterisation of individual components such as filters, duplexers, attenuators and amplifiers ensures there are no unexpected losses that could impair the system’s performance.

A common test for use in component characterisation is the measurement of the S-parameters (scattering parameters), which gives a good representation of the linear effects of components in the signal path. Using S-parameters, an engineer can measure gain/loss, group delay, noise figure and the phase and amplitude accuracy of microwave components.

A Vector Network Analyser (VNA) is the most suitable instrument for making S-parameter measurements. It is essentially a well synchronised transmitter and receiver in a single box. A modern VNA such as Anritsu’s VectorStar test instrument will be able to characterise components using both continuous wave and pulsed input signals. Pulse measurements are a particularly important requirement in radar systems, which commonly operate through the transmission of short, powerful pulses. The VectorStar VNA also has a built-in noise source for accurate noise figure measurements.

Figure 2: VectorStar enables users to get a true view of their device pulse performance and see behaviour they may have been missing

2: Antenna tests
Antennas are a crucial component of modern radar systems - their performance directly affects the speed and accuracy of a radar system’s performance.

Two common antenna test configurations are near-field measurements and far-field measurements. The parameters for these tests include polarisation, side lobes, frequency/phase performance, voltage standing wave ratio (VSWR) and gain. In addition, antenna pattern measurements essentially involve making gain and phase measurements relative to a reference antenna while varying the angular position of the antenna under test.

The same test instrument - the VNA - used in component characterisation, is also suitable for comprehensive antenna testing. The frequency and sensitivity requirements of the antennas will determine the specifications that the VNA must match or exceed.

An RF signal generator may be used in conjunction with a VNA as a secondary external source. The Anritsu MG3690C is an example.

Fig. 3 A typical set-up for Antenna measurements

3: RCS measurements
RCS is the measure of the extent to which a target reflects radar signals - in other words, it is a measure of the ratio of backscatter power per steradian (from the target to the radar receiver) to the power density that is intercepted by the target.

Fig. 4: RCS measurements may be performed in the field using a handheld VNA

RCS measurements of bulky potential targets obviously do not take place in the laboratory with a benchtop instrument. Instead, a handheld VNA such as Anritsu’s VNA Master is required (see Figure 2). Time domain gating is an essential function in such an instrument. The VNA measures the S-parameters in the frequency domain, and the gating function transforms these measurements to the time domain. The magnitude of the S21 amplitude of the standard reflection is captured, and its value provides the reference for the RCS measurement.

4: Power and spectrum analysis
Modern spectrum/signal analysers can characterise the frequency, phase noise and jitter of devices such as oscillators that are critical to radar systems.

While frequency counters have long been the standard for accurate frequency measurement, spectrum analysers offer the advantage of selecting a specific frequency to be measured when multiple signals are present. On many spectrum analysers, the accuracy of the measurement of the marker frequency is based on the sweep linearity, resolution bandwidth setting and display resolution.

Spectrum analysers such as the Anritsu MS2830A incorporate true frequency counter technology. The spectrum display sweep pauses at the marker frequency and the selected signal is routed to an internal frequency counter. The result is the best of both worlds - the ability to select a single frequency, and highly accurate frequency measurement based on a highly stable reference frequency.

In addition, one of the most reliable measurements of the purity of a signal source in a radar system is phase noise. The measurement involves characterising the noise power relative to the carrier power at many different offset frequencies. The signals at the input and output of the PLLs (phase locked loops) in a radar system are often binary signals to be used in serial data streams. The noise on the binary signals is commonly characterised in terms of jitter rather than phase noise, and this jitter can be estimated by integrating the noise power over a range of frequency offsets from the carrier frequency.

Figure 5: Phase noise in the 10Hz to 10MHz frequency offset range

The power of a radar system’s transmissions is another important parameter. As the radar equation at the start of this article showed, there is a correlation between P_t and range, and so the engineer must verify that a radar system or sub-system’s transmit performance matches its specifications. Anritsu’s ML2490A peak power meter is an example of a test instrument suited to high resolution measurements of radar systems. With high bandwidth and resolution in rising edge pulse measurements, it can provide precise and accurate details about the transmitter’s power envelope. The pulse profile mode of the ML2490A, when combined with the wide band MA2411B power sensor, provides an excellent option for measuring radar pulses.