Pulsed RF radar signals must be accurately characterized to evaluate a radar system's performance,therefore using the right test tools is vital. Modern radar test instruments,with advances in DSP measurement technology, are able to provide even greater insight than their analog counterparts.The increased performance, flexibility, and functionality of modern instruments are especially useful when evaluating radars that use advanced pulse compression techniques and pulse shaping.This paper will examine the measurements and capabilities that are available for radar using modern spectrum analyzers, vector signal analyzers,and power meters.
In operation, a radar system sweeps a beam through a region of interest known as the target space. The beam traces a path called the search scan pattern, and the region covered by the scan is called the scan volume or frame. A target is detected if it is within the line of sight of the transmit antenna and if the signal echoes from the target are strong enough to rise above the system noise floor. The strength of these target echoes is inversely proportional to the range (R) of the target to the fourth power (R4). Because of the proportionality, a target's echoes grow rapidly stronger as the target approaches the radar system.
A radar's range depends on a number of different factors, including the wavelength of operation, the transmitter power, the signal-to-noise ratio (SNR) and sensitivity of the receiver, the level of background noise and clutter, the size (and gain) of the radar antenna(s), the reflection characteristics of the target, and the radar's pulse compression, integration, and processing capabilities. Radar system designers apply the well-known radar range equation to calculate the effects of different variables on
but can be essentially represented by:
Received signal energy = (PavgG σ Ae tint)/2R4> where:
Pavg is the average transmitted power;
G is the antenna gain;
σis the radar cross section of the target;
Ae is the effective antenna area;
tint is the integration time (the time the radar beam is on the target); and
R is the range to the target.
While the range to the target is beyond a designer's control, adjustment of some parameters, such as transmitted power and pulse characteristics, can significantly affect radar system performance. Accurately characterizing the radar signal can tell a designer a great deal about a radar's capability and limitations. Measurements require the right tools and, often, the right combination of tools, such as a peak power meter and a high-performance spectrum analyzer with enough digital-signal-processing (DSP) power for complete signal analysis.
Characterizing Pulses
Accurate pulsed RF radar characterizations require measurements of a variety of different signal pulse characteristics, including pulse width, rise time, fall time, pulse repetition frequency (PRF), pulse repetition time (PRT), duty cycle, peak power, and average power. (Fig. 1).
While an average-reading power meter can derive values for peak and pulse power based on measurements of average power, it cannot directly determine peak power or other pulse characteristics. A peak power meter, on the other hand, can measure pulse and peak power directly and provide informative time-domain views with flexible triggering and gating functions (much like an oscilloscope) to simplify analysis of pulsed RF radar signals. A well-designed peak power meter is based on a high-speed, continuously sampling front end that can capture transient events. This gives a peak power meter the ability to calculate results on a single pulse, or average over multiple pulses, and enables flexible time gating functions.
A peak power meter can provide a great deal of insight into a radar system's transmitted power, but it does not provide information about the transmitter's spectral behavior. A spectrum analyzer's display shows signal amplitude as a function of frequency. Built-in functions and programmable markers can simplify key spectral measurements, such as sidelobe levels, spectral symmetry, occupied bandwidth, and average power. A spectrum analyzer's zero-span mode can be used to verify pulse characteristics such as rise time, fall time, pulse width, PRF, and peak power.
Traditional spectrum analyzers have relied on a super-heterodyne receiver architecture using mixers and local oscillators (LOs) for frequency translation, and analog resolution-bandwidth filters to isolate signals of interest. Unfortunately, the group delay and accuracy of these filters can limit the time-domain performance of a spectrum analyzer, notably for pulsed RF radar signal measurements. To overcome these limitations, digital resolution-bandwidth (RBW) filters allow pulse filtering and processing in the digital domain, providing mathematically defined near-ideal filter shapes. The digital IF architecture greatly improves spectrum analyzer performance accuracy and speed over traditional analog designs. The digital IF approach maintains the advantages in sensitivity and dynamic range of a traditional swept architecture while including the flexibility to process the measured signals using DSP processing including FFT analysis.
Many of the power and spectrum tests needed for characterizing pulsed RF radar signals can be performed with a peak power meter and a spectrum analyzer with digital IF filters. But additional measurement power is needed to evaluate modulation, phase, or transient characteristics of radar. For such tasks, a vector signal analyzer (VSA) offers many measurement capabilities not available in a traditional analog or digital spectrum analyzer.
Unlike a spectrum analyzer a vector signal analyzer captures the phase and magnitude information of the measured signal and can use this information to perform more advanced analysis. VSAs are typically very flexible and can display results in the time, frequency, and the modulation domain. See Agilent Application note 15015, vector signal analyzer basics, for detailed information on how a VSA operates.
A VSA does not sweep across a wide frequency range like a spectrum analyzer. Most VSAs operate by tuning to a specific frequency, conditioning the signal, block down converting, digitizing, and processing the signal.
The primary restraint of a vector signal analyzer is its analysis bandwidth (sometimes referred to as information bandwidth or FFT bandwidth). To properly analyze a signal a significant majority of the signal's power must be contained within the analysis bandwidth of the instrument. The analysis bandwidth of a VSA is usually dictated by the ADC sampling rate and Nyquist law. The dynamic range limit of the analyzer is usually limited by the bit level of the ADC though this may vary noticeably between instruments with same bit depth depending on the sophistication of dithering techniques, image correction, and over-sampling techniques.
VSA measurement capability provides simultaneous insight into radar signals magnitude, frequency, and phase. This allows characterization of the modulation on a pulse, intentional or unintentional. In addition, a suitable VSA can analyze a radar signal on an individual pulse basis and can provide gap free analysis.
Measurement Solutions
Pulsed RF radar signal testing involves a wide range of measurements, including peak power, waveform, spectrum, phase, and modulation, implying a large rack of instruments. Surprisingly, all of these measurements can be handled by just two essential instruments, a peak power meter and a modern spectrum analyzer capable of swept and vector measurements.
Agilent's P-Series peak power meters and sensors, for example, (Fig. 2) feature a 100-MSamples/s sampling capability, and can analyze pulses with pulse widths as narrow as 50 ns and with rise times as low as 13 ns. The meters can also make peak power measurements on signals up to 40 GHz and includes a novel zero calibration technique that does not require disconnecting the meter from the DUT. The versatile meters can simultaneously measure pulse power, average power, rise time, PRF, pulse width, and peak-to-average ratio (Fig. 3).
Agilent's PSA Series spectrum analyzers (Fig. 4) offer traditional swept-measurement capability with the added power of digital IF filters. They also include extensive DSP and Fast Fourier Transform (FFT) capability for increased measurement speed with pulsed RF radar signals. By including the optional 89601A software running on a personal computer (PC), a PSA analyzer can perform VSA measurements at analysis bandwidths as wide as 80 MHz. The software adds advanced signal capture, time-gating, and measurement display functions to the extensive set of spectrum-analysis functions already in the PSA analyzers. For example, the VSA capability simplifies the analysis of the modulation on a pulse by providing a variety of displays including magnitude, phase, and frequency, versus time. (Fig 7).
The combination of capabilities in a PSA spectrum analyzer makes it a versatile measurement tool for pulsed RF radar signal testing. In the traditional swept mode, a PSA can achieve the greatest performance in sensitivity and dynamic range when evaluating spurious content and low-level distortion. It can apply FFT analysis for improved measurement speed and flexibility, and VSA capability for advanced phase and modulation measurements, using display capabilities in both the frequency and time domains to speed the search for elusive signal anomalies.
The PSA simplifies many common radar measurements. For instance, the automated burst power measurement can automatically trigger on the pulse, determine its pulse width, and measure the pulse power above a given threshold (Fig. 5). Similar built-in functions exist for channel power, and occupied bandwidth.
Using the VSA measurement tools, a PSA analyzer's capability of showing the frequency- and time-domain views of a radar waveform (or individual pulse) on the same screen (Fig. 6) simplifies analysis of pulsed radar by revealing anomalies in the time domain that may not be apparent in the frequency domain, or vice versa. Finally, even for radar signals with chirp modulation, the VSA measurement capability of the PSA analyzers can be used to demodulate a chirped radar signal and show important details about the waveform (Fig. 7).
Selecting a spectrum analyzer for pulsed RF radar testing is a matter of matching its performance specifications to the radar measurement requirements. Since spectrum measurements may consider desired signals as well as their harmonic and spurious products, the frequency range of the analyzer should be selected accordingly, at least through the second harmonic of a signal of interest. The analyzer must also provide the dynamic-range performance required to capture and display the signals generated by a pulsed radar transmitter. Spectrum analyzer dynamic range is defined at the small-signal end by the instrument's sensitivity and displayed average noise level (DANL)level below which signals can not be seenand at the large-signal end by its third-order-intercept (TOI) performance. The widest gap between values translates into the widest dynamic range.
Spectrum analyzers such as the PSA Series instruments provide versatile functionality for measurements on pulsed RF radar signals. Combined with the advanced triggering capabilities possible with the P-Series power meters (Fig. 8), the instruments form a solution that covers most radar transmitter test scenarios and one that delivers accurate, reliable test results in a variety of graphical and tabular formats for understanding and improving the performance of any radar system.
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