How to test modern radars
Darren McCarthy- November 29, 2012Using a spectrum analyzer for pulsed measurements has always required a careful understanding and knowledge of the parameters of the pulse signal, as well as the subsequent operation of the spectrum analyzer to make valid measurements. One of the primary uses of pulsed measurement techniques has been the accurate measurement of pulsed radar signals. Advances in the type of pulse and pulse train information can greatly improve the utility of the radar; however, these advances will increase the complexity of the measurements. Automatic pulse measurements have been introduced on modern spectrum analyzers to greatly simplify challenging new measurements.
This article looks at traditional pulse measurements using a typical spectrum analyzer with a traditional swept-tuned architecture, discusses some of the advances in radar waveforms, and reviews some of the important considerations for reducing the measurement uncertainty of these advanced pulse measurements on more modern spectrum analyzer architectures.
Basic Pulse Measurements
The main advantage of using a traditional spectrum analyzer is that it can be used to test frequency-dependent power components with a wide dynamic range. Simple measurements, such as checking the symmetry of the pulse spectrum, can validate the radar transmitter. An asymmetrical spectrum can waste power, generate unwanted spurious emissions, and degrade the overall performance of the radar.
When making measurements using the spectrum analyzer, particularly on signals with low duty cycles, one needs to be familiar with the parameters of the expected pulse and the important spectrum analyzer settings such as resolutions bandwidth (RBW), span, and sweep time in order to produce informative results.
Figure 1 shows the traditional swept-tuned architecture of the spectrum analyzer. A signal is filtered and downcoverted to an IF frequency where various resolution bandwidth and video filters are applied to the signal while the local oscillator is swept across a frequency span. The resulting energy and frequency is plotted on the display.
Figure 1: Traditional Swept Tuned Spectrum Analyzer Architecture.
Since a pulsed signal is not on at all times, the energy will not completely “fill” the spectrum on a single sweep. Figure 2 shows the spectral characteristics of a simple pulsed RF signal with a pulse width and the pulse repetition interval Τ. The amplitude of the spectral lines are determined by the envelope curve about the center frequency f0.
Figure 2: Typical display of pulse signal showing pulse width τ and pulse interval Τ
When measuring the spectrum using a spectrum analyzer, it is possible to display the individual spectral lines or the envelope curve of the pulse spectrum depending on the instrument settings. The RBW should be set to a value significantly less than the pulse repetition frequency (= 1/Τ). The line spacing is equal to the pulse period (pulse repetition interval) and is independent of the setting for the sweep time on the analyzer. The height of the individual spectral lines is also independent of the RBW.
The largest spectral line displayed in the spectrum display is below the pulse amplitude of the actual pulse by the pulse desensitization factor”(PDF). The PDF is dependent upon the pulse width to the pulse period ratio:
PDF = 20 * log(τ/Τ)Using the line spectrum, the peak power of the pulse signal can be calculated when placing a marker on the tallest spectral line (as shown in Figure 3):
Peak power = marker reading – PDF = marker reading – 20*log(τ/Τ).
Figure 3: Shows the line spectrum using 50 Hz RBW for 1000 Hz pulse interval
When using the maximum peak detection method, if the RBW of the analyzer is increased such that it is greater than the reciprocal of the pulse period (but still smaller than the reciprocal of the pulse width), the spectrum analyzer will display the spectrum envelope. The amplitude of the envelope increases linearly with the RBW, thus doubling the RBW produces a 6dB increase in the amplitude.
By continuing to increase the RBW until the RBW is greater than the reciprocal of the pulse width, the spectrum analyzer can approximate the peak power of the pulse signal within the limitations of the resolution bandwidth of the traditional spectrum analyzer.
To demonstrate this limitation, Figure 4 shows the zero-span capture of three different pulse widths using a 10MHz RBW filter. While accurately representing the pulse widths for 500 and 200ns durations, when the pulse width of the signal is decreased to 100ns, the peak amplitude becomes reduced due to the filter bandwidth of the RBW filter. As the pulse width gets shorter, the limitations of the traditional spectrum analyzer impact the measurement uncertainty.
Figure 4: Zero-span measurements of three different pulse widths using the zero-span mode of a typical spectrum analzyer (RBW = 10 MHz)
Increasing Complexities of Radar
Many modern types of radar have advanced beyond the simplistic traditional functions of range detection to improve range resolution, mitigate operational limitations, and improve function. The impact of these modern radar types increases the complexity and performance requirements of the traditional spectrum analyzer.
Pulse-Doppler radar provides radial speed information about the target in addition to range and direction. Using a typical coherent transmitter and receiver, the speed information can be derived from the pulse-to-pulse variations in the received signal. Pulse-to-pulse transmitter stability verification test has become much more demanding on the performance of the measurement equipment as phase information is not collected in a traditional spectrum analyzer.
Pulse compression radars are used to improve range resolution. Frequency modulation on pulse (FMOP) and phase modulation on pulse (PMOP) can substantially improve the ability to resolve multiple targets at greater distances. A 2GHz bandwidth FM chirp can resolve target variances less than 10cm apart. Some of the typical FMOP/PMOP techniques include: Linear frequency modulation (FM Chirp); non-linear frequency modulation; encoded pulse phase modulation (e.g. Barker codes); and polyphase modulation and time-frequency coded modulation. Not only do pulse compression radars tend to increase the need for analysis bandwidth with fast risetimes and reduced pulse widths, but to check for transmitter stability requires different types of waveform analysis not available on traditional spectrum analyzers. To collect the necessary phase information, a baseband I/Q conversion is required to perform the type of analysis needed to validate these types of radar transmitters.
An example of an advanced radar technique and associated measurement might be the use of a staggered pulse repetition interval (PRI). A staggered PRI technique is used in most modern radars to overcome the limitations of a constant PRI. Constant PRI frequency radars are susceptible to self-jamming, blind speeds, false target recognition due to double echo returns, and can also be susceptible to jamming or spoofing.
Figure 5 shows the analysis of a multi-rate PRF transmitter. Not only is the PRI varied in this case, but the pulse widths are also varied on a pulse-to-pulse basis. This type of analysis on a traditional spectrum analyzer would not be possible as many of the measurements that were described earlier require a constant and stable PRI for timing and spectrum measurements.
Figure 5: Measurements of Varying PRI Pulse Train Signal using the Rohde & Schwarz FSW Spectrum Analyzer and K6 Pulse Analysis Software
>>Modern spectrum analyzer architectures for testing advanced radars
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