Methods to Conduct Efficient Radar and Transmitter Measurements
Radar technology is used well beyond traditional military applications today. Engineers are designing radar systems used in many commercial environments, including safe and efficient operation of transport systems, weather forecasting, and 5G use cases. The expansion of advanced radar systems requires periodic maintenance tests of radar transmitters and effective measurement methods to assure stable operation and compliance with relevant laws.
Table 1 lists the frequency bands used by radar. In relative terms, the lower frequency bands can measure longer distance ranges, while higher frequency bands are better for measuring shorter distance ranges with higher resolution. Radar systems operating at millimeter wave (mmWave) bands, such as 24 GHz, 60 GHz, 76 GHz, and 79 GHz are also gaining prominence. mmWave is particularly in use in many 5G use cases that utilize intelligent transport systems (ITS), such as autonomous vehicles and telehealth.
Radar Methods
Radar methods are typically divided into pulse and FM-CW, based on the different signal modulation methods (Figure 1). Recent, high-performance radar uses a combination of multiple technologies for high-resolution detection and position measurement over a wider range.
Pulse radar sends repeated square-wave pulses at fixed time intervals to determine the range from the time difference between received signals reflected by the target object. Pulse-radar technologies specified as long-pulse Q0N (where Q represents the angular modulation, 0 is an unmodulated signal, and N is no data), and short-pulse P0N (where P represents an unmodulated pulse, 0 is an unmodulated signal, and N is no data).
FM-CW radar uses a frequency modulated (FM) continuous wave (CW) signal to measure the distance from the change in the FM frequency as well as the movement speed from the received frequency Doppler shift from the transmitted frequency. Since FM-CW radar achieves a high signal to noise (S/N) ratio without requiring a high Tx signal power compared to pulse-radar, it is used in a wide range of applications, such as advanced compact aerospace implementations using semiconductors, meteorological radar, and automotive collision prevention radar.
Measuring Radar Transmitters
There are five main radio specifications for pulse-radar transmitters:
- Peak power/average Tx power
- Pulse duration/pulse width
- Pulse rise time/fall time
- Tx frequency/frequency deviation
- Necessary frequency bandwidth/40 dB bandwidth
The main factors affecting radar performance are Tx frequency, Tx power, pulse width, and pulse cycle. Accurate measurement of these radio characteristics is required to assure stable operation and maintenance of radar systems.
Instruments to Test Radar Transmitters
When measuring the radio characteristics of a radar transmitter, the transmission power is measured with a power meter/sensor, the transmission frequency and frequency deviation are measured with a frequency counter, and the pulse duration transitions are measured with an oscilloscope. Measurements in the frequency domain, such as necessary bandwidth, and unwanted emissions in the Out-of-Band (OoB) and spurious domains are usually measured with a spectrum analyzer. One reason for this is the many previous definitions for measurement methods using basic instruments as well as standards and regulations for each radio type.
Advances in measurement instrument technology allow all these measurements to be performed efficiently using a single measuring instrument supporting a wide range of measurement items. It is now possible to configure a measurement system at much lower cost than previously by understanding the features and characteristics of each instrument and the correlation with measurement results of previous instruments to choose the best system.
A power meter/sensor is used as a reference standard for measuring the power of other instruments and measures power with excellent accuracy. When measuring a pulse-radar signal, a power sensor covering a wide bandwidth of more than 50 MHz and supporting fast rise times of better than 1 μs should be selected. There are models for direct measurement of pulse width and rise time.
Frequency counters measure the RF signal frequency and frequency deviation. They have a built-in high-stability reference oscillator and open a gate at precise time units to pass and convert the signal to be measured to a pulse signal that can be counted to find the frequency. Certain models detect the radar-signal pulse to measure the pulse width by calculation from a clock count.
The oscilloscope performs A/D conversion of signals to measure the change in voltage and amplitude components over time. The oscilloscope must have excellent time resolution to measure extremely short pulse widths with very fast rise times. When measuring radar signals with an oscilloscope, a detector is inserted at the first stage to convert the RF signals to voltage.
A signal/spectrum analyzer converts the input signal to an IF signal using the superheterodyne method before sampling the A/D-converted direct signal (signal analyzer function) by sweeping the specified frequency range in the reference bandwidth (spectrum analyzer function). If the required conditions are met, one signal/spectrum analyzer can perform all key radar transmitter measurements for easy testing and maintenance.
A signal/spectrum analyzer can conduct measurements in the frequency and spurious domains. The spurious domain is the region outside the OoB domain where spurious emissions occur. The OoB domain is the region immediately outside the necessary frequency bandwidth with the modulated signal required for data transmission excluding the spurious domain. The OoB domain is the region where unwanted emissions are dominant (Figure 2).
The signal analyzer function samples the radar RF signal at a specific time and span at the set center transmission frequency. The IQ signal with these spectral components is converted to digital data using a high-speed processor to measure the Tx power, Tx frequency, pulse width, and pulse rise time. The pulse-width measurement analysis function is determined by the set span. As a general rule, it is 0.02 μs when the analysis bandwidth is 31.25 MHz (50 MHz sampling rate), and 0.8 ns when the analysis bandwidth is 1 GHz (1300 MHz sampling rate).
When measuring unwanted spurious using the spectrum analyzer function, one pulse cycle is included in the unit sweep time and the specified frequency range is swept. The performance of the measurement system, internal and external attenuators, and instrument display average noise level (DANL) has a large impact on the unwanted emissions measurement margin. The maximum permissible power at the RF input terminal of a general signal/spectrum analyzer is 1 W (+30 dB). In particular, when measuring a high-output radar signal, it is necessary to insert a sufficiently large attenuator in the signal path at the first stage of the measuring instrument to prevent instantaneous input of an excessively large power.
When using the direct method, the measuring instrument external attenuation value is determined by the loss in free space and gain of the test Rx antenna. If the indirect method and monitor terminal are used, the external attenuation value is determined by referencing the specifications of the output terminal.
When measuring high-output radar signals, the unwanted emissions measurement margin can be obtained easily by separating out measurement items other than unwanted emissions and measuring with a separate system. When measuring only unwanted emissions, the measurement margin can be increased by reducing the internal attenuation even if the radar-signal waveform is slightly distorted, because there is no impact on measurement. Moreover, when attempting to increase the measurement margin, the external attenuation amount can be reduced by using a notch filter to attenuate the power of the radar bandwidth upstream of the external attenuator.
Conclusion
The expansion of radar technology into more commercial applications – including those in mission critical environments—places greater emphasis on proper test equipment. Advances in test now allow for single instrument solutions that can accurately and efficiently verify radar system designs.
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