AWR White Papers

Radar Systems

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The linear chirp source (first block to the far left of the system diagram) generates a linear FM chirp signal, also known as a PD signal. The linear chirp pulse source consists of basic parameters that can be configured according to user specifications, such as pulse repetition frequency (PRF), pulse duty cycle, start/stop frequency, and sampling frequency. The pulse repetition interval (PRI) denotes the time difference between the starts of two consecutive pulses (Figure 2). The chirp duration (pulse on) is a function of duty cycle and PRI, and it is calculated as the product of the two; the duty cycle is a percentage and can take any non-negative value up to and including 100%. Figure 2. Control parameters defining the linear chirp generator output signal During the active portion of the chirp, this block outputs a signal with an instantaneous frequency that changes linearly between the start and stop frequency parameters. These two parameters can have any valid frequency value, resulting in signals that can have either increasing or decreasing frequencies at the start of the chirp. Designers are also able to specify the ratio of rise and pulse on. This parameter is a percentage and can have any non-negative value up to and including 100%. The signal power during the active portion of the chirp is set by the peak power parameter of the linear chirp signal generator. A non-zero initial delay may be defined for the chirp pulse; this delay may take on any non-negative value, and a warning is generated if this delay is greater than PRI. The center frequency of the chirp signal may be user-defined. If left empty, it is set to the average of the start and stop frequencies. Similarly, the sampling frequency may also be user-defined; if left empty, it is calculated based on the global variable "_SMPFRQ". In this example, the chirp signal level is set to 0 dBm, PRF = 2 kHz, and DUTY = 25%. The next block in the chain, a coupled correlator block, is commonly used for pulse compression in radar receivers. Pulse compression is a signal processing technique commonly used to increase the range resolution, as well as the signal-to-noise ratio (SNR), by modulating the transmitted pulse and then correlating the received signal with the transmitted pulse. In this example, a block performs a correlation between the signal reflected from a radar target and the transmitted signal. This requires the coupled correlator to buffer enough samples to accommodate a full PRI before it can process the chirp. To ensure a successful simulation of such scenarios, the sampling frequency should be carefully selected. The minimum value for the sampling frequency parameter would be the bandwidth of the radar signal (FSTART-FSTOP). If spectral measurements are desired, the sampling frequency can be set to a larger value. The signal next passes through the RF transmitter responsible for frequency up-conversion, filtering, and signal amplification before being radiated through the antenna toward the target. Both the RF transmitter and receiver sub-circuits define the single-stage upconverter and downconverter that are each composed of an oscillator, mixer, amplifier, and filter, as shown in Figure 3. Users may replace these subcircuits with their particular implementations. Figure 3. Components defining the RF transmitter subcircuit include an oscillator (tone source generates one or more sinusoidal tones), mixer (upconversion), filter, and amplifier Radar Systems 4 www.cadence.com/go/awr

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