1 General Information
Being the interface circuits that process radio frequencies for Radars, depending on applications, there are different types of Radar front ends that can be selected to be most suitable for the requirements. Radar front ends may be categorized by the operating frequencies, the way they interface with antennas and the way they process the information. Some generic Radar front ends are described here.
2 Continuous-Wave (CW) Radar Front Ends
CW Radar is a basic radar structure that uses a continuous frequency source for radio wave transmission. Therefore, it relies on the Doppler effect in the detection of an object. Since the signal from the frequency source has to be continuously transmitted, the CW Radar has an advantage in terms of the effective transmitted power compared to pulsed Radars. The frequency of the source can be unmodulated or modulated. If not modulated, only the velocity of a moving object can be detected based on the Doppler frequency shift. If the frequency of the signal source is modulated (called FMCW: Frequency-Modulated Continuous-Wave) by means of varying the tuning voltage of an oscillator, the distance of an object can also be measured. In general, the modulation signal on the tuning voltage of an oscillator can be saw-tooth or triangular waves. This can be done conveniently by interfacing a Radar front end (via frequency divider output) with an off-chip phase-locked loop (PLL) that can generate the modulation signal. At Silicon Radar, the 24-GHz Radar front end TRX_024_006 and 007 can be operated both in CW and FMCW modes.
3 Phased-Array Radar Front Ends
Phased-array radar is based on the beam-forming concept that the phases of the electrical signals that are sent to the antennas in an array are appropriately adjusted in such a way that, when combined spatially, it increases the intensity of the signal in the desired direction. This process can be done by using phase shifters as shown in the diagram in Figure 2. In this diagram of a 120-GHz Radar, the phase shifters can be found together with the low-noise amplifier (LNA) in the receiver and with the power amplifier (PA) in the transmitter.
4 MIMO Radar Front Ends
MIMO (Multiple-Input Multiple-Output) Radar front ends are similar to the phased-array Radars in the sense that they are both multiple-antenna systems. MIMO Radars utilize special digital signal processing techniques on a multiple units of Radars to generate a virtual antenna array. Basically, a bigger virtual array representing more receive and transmit units can be created from a smaller number of actual receive and transmit units. Therefore, by processing the mutual signals between multiple units, a more powerful Radar system resulting from an array of transmitters and receivers can be realized without physically using corresponding number of receivers and transmitters. A basic diagram of a MIMO Radar front end is shown in Figure 3.
5 Pulsed Radar Front Ends
Pulsed Radar is a time-domain Radar system, in which a high-power and short electrical pulse is transmitted and the reflected pulse from an object is detected. By processing the time between the transmitted pulse and the reflected pulse, the distance of that object can be measured. The system may start the measurement of the time at the beginning of the transmitted pulse and finished counting the time after detecting the reflected signal. As shown in the basic diagram of Figure 4, the pulse control unit generates square-wave pulse where the pulse former is used to re-shape to pulse to be suitable for transmission. In the receiver part, the same pulse shape is required as a template to down-convert the received pulse to be suitable for the sample and hold (S&H) circuit.