Basic Continuous-Wave Radar System

Figure 2-2.—Coherent MTI with a phase-locked COHO oscillator. With the development of power amplifier kly- strons, traveling-wave tubes, and crossed-field ampli- fiers, a much better method evolved for coherency. A pulse-power amplifier fed by a stabilized master oscil- lator (STAMO) makes up the transmitted signal. The STAMO signal is mixed with the IF oscillator to pro- vide an RF mixer input. The receiver then detects the Doppler shift and produces the video signal. The video signal from either type of coherent radar is usually bipolar (both positive and negative). The bipolar video that is detected in a coherent receiver is caused by the phase and frequency differ- ences of the return signals. Stationary targets, such as land, produce the same phase/frequency return on each pulse, whereas moving targets produce changing phase returns on each pulse. The MTI systems use pulse-to-pulse cancellation to suppress the stationary target returns by subtracting the previous return from the current return. Figure 2-3 is a diagram of a co- herent MTI with a STAMO oscillator. CONTINUOUS-WAVE RADAR SYSTEMS The continuous-wave radar systems include the basic continuous-wave radar and the FM-CW radar. figure 2-3.—Coherent MTI with a STAMO oscillator. Basic Continuous-Wave Radar System A basic continuous-wave (CW) radar employs a continuous transmission that results in a continuous echo signal from a target. With a continuous echo signal, determination of the target range is impossible, since there is no distinguishing start and stop of the echo signal. Leakage from the transmitter into the receiver has the same form and could be classified as a target. If the target is moving radially with respect to the transmitter, then a shift in frequency occurs. This shift is called the Doppler shift or Doppler frequency. Most CW radars use a separate antenna to receive, since there is no transmit rest period as in pulse radars. This also improves the isolation between the transmitted signal and the received echo signals. Basic CW radars with a single antenna use a fer- rite circulator to act as a duplexer. These circulators are limited to lower-power CW radars, because the amount of leakage from the transmitter to the receiver is about 20 to 30 decibels (dB) down from the trans- mitter power level. For a 1-watt transmitter, this would be 1 milliwatt of leakage. More leakage than this could easily damage the receiver. 2-4