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CW and Frequency Modulated Radar » Doppler Effect » CW Radar - Block Diagram » Isolation between Transmitter and Receiver » Non-zero IF Receiver » Receiver Bandwidth Requirements » Applications of CW radar FM-CW Radar: » Range and Doppler Measurement » Block Diagram and Characteristics » FM-CW altimeter » Multiple Frequency CW Radar. » Illustrative Problems — = ie = ae = ~ »A technique for separating the received signal from the transmitted signal. » When there is relative motion between radar and target is based on recognizing the change in the echo-signal frequency caused by the Doppler effect. »It is well known in the fields of optics and acoustics that if either the source of oscillation or the observer of the oscillation is in motion, an apparent shift in frequency will result. » This is the Doppler effect and is the basis of CW radar. » If R is the distance from the radar to target, the total number of wavelengths A contained in the two-way path between the radar and the target are 2R/A. » Since one wavelength corresponds to an phase angle excursion of 27 radians, the total phase angle excursion @ made by the electromagnetic wave during its transit to and from the target is 42%R/A radians. » If the target is in motion, R and the phase © are continually changing. A change in @ with respect to time is equal to frequency. CW and Frequency Modulated Radar » Doppler Effect » CW Radar - Block Diagram » |solation between Transmitter and Receiver » Non-zero IF Receiver » Receiver Bandwidth Requirements » Applications of CW radar » Illustrative Problems CW Radar - Block Diagram : » Consider the simple CW radar as illustrated by the block diagram of Figure below. (@a)Simple CW radar block diagram : : Frequency (b) response characteristic of beat-frequency amplifier. -— = (oo ~~» The received echo signal at a frequency fo +/- fd enters the radar via the antenna. » It is heterodyned in the detector (mixer) with a portion of the transmitter signal fo to produce a Doppler beat note of frequency fa. » The sign of fa is lost in this process. » The purpose of the Doppler amplifier is to eliminate echoes from stationary targets and to amplify the Doppler echo signal to a level where it can operate an indicating device. — 1h ae di sist » The low-frequency cut-off must be high enough to reject the d-c component caused by stationary targets, but yet it must be low enough to pass the smallest Doppler frequency expected. » Sometimes both conditions cannot be met simultaneously and a compromise is necessary. » The upper cut-off frequency is selected to pass the highest Doppler frequency expected. »The indicator might be a fair of earphones or a frequency meter. —— : SS a a ~~ [Isolation between transmitter and receiver: »In simple CW radars where a single antenna serves the purpose of both transmission and reception. »In principle, a single antenna may be employed since the necessary isolation between the transmitted and the received signals is achieved via = ia in frequency as a result of the Doppler Effect. »In practice, it is not possible to eliminate completely the transmitter leakage. » However, transmitter leakage is neither always undesirable. » A moderate amount of leakage entering the receiver along with the echo signal supplies the reference necessary for the detection of the Doppler frequency shift. » Ifa leakage signal of sufficient magnitude were not present, a sample of the transmitted signal has to be deliberately introduced into the receiver to provide the necessary reference frequency. Antenna fe =! Oseillating . Firin | i q era iF detector | fy Aripitier circuit Detonator ——— a= a =Timitation of Zero IF receiver: »Receivers of this type are called homodyne receivers, or super heterodyne receivers with zero IF. »The function of the local oscillator is replaced by the leakage signal from the transmitter. » This simpler receiver is not very sensitive because of increased noise at the lower IF caused by flicker effect. » Flicker-effect noise occurs in semiconductor devices such as diode detectors and cathodes of vacuum tubes. ———— — EE > The noise power produced by the flicker effect varies as 1/fa where « is approximately unity. » This is in contrast to shot noise or thermal noise, which is independent of frequency. » Thus, at the lower range of frequencies (audio or video region), where the Doppler frequencies usually are found. » The detector of the CW receiver can introduce a considerable amount of flicker noise. ae : ~ Non-zero IF Receiver : » The effects of flicker noise are overcome in the normal super heterodyne receiver by using an intermediate frequency. » This results from the inverse frequency dependence of flicker noise. »Instead of the usual local oscillator found in the conventional super heterodyne receiver. » The local oscillator is derived in the receiver from a portion of the transmitted signal mixed with a locally generated signal of frequency equal to that of the receiver IF. » The block diagram of a CW radar whose receiver operates with a nonzero IF is shown in figure. Teoreeetiig ontenno cw +34 transmitter fo fo + F Mi | Oscillator iner Ty fy ] Vos tot fir fo~ fie) Sideband filter Receiving fot fir antenna IF Receiver F het amplifier mixer htt Block diagram of CW doppler radar with nonzero IF receiver, sometimes called sideband superheteradyne. 
