RECEIVERSThe RF echo pulses reflected by a distant objectare similar to the transmitted pulses, but they are con-siderably diminished in amplitude. These minute sig-nals are amplified and converted into video pulses bythe receiver. A voltage amplification of 10/10th isrequired to produce a video pulse of sufficient ampli-tude to intensify the beam of a CRT. The receivermust accomplish this amplification with a minimumintroduction of noise voltages.In addition to having a high-gain and low-noisefigure, the receiver must provide a sufficient band-width to pass the many harmonics contained in thevideo pulses to minimize distortion of the pulses. Thereceiver must also accurately track the transmitter infrequency, since drift diminishes the reception of theecho signal. The receiver tuning range need only beequal to that of the transmitter.This section discusses receiver system functionalareas and radar displays.RECEIVER SYSTEM FUNCTIONAL AREASAs you study this section, refer to figure 2-11 (onpage 22), which is a simple block diagram of radarreceiver general functions based on the superhetero-dyne principle. The superheterodyne receiver is usedexclusively in radar systems.The echo signal enters the system through theantenna. It then passes through the duplexer and isamplified by the low-noise amplifier (LNA). (TWTs,parametric amplifiers, and masers are representativedevices that are used as low-noise, high-gain RF am-plifiers.) When external noise is negligible, the noisegenerated by the input stage of the receiver largelydetermines the receiver sensitivity.In many receivers, an LNA is not used and themixer is the first stage (as indicated by the dashedpath in figure 2-11). The function of the mixer stage,or the first detector, is to translate the RF to a lowerintermediate frequency—usually 30 or 60 MHz—byheterodyning the returning RF signal echo with a localoscillator signal in a nonlinear device (mixer) andextracting the signal component at the difference fre-quency. By using the IF, the necessary gain is easierto obtain than by using the higher RF. It is also easierto develop the response function (or bandpass charac-teristic) of the receiver IF stages.One of the requirements of the radar receiver isthat its internal noise be kept to a minimum. It isimportant, therefore, that the input stages of receiversbe designed with low-noise figures. If the mixer isthe first stage, its crystal characteristics will includelow conversion loss and a low-noise-to-temperature-change ratio. Any noise generated by the local oscil-lator must be kept out of the mixer stage, either by theinsertion of a narrowband filter between the localoscillator and the crystal, or by a balanced mixer.Since the bandwidth of the RF portion of thereceiver is relatively wide, the frequency-responsecharacteristic of the IF amplifier determines the over-all response characteristic of the receiver. It is in thedesign of the IF portion of the receiver that the re-sponse characteristics are accomplished, in the samemanner that the signal-to-noise ratio is accomplished.The receiver system fictional areas discussed inthis section include the automatic frequency controlsystem, local oscillators, frequency synthesizers, radarreceiver mixers, IF amplifiers, gain controls, logarith-mic IF amplifiers, detectors, and pulse compressions.Automatic Frequency Control SystemThe automatic frequency control (AFC) systemnormally used to keep the receiver in tune with thetransmitter is called the difference frequency system.A portion of the transmitter signal is coupled into theAFC mixer and is heterodyned with the local oscil-lator signal. If the transmitter and the receiver arecorrectly in tune, the resultant difference frequencywill be at the correct IF. However, if the receiver isnot in tune with the transmitter, the difference fre-quency will not be correct.Any deviation from the correct IF signal is de-tected by the AFC frequency discriminator, which, inturn, generates an error voltage. The error voltage2-21
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