RECEIVER SYSTEM FUNCTIONAL AREAS

RECEIVERS The RF echo pulses reflected by a distant object are similar to the transmitted pulses, but they are con- siderably diminished in amplitude. These minute sig- nals are amplified and converted into video pulses by the receiver. A voltage amplification of 10/10th is required to produce a video pulse of sufficient ampli- tude to intensify the beam of a CRT. The receiver must accomplish this amplification with a minimum introduction of noise voltages. In addition to having a high-gain and low-noise figure, the receiver must provide a sufficient band- width to pass the many harmonics contained in the video pulses to minimize distortion of the pulses. The receiver must also accurately track the transmitter in frequency, since drift diminishes the reception of the echo signal. The receiver tuning range need only be equal to that of the transmitter. This section discusses receiver system functional areas and radar displays. RECEIVER SYSTEM FUNCTIONAL AREAS As you study this section, refer to figure 2-11 (on page 22), which is a simple block diagram of radar receiver general functions based on the superhetero- dyne principle. The superheterodyne receiver is used exclusively in radar systems. The echo signal enters the system through the antenna. It then passes through the duplexer and is amplified by the low-noise amplifier (LNA). (TWTs, parametric amplifiers, and masers are representative devices that are used as low-noise, high-gain RF am- plifiers.) When external noise is negligible, the noise generated by the input stage of the receiver largely determines the receiver sensitivity. In many receivers, an LNA is not used and the mixer is the first stage (as indicated by the dashed path in figure 2-11). The function of the mixer stage, or the first detector, is to translate the RF to a lower intermediate frequency—usually 30 or 60 MHz—by heterodyning the returning RF signal echo with a local oscillator signal in a nonlinear device (mixer) and extracting the signal component at the difference fre- quency. By using the IF, the necessary gain is easier to obtain than by using the higher RF. It is also easier to develop the response function (or bandpass charac- teristic) of the receiver IF stages. One of the requirements of the radar receiver is that its internal noise be kept to a minimum. It is important, therefore, that the input stages of receivers be designed with low-noise figures. If the mixer is the first stage, its crystal characteristics will include low 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 the insertion of a narrowband filter between the local oscillator and the crystal, or by a balanced mixer. Since the bandwidth of the RF portion of the receiver is relatively wide, the frequency-response characteristic of the IF amplifier determines the over- all response characteristic of the receiver. It is in the design of the IF portion of the receiver that the re- sponse characteristics are accomplished, in the same manner that the signal-to-noise ratio is accomplished. The receiver system fictional areas discussed in this section include the automatic frequency control system, local oscillators, frequency synthesizers, radar receiver mixers, IF amplifiers, gain controls, logarith- mic IF amplifiers, detectors, and pulse compressions. Automatic Frequency Control System The automatic frequency control (AFC) system normally used to keep the receiver in tune with the transmitter is called the difference frequency system. A portion of the transmitter signal is coupled into the AFC mixer and is heterodyned with the local oscil- lator signal. If the transmitter and the receiver are correctly in tune, the resultant difference frequency will be at the correct IF. However, if the receiver is not 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, in turn, generates an error voltage. The error voltage 2-21