eXe

Receiver

Basic structure of a weather radar

The receiver shown in the above figure, like most radar receivers, is of the "superheterodyne" type. The first stage of the receiver, called the "front end," may consist of a radio frequency (RF) amplifier as shown. However, most radar receivers have no RF amplifier, and the mixer becomes the front end. The mixer contains a nonlinear element, usually a crystal diode, used to convert the RF echo to a lower frequency called the "intermediate frequency" (IF). This process of converting a signal to another frequency is called "heterodyning." The conversion is made because it is easier to amplify the signal at the lower frequency. The IF output of the mixer contains the same information (in both amplitude and phase) as does the incoming RF signal.

To accomplish the conversion, the mixer must be supplied with a continuous-wave (CW) signal from an RF oscillator called the "local oscillator" (LO). The power required from this oscillator is only a few milliwatts, and it must operate at a frequency close to that of the transmitter. The output of the mixer contains the sum and difference frequencies of the two inputs, but the difference frequency is selected as the intermediate frequency of the receiver. Intermediate frequencies commonly used in radar systems are 30 MHz and 60 MHz.

As the IF amplifier is designed to operate at a fixed frequency, the difference between the transmitter and local oscillator frequencies must be maintained at a constant value equal to the desired IF. In radars with magnetron transmitters, a servomechanism is used to adjust the local oscillator frequency so as to maintain the proper difference. This is called an "automatic frequency control" (AFC) system, and it operates by sampling each transmitted pulse and comparing its frequency with that of the local oscillator. If the difference is not equal to the desired intermediate frequency, the AFC system adjusts the local oscillator frequency to obtain the proper value.

In radars with klystron transmitters, no automatic frequency control is needed because the transmitter frequency is derived from the local oscillator frequency by a heterodyning process that maintains a constant difference between the two.

The waveform shown in the bottom figure (a) represents an echo as it might appear before the signal enters the mixer. The IF component in the output of the mixer is amplified by the IF amplifier (top figure) to a level suitable for subsequent processing. At the output of the IF amplifier, the echo still consists of modulated sine waves, but the frequency of the sinusoidal oscillations is now the intermediate frequency instead of the radio frequency. Moreover, the shape of the "modulation envelope" containing the oscillations is usually altered somewhat because of the response characteristics of the IF amplifier. These changes are illustrated by (b).

The sinusoidal oscillations within the echo in (a) and (b) are of no interest to the user of a pulse radar system, because the information about the presence and location of the target is contained in the amplitude or modulation envelope of the echo.

This envelope is extracted by passing the IF signal through the detector, a process called "demodulation." The output of the detector, illustrated in (c), is called the "video" signal by analogy with the picture information contained in an ordinary television signal. The video signal contains the amplitude information in the original RF echo, but the phase information is lost in the demodulation process. It is "unipolar" that is, only one half (it can be either the positive or the negative half) of the modulation envelope appears in the video. The video signal is amplified as required by a video amplifier as shown in the top figure.

Waveforms of a radar echo at various points in the receiver section (in a μsec interval there are 3000 cycles of a 3 GHz RF sinusoid but only 60 cycles of a 60 MHz IF sinusoid.)