The energy that a distant object reflects back to the antenna in a radar system is a very small fraction of the original transmitted energy. The echoes return as pulses of rf energy of the same nature as those sent out by the transmitter. However, the power of a return pulse is measured in fractions of microwatts instead of in kilowatts, and the voltage arriving at the antenna is in the range of microvolts instead of kilovolts. The radar receiver collects those pulses and provides a visual display of object information.
Information about the position of the object is present visually when the reception of an echo causes the movement or appearance of a spot of light on a cathode-ray tube (crt). The crt requires a signal on the order of at least several volts for proper operation and will not respond to the high frequencies within a return pulse. Therefore, a receiver amplifier and detector must be used that are capable of producing a visible indication on the cathode-ray tube under the following conditions: (1) when the input signal to the amplifier is in the form of pulses of extremely high-frequency, (2) the amplitude of the pulses is in the microvolt range, and (3) the pulses last for only a few microseconds.
The radar receiver evolved directly from the simple radio receiver. The radar receiver operates on exactly the same principles as the radio receiver. However, the overall requirements and limitations of a radar receiver differ somewhat from those of a radio receiver because of the higher frequencies involved and the greater sensitivity desired.
In studying the radar receiver, we will first examine the overall requirements of a radar receiver. Second, we will examine a typical radar receiver that satisfies these requirements. Finally, we will discuss the individual components of the receiver.
RADAR RECEIVER REQUIREMENTS
The following characteristics determine the design requirements of an effective radar receiver:
The word NOISE is a carryover from sound-communications equipment terminology. Noise voltages in sound equipment produce actual noise in the loudspeaker output. In radar, noise voltages result in erratic, random deflection or intensity of the indicator sweep that can mask small return signals.
Were it not for noise, the maximum range at which an object would be detectable by radar could be extended almost infinitely. Objects at great range return exceedingly small echoes. However, without noise, almost any signal could be amplified to a usable level if enough stages were added to the receiver. Because of noise, the signal detection limit or sensitivity level of a receiver is reached when the signal level falls below the noise level to such an extent as to be obscured. A simple increase of amplification is of no help because both signal and noise are amplified at the same rate.
In the radar portion of the rf spectrum, external sources of noise interference are usually negligible; consequently, the sensitivity that can be achieved in a radar receiver is usually determined by the noise produced in the receiver. Not only must noise be kept down, but everything possible must be done to minimize attenuation of the video signal (echo) before it is amplified.
The GAIN of a radar receiver must be very high. This is because the strength of the signal at the antenna is at a level of microvolts and the required output to the indicator is several volts. The gain of a radar receiver is roughly in the range of 106 to 108. FEEDBACK, or REGENERATION, is one of the most serious difficulties in the design of an amplifier with such high gain. Special precautions must be taken to avoid feedback. Such precautions include careful shielding, decoupling (isolation) between voltage supplies for the different tubes, and amplification at different frequencies in separate groups of stages.
The radar receiver requires a limited tuning range to compensate for transmitter and local oscillator frequency changes because of variations in temperature and loading. Microwave radar receivers usually use automatic frequency control (afc) for this purpose.
If distortion occurs in the receiver, the time interval between the transmitted pulse and the received pulse changes, thereby affecting range accuracy.
BLOCKING refers to a condition of the receiver in which the voltage pulse at the receiver input is too large. As a result, for a short time after the pulse, the receiver is insensitive or blocked to signals below a certain level. This condition results from one or more of the amplifier stages in the receiver being overdriven. After a strong pulse, the receiver may be biased to a point at which it will not amplify small signals. Recovery after blocking may be only a fraction of a microsecond, or it may take several hundred microseconds, depending upon the point in the receiver at which blocking occurs. To detect a weak echo immediately following a strong one, the receiver must have a short BLOCKING RECOVERY TIME. The blocking itself must be minimized as much as possible. If a portion of the transmitted pulse leaks into the receiver input, then the receiver may be blocked and not show small, nearby objects. In most receivers, blocking is minimized from this cause by a duplexer. The duplexer protects the receiver by isolating it during the transmitted pulse.
RECEIVER BLOCK DIAGRAM
The SUPERHETERODYNE receiver is almost always used in microwave radar systems. A typical superheterodyne radar receiver is shown in figure 2-23. A receiver of this type meets all the requirements listed above. Signals from the antenna enter the receiver via the duplexer. A low-noise rf amplifier is usually the first stage of modern radar receivers. Some receivers, however, send the antenna signal directly to the mixer, as shown by the dashed path. The low-noise amplifiers used in modern systems are usually solid-state devices, such as tunnel-diode, parametric, or microwave transistor amplifiers.
Figure 2-23. - Typical superheterodyne radar receiver.
The MIXER stage is often called the FIRST DETECTOR. The function of this stage is to convert the received rf energy to a lower, intermediate frequency (IF) that is easier to amplify and manipulate electronically. The intermediate frequency is usually 30 or 60 megahertz. It is obtained by heterodyning the received signal with a local-oscillator signal in the mixer stage. The mixer stage converts the received signal to the lower IF signal without distorting the data on the received signal.
After conversion to the intermediate frequency, the signal is amplified in several IF AMPLIFIER stages. Most of the gain of the receiver is developed in the IF amplifier stages. The overall bandwidth of the receiver is often determined by the bandwidth of the IF stages.
The output of the IF amplifiers is applied to the SECOND DETECTOR. It is then rectified and passed through one or more stages of amplification in the video amplifier(s). The output stage of the receiver is normally an emitter follower. The low-impedance output of the emitter follower matches the impedance of the cable. The video pulses are coupled through the cable to the indicator for video display on the crt.
As in all superheterodyne receivers, controlling the frequency of the local oscillator keeps the receiver tuned. Since this tuning is critical, some form of automatic frequency control (afc) is essential to avoid constant manual tuning. Automatic frequency control circuits mix an attenuated portion of the transmitted signal with the local oscillator signal to form an IF signal. This signal is applied to a frequency-sensitive discriminator that produces an output voltage proportional in amplitude and polarity to any change in IF frequency. If the IF signal is at the discriminator center frequency, no discriminator output occurs. The center frequency of the discriminator is essentially a reference frequency for the IF signal. The output of the DISCRIMINATOR provides a control voltage to maintain the local oscillator at the correct frequency.
Different receiving systems may vary in the type of coupling between stages, the type of mixer, the detector, the local oscillator, and the number of stages of amplification at the different frequencies. However, the receiver is always designed to have as little noise as possible. It is also designed to have sufficient gain so that noise, rather than lack of gain, limits the smallest visible signal.
This section will analyze in more detail the operation of the receiver circuits mentioned above. The circuits discussed are usually found in some form in all radar superheterodyne receivers.
LOW-NOISE AMPLIFIERS, sometimes called PREAMPS, are found in most modern radar receivers. As previously mentioned, these amplifiers are usually solid-state microwave amplifiers. The most common types are tunnel diode and parametric amplifiers. These amplifiers are discussed in detail in NEETS, Module 11, Microwave Principles. Some older systems may still use a traveling-wave tube (twt) as a low-noise first stage amplifier. However, the solid-state amplifiers produce lower noise levels and more gain.
Most radar receivers use a 30 or 60 megahertz intermediate frequency. The IF is produced by mixing a local oscillator signal with the incoming signal. The local oscillator is, therefore, essential to efficient operation and must be both tunable and very stable. For example, if the local oscillator frequency is 3,000 megahertz, a frequency change of 0.1 percent will produce a frequency shift of 3 megahertz. This is equal to the bandwidth of most receivers and would greatly decrease receiver gain.
The power output requirement for most local oscillators is small (20 to 50 milliwatts) because most receivers use crystal mixers that require very little power.
The local oscillator output frequency must be tunable over a range of several megahertz in the 4,000-megahertz region. The local oscillator must compensate for any changes in the transmitted frequency and maintain a constant 30 or 60 megahertz difference between the oscillator and the transmitter frequency. A local oscillator that can be tuned by varying the applied voltage is most desirable.
The REFLEX KLYSTRON is often used as a local oscillator because it meets all the requirements mentioned above. The reflex klystron is a very stable microwave oscillator that can be tuned by changing the repeller voltage.
Most radar systems use an automatic frequency control (afc) circuit to control the output of the local oscillator. A block diagram of a typical afc circuit is included in figure 2-23. Note that the afc circuits form a closed loop. This circuit is, in fact, often called the afc loop.
A sample of the transmitter energy is fed through the afc mixer and an IF amplifier to a discriminator. The output of the discriminator is a dc error voltage that indicates the degree of mistuning between the transmitter and the local oscillator. In this particular example let's assume that the IF is 30 megahertz. If the output of the mixer is correct, the discriminator will have no output. If the mixer output is above 30 megahertz, the output of the discriminator will be positive dc pulses; if the mixer output is below 30 megahertz, the discriminator output will be negative dc pulses. In either case, this output is fed through an amplifier to the control circuit. The control circuit adjusts the operating frequency of the local oscillator so that no mistuning exists and the IF is 30 megahertz. In this example the local oscillator is a reflex klystron and the control circuit provides he repeller plate voltage for the klystron; thus, the klystron directly controls the local oscillator frequency. In this manner the local oscillator is maintained exactly 30 megahertz below the transmitter frequency.
Q.34 What is the greatest limiting factor in a receiver's detectable range?