In studying the radar receiver requirements, we will first examine the overall needs of a radar receiver. Second, we will examine a typical radar receiver that satisfies these re-quirements. Finally, we will discuss the individual components of the receiver.
The following characteristics determine the design of effective radar receiver requirements .
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 the radar receiver requirements are 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 radar receiver requirements 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 10 8. 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.
Radar receiver requirements for tuning are a
limited 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 radar receiver requirements 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 receiver requirements. A typical superheterodyne radar receiver is shown in the figure below. 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.
Typical superheterodyne radar receiver.
The MIXER stage in radar receiver requirements 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.