2-31
Noise
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.
Gain
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 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.
Tuning
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.
Distortion
If distortion occurs in the receiver, the time interval between the transmitted pulse and the received
pulse changes, thereby affecting range accuracy.
Blocking
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.
