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 frequencyusually 30 or 60 MHzby
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