Radar System Engineering

SEC.12.3] SPECIAL PROBLEMS IN RADAR RECEIVERS 441
If elevation angle is substituted for azimuth, the device becomes an
RHI. Usually, in this case, the gain of the vertical amplifier is made
greater than that of the horizontal, so that the display is “stretched” in
the vertical direction. Since direct-coupled push-pull sweep amplifiers
are usually used, they can be biased in such a way as to produce the
“off-centering” shown.
THE RECEIVER
BY W. H. JORDAN
12.3. Special Problems in Radar Receivers.—Radar receivers,
though similar inprinciple toall radio receivers, differ from them in some
respects. This fact ischiefly duetoadifference inemphasison some of
the functions. For example, it would be foolish to design a broadcast
receiver with the ultimate in sensitivity when the weakest signal that can
be detected is determined largely by man-made and natural static. In
the radar portion of the spectrum, on the other hand, external sources of
interference are normally negligible, and consequently the sensitivity
that can be achieved in a radar receiver is normally determined by the
noise produced in the receiver itself. Methods of reducing this noise are
of prime importance in radar receiver design. Not only must noise be
kept down, but everything possible must be done to minimize attenuation
of the signal before it is amplified. How this has influenced the
design of r-f components has already been seen. The effect on the design
of i-f amplifiers will be developed in this section.
The lack of low-noise r-f amplifiers or converters in the microwave
region has meant that most microwave receivers convert the r-f signal
to an i-f signal directly in a crystal mixer and then amplify the i-f signal.
This lack of previous amplification complicates the design of the i-f
amplifier, as is shown by the following expression, given in Chap. 2, for
the over-all receiver noise figure.
N..,rd, = ~ (Ni-[ + T’ – 1)1
(1)
where
g = gain of the converter,
T = noise temperature of the converter,
Ni., = noise figure of the i-f amplifier.
A good crystal in a well-designed mixer will have a noise temperature
T that is only slightly more than 1. This means that the over-all noise
figure is reduced in direct proportion to the reduction in i-f noise figure.
Hence it is important to design the i-f amplifier to have a noise figure that
approaches as nearly as possible the theoretically perfect value of 1.