4-10
maintenance are considered. Important factors are (1) receiver sensitivity, which includes noise figure
determination and minimum discernible signal (mds) measurement; (2) tr recovery time; and (3) receiver
bandwidth.
Many radar systems contain circuits that serve special functions. Three of these special circuits are
instantaneous automatic gain control (iagc), sensitivity time control (stc), and fast time constant (ftc).
These circuits may be found in combination or alone, depending on the purpose of the radar. When the
test methods and procedures about to be described are used, these special functions should not be used. If
an automatic frequency control (afc) circuit is included in the radar, it may be permitted to operate during
receiver tests. A good way you can check afc circuit operation is to complete the tests specified for
manual tuning and then switch to afc. If the afc circuit operation is normal, test indications should not
differ.
RECEIVER SENSITIVITY
Insufficient detection range in a radar system can be caused by decreased sensitivity in the radar
receiver. This condition results mainly from the great number of adjustments and components associated
with the receiver. A decrease of receiver sensitivity has the same effect on range performance as does a
decrease of transmitter power. For example, a 6 dB loss of receiver sensitivity shortens the effective range
of a radar just as much as a 6 dB loss in transmitter power. Such a drop in transmitter power is evident
and is easy to detect. On the other hand, a 6 dB loss in receiver sensitivity, which can easily result from a
slight misadjustment in the receiver, is difficult to detect unless accurate measurements are made.
Figure 4-8 shows a comparison of radar system performance versus maximum range. The system
performance loss in dB includes both transmitter and receiver losses. You should note that with a loss of 5
dB in both receiver and transmitter (a total of 10 dB), only 55 percent of the maximum range of the
system is realized.
