stant amplitude signal out of the IF amplifier, inde-
pendent of range.
Logarithmic IF Amplifiers
When radars are operated where interference and
clutter are encountered or where jamming may occur,
techniques can be used to adjust the receiver sensi-
tivity as the interference varies in intensity. Digital
processing uses a threshold criteria similar to that
where an operator adjusts the sensitivity to the point
where interference is just barely detectable. This is the
process of constant false-alarm rate (CFAR) when the
radar receiver makes the adjustments automatically.
Logarithmic amplifiers rely on the CFAR principle to
amplify signals that exceed a certain threshold and
provide only unity gain for those below the threshold.
The threshold is automatically adjusted to maintain
the same false-alarm rate.
Logarithmic IF amplifiers are widely used in elec-
tronic attack (EA) receivers and monopulse angle-
tracking channels. They are also used in digital pulse
compression and phase detection.
Detectors
The detector in a basic radar receiver converts the
IF signal into a video signal to be displayed and/or
processed for tracking. There are many forms of
detection, and they vary depending on the type of
radar and method of coherency used.
Detectors include linear (diode) detectors, log-
arithmic detectors, and phase-sensitive detectors.
LINEAR (DIODE) DETECTORS.— The sim-
plest form of detectors, and still commonly used, are
the diode detectors. They are classified as linear de-
tectors because their output is directly proportional to
the input.
LOGARITHMIC DETECTORS.— The loga-
rithmic detectors, commonly called log detectors,
have outputs proportional to the logarithm of the IF
envelope input. Logarithmic detectors are fairly com-
mon in EA receivers. They usually have multiple
stages where the overall gain varies logarithmically.
PHASE-SENSITIVE DETECTORS.— A syn-
chronous/coherent detector for MTI was shown in
figure 2-11. This type of detector has several possible
configurations, based on the type of signal desired as
an output.
A phase (0) detector is one form of synchron-
ous/coherent detector; it has an output that has only
phase information. A synchronous detector has both
phase and amplitude information in its output. A
balanced mixer has phase, amplitude, and frequency
information in its output.
Phase-sensitive detectors (0 detectors) are key
elements in MTI radars. The 0 detector detects the
phase-shift information caused by a moving target.
The detector receives inputs of the IF signal and the
COHO signal, and it produces a video signal whose
amplitude and polarity varies with phase differences
only.
The 0 detector can be used in monopulse-tracking
radar receivers to determine the angular error of the
target from the boresight center. Two identical
receiver channels are required to provide azimuth and
elevation error detection. The angle channel phase is
compared to the range channel reference phase to pro-
duce a signal, whose amplitude and polarity indicate
the amount and direction of phase error, which is
directly related to the angle offset.
A synchronous detector can provide the same
information as a phase detector in monopulse radars.
The difference is that the phase comparison deter-
mines the polarity of the output signal only, while the
amplitude comparison determines the amount of error.
A balanced mixer is more suited for use in CW
and Doppler signal processing since frequency infor-
mation is also contained in the output of balanced
mixers. This signal is then fed to the filter bank for
Doppler shift detection.
2-27
