2-48
(A + C) - (B + D)
and mixing the difference with the local oscillator signal. The result is a bearing IF signal. Elevation
information is obtained in the same way, except the energy from horns C and D is subtracted from the
energy from horns A and B:
(A + B) - (C + D)
If the target is on the elevation and bearing axis, the summations will both be zero; therefore, neither
the bearing nor elevation channels will receive an input signal. If either of the bearing or elevation signals
is off the axis, an input to the IF channel is produced. This input is subsequently converted to an IF signal
in the appropriate channel.
The major difference between the monopulse receiver and the conventional receiver is the
requirement for a dc error voltage output from the bearing and elevation channels. The range channel of a
monopulse receiver is sent to a conventional ranging circuit for presentation are on an indicator or for use
by a range-tracking circuit. However, since most monopulse radars are automatic tracking radars, the
outputs of the bearing and elevation channels must be converted to dc error signals for use by automatic
bearing and elevation tracking systems. The dc error voltages are applied to the antenna bearing and
elevation positioning servos. These servos reposition the antenna until the errors are nulled.
The phase detectors compare the phase of the bearing and elevation IF with a reference IF from the
range channels. This comparison produces the dc error pulses needed to drive the antenna servos. The
signals from both the bearing and elevation channels are the result of a summation process. They can be
either positive (in-phase) or negative (180-degrees out of phase) when compared to the reference IF
signal. For example, if the output of horns A and C is smaller than the output of horns B and D, a negative
or 180-degree-out-of-phase signal is produced by the bearing channel (A + C) - (B + D). If output A + C
is greater than output B + D, a positive or in-phase signal is produced by the bearing channel. The phase
of the bearing and elevation output signals determines the direction in which the antenna moves; the
magnitude of the signal determines the amount of movement. Since two signals must be present at the
phase detector to produce an output, an error signal occurs only when a return echo is not on the antenna
beam axis.
This technique produces an error signal when the target moves off the radiated beam axis in either
bearing or elevation. The error signal causes the antenna to move in the proper direction and for the
proper duration to cancel the error signal. This method of automatic tracking is commonly used by
weapons-control tracking radar systems.
Q47. When a large signal and a small signal are applied to a lin-log amplifier at the same time, what
is the effect on the small signal?
Q48. What happens to the overall gain of a lin-log amplifier as each stage saturates?
Q49. A monopulse receiver has how many separate channels?
Q50. If a target is on the bearing axis of the radiated beam, what is the input to the bearing IF
channel?
Q51. What characteristic of the bearing and elevation output signals determines the direction of
antenna movement?
