Radar System Engineering

632 MOVING TARGET INDICATION [SEC. 16.3
repetition rate has “time jitter” (see Vol. 19, Chap. 25 of the Radiation
Laboratory Series).
16.3. A Practical MTI System.—Figure 169 is a block diagram of a
practical ,MTI system. It can be regarded as a refinement of the arrangement
of Fig. 16.6; instead of beating the signals with an r-f reference
signal, the same thing is now done at intermediate frequency. This is
accomplished by applying the superheterodyne principle both to the
locking pulse and to the signals. The same stable local oscillator provides
a signal to two mixers, one of which reduces the frequency of the
locking pulse, the other the frequency of the signals, to 30 Me/see.
The i-f locking pulse from the first mixer is applied to an i-f oscillator
which is thereby rendered coherent in phase with the locking pulse.
Transmitter TR switch }
R.f
locking pulse
1
R.f signals
Modulator Mixer — Stable local
oscillator
Mixer
Pfl
I
Locking pulse l.f ignals
Trigger Coherent [f Cw reference
Receiver
generator
oscillator
signal
!
FIG. 16.9.—Practical MT1 system.
The circuits of Figs. 16.6 and 169 are entirely equivalent as far as the
output of the receiver is concerned. The phase of the i-f echo signal
from a stationary target depends on the starting phase of the transmitter,
the starting phase of the local oscillator, and the range (which determines
the number of cycles executed by the local oscillator while the transmitted
pulse travels to the target and back). The i-f reference signal provided
by the coherent oscillator has a phase that depends on the starting
phases of the tr nsmitter and of the local oscillator, and on the range
(which determines the number of cycles executed by the coherent oscillator
during the echo-time). The starting phases of the transmitter and
the local oscillator cancel out when the i-f echo signals and the i-f reference
signal beat against each other, so that the phase of the receiver output
depends only on the number of cycles executed by the local oscillator
and by the coherent oscillator. Both of these oscillators are made to be
stable; consequently the beat signal from a stationary target has a
steady amplitude from pulse to pulse. When the target is moving, its
range will change from pulse to pulse and a fluctuating output signal
results from the corresponding change in the phases traversed by the
oscillators during the echo-time.