Figure 2-2.—Coherent MTI with a phase-locked
COHO oscillator.
With the development of power amplifier kly-
strons, traveling-wave tubes, and crossed-field ampli-
fiers, a much better method evolved for coherency. A
pulse-power amplifier fed by a stabilized master oscil-
lator (STAMO) makes up the transmitted signal. The
STAMO signal is mixed with the IF oscillator to pro-
vide an RF mixer input. The receiver then detects the
Doppler shift and produces the video signal. The
video signal from either type of coherent radar is
usually bipolar (both positive and negative).
The bipolar video that is detected in a coherent
receiver is caused by the phase and frequency differ-
ences of the return signals. Stationary targets, such as
land, produce the same phase/frequency return on
each pulse, whereas moving targets produce changing
phase returns on each pulse. The MTI systems use
pulse-to-pulse cancellation to suppress the stationary
target returns by subtracting the previous return from
the current return. Figure 2-3 is a diagram of a co-
herent MTI with a STAMO oscillator.
CONTINUOUS-WAVE RADAR SYSTEMS
The continuous-wave radar systems include the
basic continuous-wave radar and the FM-CW radar.
figure 2-3.—Coherent MTI with a STAMO
oscillator.
Basic Continuous-Wave Radar System
A basic continuous-wave (CW) radar employs a
continuous transmission that results in a continuous
echo signal from a target. With a continuous echo
signal, determination of the target range is impossible,
since there is no distinguishing start and stop of the
echo signal. Leakage from the transmitter into the
receiver has the same form and could be classified as
a target. If the target is moving radially with respect to
the transmitter, then a shift in frequency occurs. This
shift is called the Doppler shift or Doppler frequency.
Most CW radars use a separate antenna to receive,
since there is no transmit rest period as in pulse
radars. This also improves the isolation between the
transmitted signal and the received echo signals.
Basic CW radars with a single antenna use a fer-
rite circulator to act as a duplexer. These circulators
are limited to lower-power CW radars, because the
amount of leakage from the transmitter to the receiver
is about 20 to 30 decibels (dB) down from the trans-
mitter power level. For a 1-watt transmitter, this
would be 1 milliwatt of leakage. More leakage than
this could easily damage the receiver.
2-4
