Radar System Engineering

662 MOVING TARGET INDICATION [SEC.16.16
Equation (3), giving the allowable rate of frequency drift in the local
oscillator, was based on the assumption that the frequency was changing
at a constant rate. Most frequency deviations are not of this simple sort.
They are more likely to be periodic or random, with a superimposed uniform
drift. The rate of change of frequency can be expressed by a
Fourier spectrum and the total phase error determined by superimposing
the contributions resulting from each of the Fourier components. These
components are not all equally harmful to MTI performance. A frequency
modulation exactly synchronized with the PRF or any multiple
thereof would cause no MTI phase error whatsoever. The phase of
ground clutter at a particular range would be shifted by a given amount
from its value in the absence of the frequency modulation, but the shift
would be constant for successive transmitted pulses.
It has been shown’ that the rate of change of oscillator frequency
averaged over all modulation frequencies can be used in place of the
simple d~/dt of Eq. (3) as a figure of merit for local-oscillator stability.
Table 16.5 shows some typical figures on local-oscillator stability.
16.16. The Coherent Oscillator.-Locking at radio frequent y has
been accomplished at 10 cm, but its feasibility for field service is yet to be
investigated. The relative ease and dependability of i-f locking have
resulted in its use for all coherent microwave systems of the past. This
section will discuss oscillators that receive an i-f locking pulse and deliver
an i-f reference signal according to the arrangement of Fig. 16.9.
The Locking Pulse.—If the phase-locking is to be constant from one
pulse to the next with a precision of 1°, the carrier-frequent y packet that
does the locking must be unusually free of random components. These
may arise from several sources, of which the most likely is the transmitreceive
switch. The gaseous arc discharge in the TR tube generates noiselike
oscillations. These may reach the coherent oscillator by passing
from the signal mixer, via the local oscillator, to the locking-pulse mixer.
The available remedies are the use of loose mixer coupling, lossy localoscillator
cable, and separate pickup loops in the local-oscillator cavity.
Another cause of random components in the locking pulse is excessive
r-f pulse amplitude at the locking mixer. If the r-f pulse exceeds the
level of the local oscillator, self-rectification will take place, yielding a
video pulse that will have noncoherent components at the intermediate
frequency. Even though an effort has been made to reduce the r-f pulse
amplitude, this trouble may persist in a subtle form. If the attenuator
preceding the locking mixer is of the simple ‘‘ waveguide-beyond-cutoff”
variety, it may allow harmonic frequencies, generated by the transmitter,
1S. Roberts, “A Method of Rating the Stability of Oscillators for MTI,” RL
Report No. 819, Oct. 16, 1945.