Radar System Engineering

672 MOVING TARGET INDICATION [SEC.16.19
tion. This difficulty can be avoided by operating at a carrier frequency
that is far off crystal resonance but still on the flat part of the loaded
characteristic.
16.19. Delay-line Signal Circuits.-The delay line is a low-impedance
device with a large attenuation; it is mechanically awkward to mount the
line physically close to vacuum-tube circuits. These facts combine to
make the delay process electrical y expensive. The subject of delay-line
signal circuits has been treated at length in another book. 1 It will be
reviewed here.
fine-driving Circuits.-The requirement that the signal output shall
be well above the statistical noise level generally means that considerable
power must be furnished to the transmitting crystal.
If an i-f signal from the receiver is to drive the line, amplification is
required. Otherwise, a carrier frequency must be modulated with a
video signal. This problem is similar to that of the television transmitter.
Only a reasonable degree of linearity is required; the response slope should
not vary by more than perhaps 15 per cent. The real difficulty arises
from the proximity of the carrier and video frequencies and from the
requirement that the delayed and undelayed video signals must cancel
to 1 per cent in amplitude.
In simple modulators, modulation components are removed from
the modulated carrier channel by frequency discrimination. If the
modulation spectrum extends to more than half the carrier frequency,
spurious transients in the carrier pass band may arise in two ways.
The modulation spectrum may have components that lie in the carrierchannel
pass band, or modulation components may add to carrier sideband
components to give components lying within the carrier-channel
pass band. Further, spurious components may arise from frequency
modulation of the carrier oscillator by the video signal. Since the
carrier phase is random from pulse to pulse, the envelope of the summation
of all such components within the carrier pass band will vary from
pulse to pulse so that cancellation will be imperfect. These difficulties
can be avoided by the balanced modulation of a carrier amplifier driven
by an isolated oscillator. A common procedure is to use parallel-grid
modulation of a push-pull amplifier stage. Alternatively, the problem
might be solved by phase-locking the carrier oscillator to the transmitted
pulse trigger, thereby insuring a constant relative phase between each
echo and the carrier. Oscillator isolation would still be necessary, since
otherwise a strong moving target might destroy the phase coherence.
The Two-channel Amplifier.-The signal level at the input terminals
of the two-channel amplifier must be great enough so that the statistical
noise generated therein is considerably smaller than the statistical noise
1vol. 20.