Radar System Engineering

SEC. 17.6] THE PHASE-SHIFT METHOD 695
to its delay time. If the modulator is self-synchronous, its firing time
must coincide with the first coded pulse. Arangeerror in the indicators
results unless a compensating delay line is used in the video channel at one
station or the other.
If the appearance of the azimuth pulse at short range on the indicators
is not tolerable, a triggered modulator can be fired after the azimuth
pulse, the delay from the third coded pulse being precisely fixed. At the
receiver a similar delay initiated by the decoded basic pulse can be
inserted ‘ahead of the indicators. Correct adjustment of this delay can
easily be made by observing the transmitter pulse appearing with the
video signals.
No analogous method of azimuth-pulse removal exists for a selfsynchronous
modulator. It is usually satisfactory to remove the pulse by
anticoincidence methods, even though a “hole” is left in the video signal
train. Otherwise, it will be necessary to introduce between the modulator
pulse and the basic pulse a delay which is longer than the useful videosignal
interval and to transmit both pulses. 1
17.6. The Phase-shift Method.-If a sine wave is passed through a
linear full-wave phase-shifting device connected directly to the scanner,
the resulting phase shift is numerically equal to the scan angle measured
from the position of zero phase shift. By transmitting the phase-shifted
wave together with a reference signal of commensurate frequency and
fixed phase, it is possible to use the relative phases as data from which
to reconstruct the scanner angle.
C-w Methods.—A method of accomplishing this by use of c-w signals
is illustrated in Fig. 17.6. Separate subcarriers fl and f, are respectively
modulated by the phase-shifted sinusoid and by the reference signal,
which is the same sinusoid without phase shift. At the receiving end the
video signals and each of the subcarriers are separated by appropriate
filters and the phase-shifted and the reference sinusoids are obtained by
suitable demodulators. The reference wave is then passed through a
phase shifter similar to that on the scanner. This phase shifter is driven
by a servomechanism whose error signal is the output of a circuit which
compares the phase of the shifted wave and that of the reference wave.
A data transmitter geared to the phase shifter provides proper information
to the indicators.
The above method is very expensive because of the two subcarriers.
Fig. 17.7 illustrates an alternative c-w method which avoids this difficulty.
Here the phase-shifted signal is a harmonic of the reference signal and is
derived from it by a frequency multiplier. Thus the two signals can be
transmitted without confusion on the same subcarrier. At the receiving
1It is not satisfactoryto use such a long delay between the basic pulse and the
azimuth pulse since small percentages of “jitter” or drift become too important.