Radar System Engineering

SEC. 17.8] PULSE METHOD FOR RELAYING SINE AND COSINE 707
separate channel. There must now be developed a d-c voltage whose
value is at every instant proportional to the delay of the sine pulse with
respect to the basic pulse. Two of the many possible methods of accomplishing
this are illustrated. The components of Fig. 17.13a drawn in solid
lines illustrate a very simple method. The basic pulse triggers a flip-flop
having a lifetime slightly greater than the maximum delay of the sine
pulse; the resulting square wave is used to switch a sawtooth generator of
very low output impedance. Thus, within the life of the sawtooth, the
instantaneous voltage at S is proportional to time elapsed since the
occurrence of the basic pulse. When the sine pulse occurs, it momentarily
closes the “double clamp” (similar to Fig. 1326), thus connecting
point T tightly to point S so that the condenser is charged to the instantaneous
potential of S. The leakage path from T to ground is made to
have a high resistance so that the potential at T remains essentially constant
until the new cycle, at which time it will take a new value corresponding
to the new value of the time delay of the sine pulse. Thus the
potential at T will go through the same variations as the time delay and
will in the present case have the form A + B sin o as desired. This very
simple method is satisfactory provided little or no interference is encountered.
An interfering pulse will, however, cause T to take a potential
corresponding to its time of appearance. This effect can be reduced by
filtering so that no single pulse can cause any very great change, but this
filtering may cause troublesome phase lags in the desired output. A
better method of protection is to employ the output voltage to control the
opening of a switch (dotted circuits in Fig. 17.13a), through which the sine
pulse must pass, in such a way that a pulse can be admitted only during a
very narrow time interval including the time when the true pulse is
expected. (Since this device as described will work only when the tracking
is already nearly right, some means must be provided for holding the
switch open until correct conditions are once established. ) With the
addition of this protective switch, the method of Fig. 17.13a is satisfactory.
The addition makes it, however, nearly as complex as more
elegant methods of regenerative tracking. Regenerative tracking,
although more complex, provides certain very definite advantages in
compensating for errors occurring later in the circuit.
The elements of a regenerative tracking circuit are shown in Fig.
17.13b. The basic pulse triggers a delay circuit which is controlled by the
final output voltage in such a way that, assuming this voltage is right, the
delay circuit produces a pulse shortly before the arrival of the sine pulse.
The delay-circuit pulse triggers a flopover (Sec. 13.7) which in turn
sends a pulse down an improperly terminated delay line. The reflected
pulse, of opposite polarity, turns off the flopover. The resulting square