Radar System Engineering

688 RADAR RELAY [SEC. 174
long signals from straddling two or more pulse intervals and thus providing
false coincidences. It cannot, of course, prevent such blocks of
signals from saturating the receiver and excludlng the desired pulses.
A brief analysis of the effectiveness of the coding is worth while.
Periodic pulses cannot give a response from V, unless their frequency is
greater than the reciprocal of the delay times in the coding. Random
pulses or noise do, however, have a finite probability of initiating a
response. To calculate thk probability suppose that there are n random
pulses per second and let r be their length. Then the fractional time
during which the grid of Vdbis receiving signals through the delay line is m.
The probability that a signal from a given undelayed pulse will overlap
that from the delayed pulses is therefore Zrm, the factor 2 entering on the
assumption that any overlap at all will produce a result. Thus the
number of reinforced signals reaching the grid of Vi each second is 2n%.
Since the probability that one of these will coincide with a pulse on the
suppressor grid is n7,1 the total number of pulses triggering V, each
second is 2 n’rz. This is to be compared with the number n of interfering
signals originally present, the ratio of improvement being
n/2n3r2
= l/2n2r2.
If, for example, r = 1 psec and n = 10,000, this ratio is 5000. The ratio
improves for smaller n and vice versa.
If the signal being relayed is the firing time of the modulator, the
firing should, if possible, coincide with the transmission of the third pulse.
If the modulator can be triggered with sufficient accuracy, the output
connection indicated in -Fig. 17.2 can be used for this purpose. If, on
the other hand, the modulator is self-synchronous, its pulse must be used
to trigger the coder. The synchronizing pulse at the receiving station,
which must await the arrival of the third pulse, will be 6 ~sec late. If
this error cannot be tolerated, the video signals must be passed through a
6-psec delay line before they reach the indicators.
Figure 17.3 also illustrates one method for excluding all signals
except during an interval surrounding the expected time of arrival of the
desired signal. The coincidence pulse from the plate of Vi is passed, after
buffering in Vw, through a 2-Asec delay line, and is used to trigger the
flip-flop circuit formed by V* and V6b (Sec. 13.7), in which circuit
the latter tube is normally off. After the flip-flop has been triggered, the
current through VW lifts the cathode of Via, cutting this tube off by an
amount greater than the signal level. The flip-flop is timed to reverse
shortly before the next desired signal is expected. 1’1~ prevents an
appreciable pedestal from appearing on the common plate circuit
since it is turned on when VI. is turned off and vice versa. If the signals
1The factor 2 does not enteragain.