Radar System Engineering

452 THE RECEIVING SYSTEM—RADAR RECEIVERS [SEC.12.6
in theresponec curve. This value of M increases the cutoff frequency
to a value of 1.73 times that of the simple RC circuit, but produces a 2.5
per cent overshoot on the pulse (the same amount as that produced by a
transitionally coupled i-f stage).
Still greater bandwidths can be obtained by the use of series peaking’
or a combination of series and shunt peaking. These circuits are more
difficult to use, however, and the reader is referred to Vol. 18 of the Radiation
Laboratory
Series for details.
In order to preserve good response to long pulses, it is necessary that
the time constant of the coupling circuit, C,(RL + R.), be kept large compared
to the duration of the longest blocks of signals involved. In this
event, the fractional amount of droop on the top of the long pulse is just
the ratio of the pulse length to the coupling time constant. Thus if the
time constant were 50,000 Pscc while the pulse length was 1000 l,sec, the
pulse would droop 2 per cent. This is a reasonably small amount, but
if there were five such circuits in cascade between the detector and the
CRT, then the total amount of droop would be 10 per cent. Although
it is fairly easy to maintain such long time constants provided RL + R*
is large, occasionally it happens that RL + Rg is fairly small. This
means that C. must be large, which may result in a large capacity to
ground and affect the high-frequency response. For example, a l-~f
condenser may have several hundred micromicrofarads of capacity to its
shell. Mounting the condenser on insulating posts is frequently necessary
in such cases.
Another way of reducing the amount of droop on the pulse is by using
low-frequency compensation circuits. In Fig. 12”10, if R, and C’. are
chosen properly, the beginning of the pulse will be flat and the time for the
pulse to fall to 90 per cent will be greatly increased. The value of C,
must be chosen to satisfy the relation
C.RP = C.R,. (13)
The value of R, is not specified, ,but the amount of droop becomes less
as R, is increased. However, even for R, = R. the time for the pulse to
fall to 90 per cent is increased by a factor of 5 over the uncompensated
network.
The limiting of video output signal level is usually accomplished by
applying negative video pulses to the grid of a tube. When the pulses
are large enough to drive the tube to cutoff, the output pulse remains
constant in amplitude, no matter how much larger the input pulses may
be. If the circuit shown in Fig. 12.10 were connected to the detector of
Fig. 129, it would serve very well as a limiter-amplifier. With a sharpcutoff
tube whose screen is operated at low voltage (75 to 100 volts),
I In seriespeakinga compensatingnetworkis placed in serieswith the two stages.