Radar System Engineering

450 THE RECEIVING SYSTEih—RADAR RECEIVERS [SEC. 126
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~ m ‘-
S+=
L, c R
FIG. 12.9.—Diode detector.
monly used second detectors is the diode detector shown in Fig. 12.9.
The diode is usually a tube such as the 6H6 or the 6AL5, although many
other tubes have been used and recently crystals have been developed
for the purpose. The i-f bypass condenser C usually has around 10
p~f of capacity; it must be considerably larger than the plate-to-cathode
capacity of the diode to get good detector efficiency, but it must not be so
To,i~m large as to spoil the high-frequency
L2 ~mplifier response of the video amplifier. Theoretically,
the high-frequency response
would be maintained, however large
C, by making R proportionally smaller.
If R is too small, however, the detection
efficiency is lowered and the maximum
voltage that the detector can
produce becomes too low. Values of R from a few hundred to a few
thousand ohms are usual. Inductance Lz is chosen to offer a high
impedance at the intermediate frequency and is sometimes made resonant
at this frequency.
12.6. Video Arnpliliers.-One or more stages of video amplification
follow the detector. The form of the video amplifier is determined largely
bv the number and location of the indicator tubes. In an airborne radar
set there may be only a single indicator located near the receiver, in which
case the video amplifier is very simple. In large ground and ship sets
there will be many indicators located far from the receiver, and the video
amplifiers must be more complex. In the latter case, the video amplifier
following the detector drives a terminated line at a level of a few volts,
and individual indicators are driven by video amplifiers bridged across
this line.
The requirements placed on a video amplifier can be stated quite
generally.
1. A signal of a few volts from the detector must be amplified and
transmitted to the indicator, the signal level at the indicator being
usually about 20 volts, although some types of indicator require a
much higher voltage.
2. The amplifier must have good transient response, as defined by
the following: it must pass long pulses with little “ droop” on the
top of the pulse; this requires good low-frequency response. The
rise timel must be definitely shorter than the pulse length, and the
overshoot on a pulse should be held to 10 per cent or less; these
factors are determined by the high-frequency response. The
] Rise time is conveniently defined as the time required for the output voltage of
the amplifier tmr&e from 10 per cent to 90 per cent of its final value when a square
pulse is applied to the input,