Radar System Engineering

414 RF COMPONENTS [SEC.11.7
crystals are tested by sending through them a d-c pulse of duration
2.5 X 10–gsec and total energy of the order of an erg. Crystals which
pass this test are safe in a properly operating radar set.
Table 11.2 lists for comparison the rejection-limit specifications of the
most widely used crystals in the three radar bands. The bearing of these
figures on the over-all sensitivity limit of microwave receivers is discussed
in Chap. 12.
In the manufacture of crystals to meet the specifications (which are a
considerable advance over those met by the crystals used in early microwave
radars) careful attention must be given to the original purity of the
silicon, to “doping” with small amounts of the proper impurity, to the
preparation of the crystal face, and to the sharpening and adjustment of
the whisker. Crystals are stored in metal containers to avoid burnout
from stray r-f fields. They can be burned out easily at the time of installation
by an accidental discharge of static electricity through them.
TAJWB 11.2.—SPECIFICATIONS OF CONVERTER-TYPE CRYSTALS
Type
Wavelength Conversion Noise tempera- Burnout
band (cm) ]0SS(db) ture (times) test (ergs)
1N21B 8-11 6.5 2,0 2.0
1N23B 3.1-3.5 6.5 2.7 0.3
1N26 1.25 8.5 2.5 0,1
11.7. The LocaI Oscillator.-Reflex klystrons are, with very few
exceptions, used as local oscillators in microwave receivers. Figure
11.22 shows a schematic view of a typical tube. The resonant circuit is a
reentrant, doughnut-like cavity
with grids across the central portion.
An electron gun with a proper focusing
electrode directs a stream of
electrons through the grids. Upon
arrival at the first grid the electrons
have a velocity corresponding to 300
volts. It will be assumed that oscillations
exist in the cavity. Elec-
+300volts
FIG. 11.22.—Schematic of reflex klystron.
1
trons will then alternately be
H = heater
accelerated and decelerated by the K = cathode
F = focusing electrode
r-f field across the grids. An electron
that goes through just as the
C = cavity
R = reflector
O = coaxial output line.
field between the grids is passing
through zero on the way from acceleration to deceleration will not have its
velocity changed and can be taken as a reference electron. In the space
1Klystrons and “MicrowaveTriodss, Vol. 7, Radiation Laboratory Series.