Radar System Engineering

412 R-F COMPONENTS [SEC.114
order to obtain a much lower beat frequency which can be amplified.
A more complete and integrated discussion of microwave receivers is
reserved for the next chapter, but for the sake of continuity the components
of the receiver which handle microwaves are treated here in conjunction
with other r-f components.
11.6. The Mixer Crystal.-Since no satisfactory amplifier for microwaves
exists, the conversion to intermediate frequency must be made at
—
“m=
.1
a. Pin end
a. Sylvania b, Western Electric
FIG. 11. 19.—Microwave mixer crystals.
the low level of received signal powers. It is important, therefore, that
the nonlinear element be as efficient as possible, and that a minimum
amount of added noise be introduced. The most satisfactory device
that has been found is the rectifying contact between a metallic point
and a crystal of silicon.’ For protection and stability the silicon and the
“cat whisker” are sealed up in a cartridge. The term “crystal” ordinarily
refers to the whole assembly, a cross-section view of which is
shown in Fig. 11”19.
The d-c characteristic of a crystal has the form shown in Fig. 11“20.
An equivalent circuit that accounts for the r-f properties is shown in
Fig. 11.21. The nonlinear resistance of the rectifying contact is denoted
by r.. In parallel with it is the capacity C of the boundary layer of the
1CrystalRectifiers, Vol. 15. lbdiat
ion Laboratory Series.