As known as THE MAGIC EYE. It is in principle a cathode
ray tube used to indicate whether the receiver is correctly
tuned. Basically the magic eye is a thermionic valve
consisting of three electrodes.
In this way, it has the cathode, a metal anode that
is coated with a fluorescent material like a screen,
which under the influence of the electron current will
light up. The third electrode, in a special shape is
mounted between the cathode and the anode, in order
to focus the electron beam according to the voltage
applied to it.
In the receiver, the magic eye is connected to the automatic
gain control stage (AGC). It supplies the suitable control
voltage that is applied directly to the focussing electrode
in such away the light spot area changes proportinally
as the frequency signal is stronger or weaker. (a)
When compared with the first types of electro-mechanical
tuning devices, the magic eye as a kind of thermionic
valve had many advantages. The most important one was
its inertia-free operation. It is interesting to note
that the company RCA originally introduced in 1930 the
term: MAGIC EYE, as a kind of trade name.