Radio Broadcast - 1925, February - 113 Pages ... - VacuumTubeEra

in
718 Radio Broadcast
R
B
oscillate.
Such a system will absorb considerable
power on the high frequencies, and practically
none at all on the low frequencies,
which approaches the ideal condition outlined
a few paragraphs above.
A most simple and illustrative example of
differential absorption is found in a slight
modification of circuit B in Fig. i. If, instead
of short circuiting the terminals of the
stabilizing coil L, a fixed condenser is shunted
across it, as in LC of Fig. 2 (A), an oscillatory
circuit will be formed which will absorb energy
FIG.
Examples of straight stabilization
sorption." Straight absorption refers to systems
that, without consideration of the changing
tendency to oscillate, absorb over the full
wave range. This arrangement is obviously very
inefficient, for if the system is adjusted greatly
to dampen oscillations on the higher frequencies,
where a comparatively large amount
of absorption is required it will dissipate much
more power than is
necessary on the higher
waves, where almost any set is self stabilizing.
Examples of this type of absorption are
numerous and are found in many amateur
receivers and in a few commercial sets. The
most common procedure is to place a resistance
of from fifty to a thousand ohms in the
grid circuit of the radio-frequency amplifier,
;
(A, Fig. i). Another and widely advocate^
arrangement is to wind from two to
eight turns of wire (L) with a diameter of
about two to three inches, placing the coil
I
only about the wave to which it is tuned. If
LC is given a resonance or tuned point at about
two hundred and seventy-five meters, it will
absorb sufficient power to stabilize the receiver
on the short waves, having little or no i
effect on the higher waves where external
stabilization is not required. This is the
system first introduced by Austin, in 1916, for
the control of oscillations in long wave heterodyne
reception, later used by Mr. L. M.
Cockaday in a receiver designed by him.
This method is at present employed in the
two-stage r. f. "Starr" set.
The method just described is
probably the
most efficient of all absorption methods, and
is very easily applied to any
r. f. receiver in
the course of construction or quite completed.
It is
only necessary to wind the absorption
coils and place them in inductive relation to
the secondary. The calculation of the correct
number of turns is
not difficult.
close to the primary and secondary of the
radio-frequency transformer (Fig. i, B). As
might be expected both of these systems work
satisfactorily, on the low waves where the
losses are just sufficient to stabilize the receiver,
regeneration with good signal strength
and selectivity being permitted. However,
on the upper side of the scale, the last trace
of regeneration is
dampened and the signal
itself is absorbed. Variable controls, for obvious
reasons, are only an impractical solution.
DIFFERENTIAL ABSORPTION
(SHOWING SECOND R.F. STAGE)
SECOND R.F. STAGE
TO DET.
P\IFFERENTIAL absorption refers to
L-/ those systems that discriminate between
the higher and lower wavelengths, and more or
less adjust the amount of absorption in proportion
to the tendency to over-regenerate and
60-90 V.
FIG. 2
Preferred and efficient methods of stabilization.
The series resistance system (C) reduces the B battery
consumption of the r.f. tubes, adding economy
to its other desirable characteristics