San Antonio light. [volume] (San Antonio, Tex.) 1911-1993, 1925-10-25
beats" EXPLAINED.

tenu 10 weed them out from the trans
mitted signal.
beats" EXPLAINED.
The current is now rectified by
detector as usual, and may be further
amplified through one or more audio
frequency stages.
The interesting thing about this
circuit is the means employed for re-
ducing the incoming signal current
to the intermediate" frequency. 1t
is done by the simple method of
beats same kind of beats which
are heard when we strike a key on
the piano, two strings of which are
not quite in tune with each other.
If, for example, the note should
vibrate exactly 435 times per Sec
ond, but only one of the two strings
is correct and the other is tuned to
the pitch of 436, then once every Sec
ond, the faster string will "gain one"
on the slower one so that if at some
particular instant the two strings are
| vibrating in phase. so as to pull to-
gether," They will, just one second
later, again come in phase with each
other, and another beat" will be
heard as the two strings reinforce
each other.
Half way between the times when
they are in phase, their effects oppose
each other. so that the sound is great
| ly weakened.
If the sharp string was still
further "out of tune" and vibrated
437 to the correct strings ABS, then
there would be two beats per second,
and so on.
NEW FREQUENCY OBTAINED
That is. by combining two fire.
quencies nearly the same. we get
frequency equal to the difference be-
tween the two, which, although not
the only one produced, is the one in
| which we are interested.
The effect is shown in the dia-
gram, where, for the sake of simple
ity, we have represented a frequency
of per second, and another of q per
second. and have shown how. when
the effects Of the two are added to-
gether, the result is the appearance
of a new frequency.
In the best superheterodyne prac-
tice we ordinarily use a beat-fre-
quency around 10000 cycles per Sec
ond 10000 meter wave length. In
order TO bring a 750-kilocycle signal
down to it, we would have to combine
with it a series of oscillations at a
rate of either $50 or 100 kilocycles
per second. This new wave must be
adjustable, so that we can tune it
to just 30 kilocycles above or below
whatever station we want to receive.
WHAT HETERODYNING is.
To produce this auxiliary wave we
use heterodyEe, or oscillator, which
is simply an electron tube connected
to a tuned circuit so that its plate
current pulsates steadily at the nat
ural frequency determined by the
tuning of the circuit. This pulsating
current is run through a coil coupled
to another coil which. in turn, is con-
nected into the tuning unit OF the
set, so that it's output contains a beat
frequency equal to the difference be-
tween the incoming frequency and the
oscillator frequency.
When it is rectified and the high
frequency oscillations damped out Tor
shunted out through n condenser), the
result is not an andio frequency. but
is the desired "iptermediate frequency
current"
After the intermediate frequency
current has been sufficiently ampli-
| fled. all we have to do is to repeat
the detecting process, using what is
called the second detector tube, and
we have our current ready for the
| snaaEa