How The SHORT WAVE OSCILLOCLAST Works
                            By R.W. Baldwin
                 Electronic Medical Digest, Spring 1948


     To understand the operation of the Oscillocalst, it is necessary
to know how a vacuum tube works since these machines are built around
two of these.

     In Figure 1, the filament heated by the battery "B" emits
electrons.  Because the plate is positive with respect to filament, the
electrons are attracted to it.  This condition is indicated by the
meter which will show a deflection.

     If we reverse the connections of the battery "B" which would cause
the plate to be negative, the electrons would be repelled and no plate
current could flow.




     In Figure 2, we have the same set of conditions as above, with the
addition of another element in the tube called the grid and the battery
"C."  The purpose of the grid is to control or limit the number of
electrons which will reach the plate.  We do this by simply making it
(grid) more or less negative with respect to the plate (see battery
"C").  The electrons being repelled will reach the plate in fewer
numbers and as a result, the meter "M" will indicate a smaller current.
In fact, we can cause the plate current to be intirely cut off by
making the negative grid potential high enough.


No diagrams



     An interesting thing about the grid is the fact that no matter how
high the negative grid potential, the grid draws no current.  It can
easily be seen that it is possible to control large amounts of power by
the use of a small voltage of only enough power to overcome insulation
losses, etc.  Tubes are usually designed so that quite small amounts of
grid voltage, compared to plate voltage, is required.

THE OSCILLATORS - In connection with vacuum tubes, an oscillating
circuit usually contains: Inductance (coil), Capacity (condenser), and
Resistance (losses in the coil, tube, etc.).







     See Figure 3 - This circuit is known as a self-excited oscillator
because the necessary grid voltage or excitation, as it is known, is
taken from the plate circuit in the following manner:
     While the condenser "C" is charging, there is no voltage in the
plate coil (L1).  The grid coil, which is inductively coupled to the
place coil receives no energy; the grid is negative, and no current
flows in either the grid or plate coil.  But when the condenser builds
up a charge large enough to overcome the resistance in the plate coil
and its circuit, it discharges through the pllate coil.  The grid coil,
because it is coupled to the plate coil, picks up energy causing grid
to be positive instead of negative, thus allowing a larger current to
flow in the plate circuit.  Once the condenser has discharged, the same
thing takes place again.  The grequency of these oscillations is
determined by the size of the condenser and the coils.

     Because the grid takes only a small amount of the power, say 10
per cent, we can use this energy for anything we wish.






     See Figure 4 - This circuit is comprised of two of the oscillators
just described.  True, the coils and condensers connected to the two
tubes have different values, but they operate for the same reason
given.  The coils and the condensers in the first tube are selectred to
produce oscillations of about 90 per minute.  This energy is used as
plate voltage for the second tube, and is transferred by the
transformer "T."

     The coils and condensers in the second tube are such that it
oscillates at about 43 megacycles (7 meters).  The first tube produces
energy 90 times a minute; therfore, the second tube can oscillate only
while plate voltage is avaiable, which, of couse, is 90 times a minute.


THE 43-MEGACYCLE WAVE

     In describing the radio frequency wave of 7 meters, we must use
the term modulation.  Modulation is described as the process whereby
the amplitude, frequency or phase of a wave is varied in accordance
with a signal wave.  The resultant wave will contain frequencies
identical with those present in the two original waves, and in
addition, new frequencies will be produced which will be made up of
various combinations of the original frequencies.

     From the above, it can be seen that the fact that the plate
voltage of tube "2" is varied from zero to maximum causes complete
modulation.  this can be demonstrated by tuning to the 7-meter wave on
a radio, and then listening to the sound produced by an earphone
connected across the terminals "1" and "2" of transformer "T" in figure
4.  The sounds produced by the radio and the sounds by the earphone
will be almost identical.

     The audio energy and the radio frequency energy are both used on
the patient at the same time.  Both the radio frequency and the audio
energy are produced at the same time (90 times per minute).  Between
the oscillations of 90 per minute, no energy is produced.