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May 1946 Radio-Craft
 [Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
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Here is another instance of an article
which, if it had been in an April magazine issue, you might be justified in thinking
it might be a gag. "FM Carrier Stabilization," a 1946 Radio-Craft feature,
centers around the use of a General Electric (GE) GL-2H21 "Phasitron" vacuum tube.
Be assured that it is a real component, developed to address the difficulties in
achieving frequency modulation (FM) requirements set forth by the Federal Communications
Commission (FCC) at what was really the dawn of the FM commercial broadcast radio
era. Only a little over a decade had passed since
Major Armstrong
announced his broadband FM invention, and radio stations were planning to adopt
the superior (to AM) form of broadcasting at a rapid rate, following the end of
World War II. The Phasitron was GE's solution to the problem of maintaining
the average carrier frequency stability requirement.
Part I - The General Electric and Federal Systems
Fig. 1 - The G-E Phasitron, structural view the system of wire
deflectors.
Fig. 4 - Phasitron, cutaway view, 1 - Cathode. 2 - Focus electrode
No. 1. 3 - Focus electrode No. 2. 4 - Neutral plane. 5 - Anode No. 2. 6 - Anode
No. 1. 7 - Deflector grid.
One of the major problems in FM broadcasting has been that of maintaining the
average carrier frequency while modulating it so that it deviates up to 75 kc on
either side of its assigned value. The FCC regulations call for a maximum drift
of only 2000 cycles from the mean, a very small percentage of the new FM carriers
(which operate on approximately 100 mc).
Amplitude modulation broadcasting takes advantage of the precise unchanging carrier
possible with crystal control. This property is a disadvantage in FM. The crystal
resists the change of frequency which is the very basis of the system.
The major electronic and radio manufacturers have devised ingenious methods whereby
the high standard of crystal control is combined with frequency modulation techniques
so that the mean transmitted carrier is maintained within the rigid requirements
while it deviates in accordance with the modulation to provide the very high fidelity
of maximum deviation of which the FM is capable.
GE Phasitron Method
The type GL-2H21 phasitron tube (Fig 1) is designed to provide wide phase excursions
at audio frequency rates in a crystal-controlled carrier. It can operate up to 500
kc. It is generally used at approximately 230 kc, a frequency at which it permits
a deviation of approximately 175 cycles per second. Multiplying these values by
432 puts the carrier in the new FM band with a maximum deviation of 75 kc as required.
Fig. 2 - Block diagram of the G-E FM circuit which uses the Phasitron
frequency modulator.
Fig. 3 - Action of three-phase voltage in the deflector grid
system.
Fig. 5 - How the Phasitron is connected to frequency modulate
the crystal's output.
Fig. 7 - Structure of Anode 1 showing maximum and minimum current
curves.
Fig. 9 - The Federal system employs the well-known Miller effect
to stabilize frequency.]
Fig. 10 - Block diagram of the stabilizing unit, showing the
frequency-dividing system.
Fig. 11 - The 12H6 output corrects frequency.
The associated circuits required by the phasitron are not complicated. They consist
essentially of a crystal oscillator operating at 230 kc and a circuit to convert
the output to 3-phase 230 kc (Fig: 2).
The tube contains a deflecting grid structure of 36 wires, the active portions
of which are horizontal. Every third wire is connected together and to a common
base pin. Each phase of the 3-phase voltage is connected to one of these base pins.
An additional deflector is connected to another base pin and constitutes the neutral
plane (Fig. 3), and is grounded through a condenser.
Electrons emitted from the cathode are attracted to anodes 1 and 2 (Fig. 4),
which are at positive potential, thus forming a tapered thin-edge disk. This electron
disk extends from cathode to anode 1 and lies between the neutral plane and the
system of wire deflectors.
Construction of the deflecting grid system and its connection to the circuit
are shown in Figs. 4 and 5. As a consequence of the 3-phase voltages, the potentials
on grids A, B, and C vary. At some instant, for example, A and B are positive and
C negative (Fig. 3-a). The latter grids repel electrons towards the neutral plane,
while A and B attract them. The periphery of the disk then assumes a sine wave pattern
(Fig. 6) which rotates at a velocity determined by the crystal frequency and the
number of deflector grids.
Anode 1 has 24 holes punched in it, 12 above the disk plane and 12 below it.
These are shown in Fig. 7. At the instant when the sine wave pattern is in the position
shown by the heavy lines, all electrons are transmitted through the holes to anode
2. One half-cycle later (dotted lines) no current can pass through the holes and
anode 2 current is zero. As the disk rotates the anode current varies sinusoidally
between these extremes.
To modulate this current a coil L is placed over the phasitron (Fig. 8). It is
supplied with the audio frequency current. The magnetic force on each electron moving
from the cathode causes the entire disk to rotate in a direction determined by the
polarity of the a.f. voltage. This effect acts to speed up or slow down the electrons
in the already-rotating disk. Therefore an angular displacement (at an audio rate)
is superimposed upon the normal disk rotation due to the 230 kc, 3-phase voltage,
and results in phase modulation of the carrier. A maximum a.f. power of 50 milliwatts
is required.
Federal's Frequency Stabilization
Fig. 6 - Pattern of the rotating disk of electrons due to the
three-phase voltage.
Fig. 8 - How audio-frequency modulation is impressed upon the
Phasitron tube.
This CFS system is based upon the Miller effect principle, generally known to
radio technicians as a difficulty to be overcome. In this case it is a useful property.
As a result of Miller effect, the input capacitance of an amplifying tube depends
upon its amplification and upon the difference of phase between a.c. grid and plate
voltages.
The modulating unit (Fig. 9) is an important part of the circuit. It contains
a Hartley coupled oscillator operating at approximately 4 mc. This frequency is
divided by 256 in suitable multivibrator circuits so that it lies within the limits
of 14.3-17.6 kc, depending upon assigned carrier. The frequency of a precise temperature-controlled
crystal oscillator is similarly divided so that the frequency is the same as that
just mentioned (Fig. 10).
The two frequencies so obtained differ in phase. They are passed through a buffer
stage and individual low pass filters and then applied to a phase discriminator
of the conventional type known to FM technicians and repairmen (Fig. 11). The magnitude
of the discriminator a.c. output voltage depends upon the difference of phase between
the two applied frequencies A and B. The polarity depends upon whether the Hartley
oscillator frequency A drifts upward or down.
The low-pass filter which follows the discriminator grounds out the rapid a.f.
modulation and permits only the varying voltage caused by gradual frequency drifts
to affect the grid bias of the Miller tube (V-903). Change of grid bias of this
tube varies the input capacitance and therefore the frequency of the Hartley tank
coil across which it is connected. The original frequency drift of the Hartley oscillator
is compensated for in this way.
The output frequency of the CFS and modulator unit is multiplied in succeeding
stages to bring it into the FM band. The center frequency of this transmitter is
maintained to within 0.001 per cent of its assigned values.
Posted June 12, 2024
(updated from original post on 5/10/2018)
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