T EVIS i - AmericanRadioHistory.Com

Converter Calibration
(Continued from page 7)
done easily by using a center- tapped
transformer with a 50,000 -ohm Helipot
across the plate leads. The center tap
of the transformer and the Helipot
slider provide the signal leads. With
the potentiometer slider centered, zero
output voltage will exist. Moving the
slider in either direction will cause a
voltage to appear; but in one direction
the voltage will be practically in -phase
with the line voltage, and in the other
direction it will be practically 180° out -
of- phase. When a transformer is used,
a phase shift of about 2° will occur between
the input and output voltages.
The output voltage can be read directly
from a calibrated dial on the Helipot,
provided that the input voltage to the
transformer is adjusted, using a Weston
Model 622 voltmeter (1 %), a vacuum
tube voltmeter (3 %), or a laboratory
dynamometer voltmeter (1s %).
The
converter output of 0 to 5 volts d.c. can
be read by the Weston 200,000 ohm /volt
voltmeter.
It might be well to note here that converters
of this type can be very tricky
to design, build, and calibrate.
Calibration of the capacity converter
(Fig. 1B) consists of connecting known
capacitors to the input of the converter
MI2THD2l3
°g euui41 Eueatd
MARCH 23 -28-IRE National Convention,
Waldorf- Astoria Hotel and Grand
Central Palace, New York, N. Y.
APRIL 1I -New England Radio Engineering
Meeting, University of Connecticut,
Storrs, Conn.
APRIL 18- Spring Technical Conference
of the Cincinnati Section, IRE,
Engineering Societies Bldg., Cincinnati,
Ohio.
APRIL 23- 24- International Symposium
on Non -Linear Circuit Analysis,
organized by Polytechnic Institute of
Brooklyn, Engineering Societies Bldg.,
33 W. 39th St., New York, N.Y.
APRIL 21.30 -Spring Technical Meet-
ing, sponsored Jointly by the U.S.A.
National Committee of the Interna-
tional Scientific Radio Union and the
IRE Group on Antennas and Propagation,
National Bureau of Standards,
Washington, D. C.
APRIL 29-MAY 1- Electronic Components
Symposium, sponsored by the
AIEE, IRE, RTMA and WCEMA,
Shakespeare Club, Pasadena, Calif.
MAY 7- 9- Acoustical Society of Amer.
Ica, 45th Meeting, featuring Sound Reproduction,
Warwick Hotel, Philadel.
phia, Pa.
MAY 11-13-National Conference on
Airborne Electronics, Hotel Biltmore,
Dayton, Ohio
AUGUST 19- 21- Western Electronic
Show and Convention, Civic Auditorium,
San Francisco, Calif.
APRIL, 1953
to simulate different capacities, indicating
-for example- different fuel levels.
The converter output is fed to a potentiometer
divider, placed across a stable
5 -volt supply. Proper voltage is then
provided by the slider. The calibration
capacitor is a special capacitor arrangement
whereby both the stator and rotor
are above ground, and ground is the
shielded box surrounding the variable
capacitor. Plug -in fixed capacitors increase
the total capacity range. This
results in a three -wire arrangement
which makes possible a system free
from errors due to changes in temperature
or moisture content of cable insulation.
Fuel gauge accuracy is also
independent of cable lengths or capacity.
Accuracy is approximately 1 %.
The potentiometer divider converts
mechanical motion into a change of po-
tentiometer slider position.
The best
method of calibration of this device is
through the use of a ratio bridge (Fig.
2A). A potentiometer divider generally
consists of a 5000 -ohm potentiometer,
and is arranged in a bridge circuit with
a Helipot having a ten -turn dial, also
rated at 5000 ohms. Voltage is applied
across the dividers and a null indicator
(D'Arsonval microammeter) is placed
between the two sliders. The actual voltage
across the two parallel potentiometers
is not critical, i.e., the voltage
can vary over several volts -say, between
4 to 6 volts- because the ratio
bridge is a null device. The Helipot is
adjusted for a null and its dial indicates
0 to 1000, which can conveniently be reduced
to 0 to 100. Therefore, the input
to the potentiometer divider can be
plotted against 0 to 100% motion. This
method of measurement is widely used
because it eliminates the necessity for
measuring and maintaining 5 volts d.c.
to better than 1% and then reading the
divider output voltage to 1 %. The overall
accuracy of a ratio bridge circuit
can be % %.
Equipment in a converter calibration
bench is also suitable for testing and
calibrating subcarrier oscillators.
subcarrier oscillator is frequency -modulated
by applying a signal between 0
and 5 volts d.c. The a.f. output deviates
from the center frequency ±7.5 %, with
2.5 volts d.c- causing the subcarrier
oscillator to oscillate at center frequencylThus,
the EPUT meter can
measure the subcarrier output frequency
and the Weston d.c. voltmeter
can measure the input voltage accurately.
The calibration bench requires power
supplies of 400 cycles, 115 volts a.c.
and 26 volts a.c., single -phase; several
hundred volts d.c.; 6.3 volts a.c. or d.c.;
and 28 volts d.c. Most of these power
requirements can be supplied by commercial
power supplies. , ®
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31