TM 9-4935-282-34
803D
provide a means of making the four precision voltages (500,
to 5 volts maximum for full scale inputs of 50 and 500 volts
50, 5, and 0.5) adjustable. Note that each string, with the
respectively.
exception of the first, parallels two resistors of the preceding
b. OPERATION. All AC measurements are made by
string. Between the two wipers of S6 (voltage dial A) then,
first converting the AC input voltage to a DC voltage. The
there is a total resistance of 40K (80K paralleled by 80K). With
converter provides a DC output of 5 volts when full range
the range switch in the 500 volt position, a total voltage of 100
voltage is applied to the 803D in each AC range. In the 5 volt
volts DC will appear across these two wipers. Also, there will
AC position, range switch sections S2G and S2H connect the
be 10, 1, and 0.1 volts DC across the wipers of S7, S8, and
input binding posts directly to the converter input. In this case,
S9, respectively. Voltage dial E (S10) picks increments of 0.01
the converter feedback is of such a value that the DC output
volt DC from the last decade. These voltages are reduced by
voltage is equal to the rms value of the converter input AC
a factor of 10 for each lower voltage range. All resistors of
voltage. In the 500 and 50 volt AC positions, an input
each decade are matched and the three most critical decades
attenuator reduces the unknown AC by a factor of 100 and 10
(40K, 8K, and 1.6K are matched for each instrument, providing
respectively. The operation of the converter is then the same
an overall accuracy of 0.005% absolute. With the null switch in
as for the 5 volt position. In the 0.5 volt AC position, range
any null range, the output of the Kelvin-Varley attenuator is
switch section S2I and section S2J provide connection to
connected in series with the vtvm attenuator thus providing the
feedback resistor that allow the converter to produce an output
accurate 0 to 500 volts reference supply required.
that equal to ten times the AC input. Thus, an output of 5 volts
f. ADJUSTMENTS. Variable resistor R121 is used
DC is provided for full scale input on any AC range.
during calibration to set the 500 volt supply to 500 volts with
c. NULL INDICATIONS. When making AC differential
calibrate control R2 set at its center of rotation. This allows the
measurements, the null range used times the applicable AC
reference supply to be adjusted to 500 volts by means of the
null multiplier must be used to represent full scale on the 803D
calibrate control at any time deemed necessary. With the
meter. This is due to the way that the converter is constructed.
operate-calibrate switch held at calibrate, a fixed percentage of
For the 500 volt AC range, the converter produces a DC output
the 500 volt supply is compared to the precise potential of an
voltage equal to 1/100 of the AC input voltage. Thus, the AC
internal standard cell or Zener reference diode in the (AG
null multiplier for the 500 volt range position is X100. For
models). Any difference in potential is fed to the null detector
example, when the range switch is set to 500 and the null
to give an indication on the meter so that the 500 volt supply
switch is set to 0.01, full scale meter deflection represents a
can be set with the calibrate control. The fixed percentage of
one volt (100 x 0.01) difference between the unknown voltage
the reference supply is set accurately during calibration by
and the amount set on the voltage readout dials. By similar
means of R318. The 50, 5, and 0.5 volt range resistor
reasoning, the multipliers for the 50, 5, and 0.5 volt AC ranges
networks are set accurately during calibration by means of
are X10, X1, and X0.1, respectively.
R323, R326, and R329.
d. ADJUSTMENTS. For 0.5 volt converter gain, R536
and R545 at the output of the converter are adjusted. The gain
3-4. AC TO DC CONVERTER
of the amplifier for the 5 volt range is adjusted by means of
R542 and R541 in the feedback network. The high frequency
a. GENERAL. The AC to DC converter is composed of
response of the amplifier input is adjusted by means of C504
an attenuator, an operational amplifier, and a rectifier-filter
while C519 adjusts the high frequency response of the 0.5 volt
circuit. A diode in the rectifier-filter circuit is used to convert
feedback circuit. The attenuation of the 500 volt attenuator is
the unknown AC into pulsating DC, which is then filtered to
adjusted with R535 and R544 and the attenuator high
obtain a DC voltage that is proportional to the average value of
frequency response is adjusted with C501. The attenuation of
the AC input voltage. The output however, is calibrated to
the 50 volt attenuator is adjusted with R503 and R533 and the
indicate the rms value of a pure sine wave. An operational
attenuator high frequency response is adjusted with C523.
amplifier containing three resistance-capacitance coupled
amplifier stages with high negative feedback is used to make
3-5. AC - DC POLARITY SWITCH
the rectification characteristics of the diodes linear and stable.
The amplifier achieves a midband loop gain of approximately
a. The AC - DC polarity switch is provided for selecting
70 db with a virtually flat frequency response from 20 cps to 10
either the AC or DC mode of operation. When the AC - DC
kc. The high negative feedback makes the amplifier practically
polarity switch is set to AC, the AC to DC converter is switched
noise free and relatively insensitive to gain changes in
into the circuit by sections S5K, S5L, S5M, and S5N. Also,
individual tubes due to aging and tube replacement. At the
sections S5D and S5E are used to switch 5 volts DC to the
output to the amplifier, full wave rectification is used to return
Kevin-Varley divider. When in the AC vtvm mode, S5F is used
negative feedback to the grid of the first amplifier tube. The
to provide proper attenuation for the DC attenuator.
attenuator is used to reduce the AC input voltage by a factor of
10 or 100, as required to restrict the operational amplifier input
3-4
