HP 400D - SECTION Ill; Instrument Turn-On; General Operating Information

T.O. 33A1-12-349-1
Section III
Paragraphs 3-1 to 3-9
SECTION Ill
OPERATING INSTRUCTIONS
3-1. INSTRUMENT TURN-ON.
3-2. The voltmeter is ready for use as received from
the
factory
and
will
give
specified
performance
after
a
few minutes warmup. See Section II for information
regarding connection to the power source and to the
voltage
to
be
measured.
Controls
are
shown
in
figure
3-1.
3-3. GENERAL OPERATING INFORMATION.
3-4.
METER
ZERO
CHARACTERISTIC.
When
the
Model
400D
and
400H
Voltmeters
are
turned
off,
the
meter
pointer
should
rest
exactly
on
the
zero
calibration
mark
on
the
meter
scale.
If it
does
not,
zero-set
the
meter
as
instructed
in
paragraph
5-7.
The
meter
supplied
in
the
Model
400L
Voltmeter
is
not
provided
with
a
mechanical
meter
zero
adjustment.
When
the
voltmeter
is
turned
on
with
the
INPUT
terminals
shorted,
the
meter
pointer
may
deflect
upscale
slightly;
this
deflection
does
not
affect
the
accuracy
of
a
reading.
NOTE
When the voltmeter RANGE switch is set to the
lowest ranges and the INPUT terminals are not
terminated or shielded, noise pickup can be
enough to produce up to full-scale meter deflec-
tion. This condition is normal and is caused
by stray voltages in the vicinity of the instru-
ment. For maximumaccuracy on the -001-volt
range, the voltage under measurement should
be applied to the voltmeter through a shielded
test lead.
3-5. METER SCALES. The two voltage scales on each
of the voltmeter models are related to each other by
a factor of 1710 (10 db). In conjunction with the calib-
rated RANGE switch steps, this provides an intermediate
range step spaced 10 db between "power of ten" ranges,
which are 20 db apart. The relationship of the DECIBELS
scale to the 0 to 1 VOLT scale is determined by making
0 db on the DECIBELS scale equal to the voltage required
to produce 1 milliwatt in 600 ohms (0.775 volts). Thus,
the DECIBELS scale reads directly in dbm (decibels
referred to one milliwatt) across a 600-ohm circuit,
and
can
be
used
to
measure
absolute
level
of
sine
wave
signals. It can also be used to measure relative levels
of any group of signals which have the same waveform,
across any constant circuit impedance. The RANGE
switch changes voltmeter sensitivity in 10-db steps
accurate to within +1/8 db. The RANGE switch position
indicates the value of a full-scale meter reading.
3-6. CONNECTIONS. Voltmeter test leads must be
provided by the user. The type of leads and probes
used will depend upon the application, as listed below:
a For connection to low-impedance signal sources,
plain wire leads often are sufficient.
00102-2
b. For high-impedance sources, or where noise pickup
is a problem, low-capacity shielded wire must be used
with a shielded, dual banana plug for connection to the
voltmeter terminals.
c. If a probe is used, it should also be shielded to
prevent pickup from the hand.
d. For signals above a few hundred kilocycles, the
capacity of the test leads must be kept to a minimum
by using very short leads, preferably unshielded. An
alligator clip should be used at the test end so that
connection can be made without adding the capacity of
the user's hands.
3-7. MAXIMUM INPUT VOLTAGE, Do not apply more
than 600 volts dc to the INPUT terminals. To do so ex~-
ceeds the voltage rating of the input capacitor.
3-8. If an applied voltage momentarily exceeds the
selected full-scale voltmeter sensitivity, a few seconds
may be required for circuit recovery, but no damage
will result.
3-9. INPUT VOLTAGE WAVEFORM. The voltmeter
is calibrated to indicate the root-mean-square value
of a sine wave; however, meter pointer deflection is
proportional to the average value of whatever waveform
is applied to the input. If the input signal waveform
is not a sine wave, the reading will be in error by an
amount dependent upon the amount and phase of the
harmonics present, as shown in figure 3-2 below.
When harmonic distortion is less than about 10%, the
error which results is negligible.
INPUT VOLTAGE
CHARACTERISTICS
METER
INDICATION
Fundamental = 100 100
Fundamental +10% 100
2nd harmonic
Fundamental +20%
2nd harmonic
100-102
Fundamental +50% 100-110
2nd harmonic
Fundamental +10%
3rd harmonic
Fundamental +20%
3rd harmonic
Fundamental +50%
3rd harmonic
Note: This chart is universal in application since
these errors are inherent in all average-respond-
ing type voltage-measuring instruments.
96-104
94-108
90-116
Figure 3-2. Effect of Harmonics on Voltage
Measurements
3-1