Impedance Correction Graph - HP 400D Organizational, Ds, Gs, And Depot Maintenance Manual

AVOID A SHORT CIRCUIT ACROSS THE POW-
ER LINE! To measure power line voltage, first
connect only the upper (red) INPUT terminal to
each side of the power line, in turn, leaving it
connected to the side that causes meter indi-
cation. Then connect the lower (black) INPUT
terminal (grounded internally) to the other side
of the line. If this procedure is not followed,
the power line may be short-circuited through
the grounded INPUT terminal of the voltmeter.
f. Read the meter indication on the appropriate VOLTS
scale, in accordance with the full-scale value indicated
on the RANGE switch. Evaluate the reading in terms
of the full-scale value indicated on the RANGE switch.
Study the following examples:
Example 1
When the RANGE switch is in the .1 VOLTS range, read
the 0 to 1 VOLTS scaIe. If the meter indicates .64 on
that scale, the voltage being measured is:
Example 2
When the WGE switch is in the 30 VOLTS range, read
the 0 to 3 VOLTS scale. If the meter indicates 1.6 on
that scale, the voltage being measured is:
3-17. MEASUREMENT OF DECIBELS.
3-18. The DECIBELS meter scale is provided for
measuring dbm directly across 600 ohms and for
measuring db ratio for comparison purposes when
each measurement is made across the same circuit
impedance. To measure signal level directly in dbm
(0 dbm equals 1 milliwatt into 600 ohms) proceed as
follows:
a. Connect the voltmeter to the a-c power source.
b. Turn the Power switch ON and allow a warmup
period of approximately five minutes.
c. Disconnect any external equipment from the OUT-
PUT terminals.
d. Set the RANGE switch to the DB range which will
give an upscale reading of the signal to be measured.
If in doubt, select a higher-level scale.
e. Connect the voltage to be measured to the INPUT
terminals.
00102-2
f. Note the meter indication on the DECIBELS scale
(-12 to +2 db). The signal level is the algebraic sum of
the meter indication and the db value indicated by the
RANGE selector. Study the following examples:
Example 1
If the indication on the DECIBELS scale is +2 and the
RANGE switch is in the +20 DB position, the level is
+22 dbm.
Example 2
If the indication on the DECIBELS scale is +1.5 and the
RANGE switch is in the -40 DB position, the level is
-38.5 dbm.
3-19. To measure db across impedances other than
600 ohms, follow the above procedure and evaluate the
results as follows:
NOTE
Since the measurement is made across other
than 600 ohms, the level obtained in step f is in
db, but not in dbm.
a. To obtain the difference in db between measure-
ments made across equal impedances, algebraically
subtract the levels being compared.
b. To obtain the reading of a single measurement
in dbm, note the impedance across which the measure-
ment is made and refer to the Impedance Correction
Graph, described in paragraph 3-20.
c. To obtain the difference in dbm between measure-
ments made across different impedances, convert each
measurement to dbm using the Impedance Correction
Graph described in paragraph 3-20. Then algebraically
subtract the dbm levels being compared.

3-20. IMPEDANCE CORRECTION GRAPH.

3-21. As the voltmeter DECIBELS scale is calibrated
to indicate dbm for measurements made across 600-ohm
circuits, a correction factor must be used when meas-
urements are made across circuit impedances other
than 600. ohms, if absolute dbm levels are desired. The
correction factor is not necessary in measuring relative
db levels (not dbm) across the same impedance, but it is
required for comparison of db levels measured across
different impedances. The Impedance Correction Graph
in figure 3-4 gives the correction factor for conversion
of the meter reading to dbm when the impedance of
the circuit under test is known. To use the graph, read
the conversion factor corresponding to the test circuit
impedance and add it to the meter reading determined
by the method of paragraph 3-17. Observe the algebraic
sign of the correction factor in making the algebraic
addition. Use the following examples:
Example 1
If the measurement is made across 90 ohms, the
indication on the DECIBELS scale is +2, and the RANGE
switch is at the +30 DB position, the level in dbm is
obtained as follows:
TM 11-6625-1514-15
Section III
Paragraphs 3-17 to 3-21
3-3