Boonton Electronics 71A, 71AR - Page 20

fixture to be used in subsequent measurements should be
attached for this adjustment.)
with the 111000 pF ZERO" and "c ZERO" controls properly
set, the instrument will be correctly zeroed for .all
ranges, assuming the "COMP ADJ
II
control is properly
adjusted.
It is advisable to check the 1 pF zero position periodi-
cally during the course of operation, particularly if
working on the 1 or 3 pF ranges.
NOTE
The front panel IIC ZERO" control is a dual ratio
control which adjusts a variable air capacitor
having a full 360
0
of rotation. It will be noted
that the "C ZERO
II
control turns very easily for
about 270
0
of rotation. At this point the torque
requirement increases abruptly, indicating a shift
from the 36:1 IIfine
ll
adjustment to the 6:1 "coarse"
adjustment. If it is found that the "c ZERO" range
is inadequate to zero out the external capacitance,
its range may be extended as outlined in paragraph
3.7.
3.4.3 Errors in Capacitance Measurements Resulting from
Series Inductance of Connecting Cables.
The measured capacitance, C
m
, of a specimen may differ from
the true capacitance, Ct, as a result of the series inductance
of connecting cables, and the inductance of the specimen.
The error will be noted as an apparent increase in capaci-
tance value in accordance with the following expression:
Where L = the combined series inductance in micro-
henries of BOTH lengths of connecting coaxial cable
and the inductance of the sample (generally small
with respect to the cable inductance).
The correction chart of Figure 3-1 indicates typical cor-
rections to be subtracted from the mea$ured value for a
pair of l-foot cables of RG-62/U at 1 MHz for test values
3-8
71A
d