GENERAL RADIO COMPANY 1608-A Operating Instructions Manual page 22

OPERATING PROCEDURE
TABLE 2-1
BRIDGE COMPONENT RATINGS
FULL-SCALE RANGE setting
Ra Ra Max Ra Max
C
G
R
L Value Voltage Current
1100 fJ-f
1100 ml) 1100mD
1100 fJ-h ID 0.71 v 710 rna
110 fJ-f
110 mD lID 11 mh IOD
2.2v
220 rna
11 fJ-f
11
mU
110D 110 mh 100D
7.1 v 71ma
1100 nf
1100 fJ-D
1100D 1100mh lkD 22v 22ma
110nf
110 fJ-D
11 kD llh 10kD
71v 7.1ma
11 nf
11 fJ-D
IlOkD
110h 100kD 220v 2.2ma
1100 pf 1100nD 1100 kD 1100h
IMD 500v 0.7ma
CENTADE - R n (Rl): 30 rna
STANDARD RESISTOR R
t
(R3): 58v, 86ma.
STANDARD CAPACITOR, C
t
(C 1): 600 v peak (425 v rms).
DETECTOR INPUT CAPACITOR (C556): 400 v peak (280 v rms).
(2)
2.4.5.4
Effect of Level on Iron-Cored Inductor Meas-
urements. Iron-cored inductors are nonlinear devices
whose inductance depends on the level of the applied
voltage.
1£
measurements are to be repeatable, the sig-
nal level must be specified. The "initial permeability"
inductance, or inductance at "zero level", is often used
as a reference (as on General Radio Type 1481 Induc-
tors). To obtain this value, plot L vs applied voltage
and extrapolate to zero voltage. The GEN LEV con-
trol permits such measurements, and it
1S
often useful
to make a level change in order to see if the unknown
inductance depends on the signal level.
2.4.6
DIFFERENCES BETWEEN AC AND DC RESIS-
TANCE MEASUREMENTS.
2.4.6.1
General. The ac resistance bridge of the Type
1608-A Impedance Bridge provides a means for extend-
ing the range and sensitivity of resistance measurements
over that possible with dc, without using a higher ap-
plied voltage or a sensitive dc amplifier. The ac re-
sistance of a resistor can differ from the dc value for
a
number of reasons. However, most of those are neg-
ligible at 1000 cps, and in some cases the use of ac
avoids undesirable effects thar can cause errors in dc
lTIeasurement.
2.4.6.2
Frequency Effects.
a. Series Inductance and Parallel Capacitance.
At audio frequencies almost all resistors except those
of very high value (see band c below) can be accurately
represented by the equivalent circuit of Figure 2-4. In
this circuit the resistor is a pure resistance and equal
to the low-level dc value unless some other effect is
appreciable. If we let
QL =
~
and Q c
=
u.:R c ' then the
R
effective series resistance of this equivalent circuit is
R
(1)
and the effective parallel conductance is
1 1
G
p
""R
x 1+QL2
Low-valued resistors have a completely negligible
Qc but
QL
can become appreciable, particularly for wire-
wound resistors. Since Qc is negligible, the value of R s
is equal to the dc value, but the value of G p
is
not equal
1
to
R;k"
However, on the Type 1608-A, if the resistor is
inductive, it can be balanced only on the R s bridge,
where there is no error.
High-valued resistors have a negligible QL but Qc
is
appreciable even if the parallel capacitance is small.
If the unknown resistor is capacitive, it can be measured
only on the G p bridge where there is no error due to
lumped parallel capacitance.
It is conceivable that both .QL and Qc could be
large enough to have an appreciable effect in the middle
17