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Section $-Accuracy
4,l
GENERAL.
Basically the 1650-B measures C, R, L, and G
t o within 1%, and D and Q t o 5%; however, a t the
range extremes the accuracy naturally decreases.
To
know when the accuracy of your instrument decreases,
consider the following:
1. What is the approximate magnitude of the im-
pedance at the measuring frequency? A common 100-
p F capacitor has a 150-mn impedance at 10-kHz (see
Reactance Chart in the Appendix).
The bridge resid-
uals of 1 mR and 0.2 pH (10 mn impedance) are in
series with this low impedance along with the self
inductance of any connecting leads t o the unknown,
thus causing the error t o approach 10%. Therefore,
be wary of low impedances.
2. Visualize the unknown a s having a reactive part
and a real part in parallel.
I f the real part i s very
small then it essentially controls the impedance and
the DQ dial will be the major balancing control. That
is, the position of the CGRL dial won't affect the
balance much, hence low accuracy. Therefore, be wary
of high-loss components.
Conversely, very low-loss
components will not be too dependent on the DQ bal-
ance and. hence will have low DQ accuracy. For exarn-
ple, measure the residual bridge capacitance.
In summary, if the impedance t o be measured is
very low, very high, or very lossy, read the relevant
paragraphs in this section and make the required cor-
rections.
Note that, by increasing the measuring fre-
quency, very small inductive impedances can often be
moved up; by decreasing the frequency, small capaci-
tive impedances can be moved up into more easily
measured areas.
4.2
DC RESISTANCE.
The accuracy of dc resistance measurements is
*1% if the CGRL dial reading i s between 1 and 11 a s
long a s there is enough sensitivity.
Below 1
a,
the
accuracy is limited t o i l O m a by the sensitivity.
Above 100
kn,
an external supply is required t o get
1% accuracy.
For low-resistance measurements, short, heavy
leads should be used a s connections t o the unknown.
The zero resistance of the leads should be measured
with the free ends connected together, and subtracted
from the bridge reading with the unknown in place. The
user should be particularly careful when using banana-
pin connections.
For best connection t o the bridge,
tighten the binding post hard enough t o notch the wire
inserted in the hole.
4.3 AC RESISTANCE.
The accuracy of the R reading is 4% if the
balance is made between 1 and 11 on the CGRL dial.
Below 1 on the dial the accuracy is
a division.
Thus the over-all accuracy is *1% or
d
milliohm,
whichever is greater, a s long a s the 1-milliohrn resid-
ual resistance is subtracted from the R reading.
ACCURACY
4-1
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