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4.7 D AND Q ACCURACY.
Figure 4
-
1.
Copocitonce to ground for copocitonce
meosurement.
serious effect.
(For measurements of grounded com-
ponents refer t o paragraph 3.10.)
The effects of s t r a y capacitances to the panel
(ground) are usually negligible in the capacitance
bridges (Figure 4-1).
Capacitance from the LOW ter-
minal t o ground ( C A ) s h u n t s the detector and c a u s e s
no error. Capacitance from the other terminal t o ground
( C B ) shunts the standard capacitor (CT) and produces
an error of
Since C T is large, it t a k e s 1000 p F t o produce a 1%
error (when D i s small).
In the inductance bridge (Figure 4-2) C A i s
across the detector and h a s no effect, but C B shunts
the CGRL rheostat.
Capacitance across t h i s rheostat
c a u s e s a D ( l / Q ) error of - w R ~ C B - The L error is
usually negligible except when Qx is very low.
D (or 1/Q) accuracy is dependent upon frequency
and the CGRL d i a l setting.
CGRL d i a l setting of 1 or above:
1-kHz or lower: kO.OO1 *5%
Above 1-kHz:
k0.001 ( f / l kHz) k5%
CGRL d i a l setting below 1:
1-kHz or lower: qO.OOl)(l/CGRL d i a l setting)
*5 %
Above 1-kHz: +(O.OOl)(l/CGRL d i a l setting)
( f / l kHz)
*5%
NOTE
The percentage accuracy, 5%, applies
directly t o Q, but t h e fixed-accuracy
term, k0.001, d o e s not apply directly
because Q
=
1/D
=
1/+0.001
=
k1000,
which is not true. Also, the corrections
for residual and lead impedances must
be taken into account (paragraph 4.10).
4.8 ORTHONULL ACCURACY.
T h e advantage of Orthonull is illustrated in
Figure 4-3, which i s a plot of the numbers of adjust-
ments n e c e s s a r y for a balance.
Not only d o e s the
Orthonull reduce the number of balances, but i t permits
1% measuremenrs that would otherwise be impossible
below a Q of 1/3, due t o the finite resolution of the
DQ rheostat.
T h i s finite resolution c a u s e s the meter
indication t o vary in jumps when Orthonull is used a t
Q's below 1/3.
However, by choosing the best null,
1% accuracy is possible with Q's of l e s s than 0.2.
As
Q i s further reduced, i t is eventually impossible t o
achieve 1% balances.
The accuracy that c a n be ex-
30
ACCURACY
4-3
I
1
I
I
I
VALSE
NULL'
-
CAUSED
BY
-
LACK
OF
RESOLUTION
\
Thus, for inductance measurements, i t i s desirable t o
25
connect the terminal with the most capacitance t o
ground to t h e UNKNOWN terminal marked LOW.
V)
I -
c
I
2 0
V)
2 z
0
-
4 2
g z
1 5 -
E
z
I
+
3 W
z w
1 0 -
I-
5
IbUL24
8.2
-
L\
-
-
-
-
-
-
SEVERAL-
DIPS
-
I
WITH ORTHONULL
1
I
I
I
0.1
0.5
1.0
2.0
3.0
Q
lbS(i32
Figure 4 -
2.
Copacitonce to ground for inductance
measurement.
Figure
4
-
3.
Number of balances vs
Q.
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