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__ T_Y_PE_16_0_8-_A_I_M_P_E_D_A_N_C_E_B_R_I_D_G_E
_
Figure
2-6.
Methods of applying
external ac.
connection to the BIAS terminals (be sure to open the
jumper strap).
See Figure 2-6a. In this connection, the
generator is ungrounded, and capacitance from its ter-
minals to ground must be considered.
Capacitance from
the negative BIAS terminal to ground can cause a large
error at high frequencies when low impedances are meas-
ured. Therefore, use a shielded cable and use the outer
conductor to connect the low generator terminal to the
positive BIAS terminal. Capacitance of over 100 pt from
the positive BIAS terminal to ground can cause appreci-
able error (refer to paragraph 2.4.5.1).
A bridge trans-
former can be used to connect a generator to the BIAS
terminals, but this has no advantage over the use of the
internal bridge transformer unless the external trans-
former has a higher voltage rating, as do the Type 578
Transformers (see Figure 2-6b).
o
100ke
Figure
2-7. D
and Q ranges vs
frequency.
10'"'-'
100
Ikc
10ke
FREQUENCY
o
Emax =+volts (f in cps), or 100 volts,
0.001
t-----+---+-7"-~_+-=-___i
O.OIItH~--+~--'='---t----I-+-t---3lo,rl
2.5.5 D AND Q RANGES VS FREQUENCY.
The
D
and
Q
ranges are functions of frequency.
Also, at frequen-
cies above 1 kc, the whole D and Q range cannot be used
without serious error in the C, G, R, or L reading. The
solid lines of Figure 2-7 give the over-all ranges of the
D or
Q
adjustments for the various bridges. The shaded
ther limit on the voltage applied to the EXT GEN ter-
minals:
whichever is larger. This transformer has a 3-to-l step-
down ratio and an equivalent resistance, referred to the
primary, of 20 ohms.
Therefore, to limit the power ap-
plied to the bridge to 1/2 watt, a series resistor of
E2
---20D should be placed in series with the external
2
supply.
ANY PANEL
L..-
- . .
CONNECTOR
TYPE 578-A
BRIDGE - TRANSFORMER
I
r+-;I~-()
I
I
BI~S
I
~I.--o
EXT
OSC
SHIELDED
LEAD
EXT~
OSCLJ
~S
2:5.4
MAXIMUM APPLI ED AC VOLTAGE.
The max!-
mum ac voltage that may be applied to the Type 1608-A
Bridge Impedance depends on:
a.
the voltage and power ratings of each compon-
ent (including the unknown),
b. the bridge circuit used,
c. the range used,
d. the position of the variable components,
e. the method of applying the voltage.
Exact limits for any specific measurement can be calcu-
lated from the data of Table 2-7 and the circuit diagrams
of Figure 1-2.
If such a maximum voltage is applied,
care· must be taken to avoid any adjustments of the panel
controls that would result in an overload.
A much simpler approach is to limit the power into
the bridge to 1/2 watt so that no bridge components can
be damaged under any conditions. If the power rating of
the unknown is less than 1/2 watt, the input power
should be reduced accordingly.
A series resistor is the
simplest way to limit the power. It should have a value
1:.
2
of R = 4P' where E is the open-circuit generator volt-
age and P the power rating of the unknown component.
The input transformer imposes the following fur-
20
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