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Operation
2-22
2.9.2 Shielded fixture construction
Since shielding is so critical for proper operation of external
feedback, it is recommended that a shielded fixture similar to
the one shown in Figure 2-16 be used to house the feedback
element. The fixture is constructed of a commercially avail-
able shielded fixture modified with the standard BNC con-
nectors replaced with triaxial female connectors. For
convenience, a banana jack can be mounted on the box to
make the necessary PREAMP OUT connection. Alternately,
a wire could be run through a rubber grommet mounted in a
hole in the side of the box. Note that input low is connected
to chassis ground within the shielded box. This connection
can be made by using a small solder lug secured with a
screw.
2.9.3 External feedback procedure
Use the following procedure to operate the Model 6512 in
the external feedback mode.
1. Connect the feedback element between the PREAMP
OUT terminal and the Input High terminal.
2. Place the instrument in the external feedback mode by
pressing the SHIFT then VOLTS buttons in that order.
The AMPS and VOLTS indicators will illuminate simul-
taneously in the external feedback mode.
3. The display will show the voltage measured at the out-
put of the input preamplifier (PREAMP OUT). Howev-
er, the “V” exponent will not appear as in the volts
mode. For example, with a 150mV output the display
will show:
150.00 m
(External feedback may be temporarily digitally cali-
brated as outlined in paragraph 7.4.16.)
4. The external feedback mode may be cancelled by press-
ing one of the four functions keys (VOLTS, OHMS,
COUL, or AMPS).
A. CONSTRUCTION
HI
B. EQUIVALENT CIRCUIT
TO 6512 INPUT JACK
TO PREAMP OUT
3
FEEDBACK ELEMENT
SOLDER LUG
1
INPUT LOW (INNER SHIELD)
INPUT HIGH (CENTER CONDUCTOR)
2
4
2
5
PREAMP OUT
6512 INPUT AMP
S
LO
TO RANGING
AMP AND A/D
GND
HI
LO
GND
SHIELDED
FIXTURE
FEEDBACK
ELEMENT
237-ALG-2
CABLE
7078-TRX-3
CABLE
FROM SIGNAL
HI
LO
PARTS LIST
ITEM DESCRIPTION
MFR. PART NUMBER
1
SHIELDED FIXTURE
POMONA #2390
2
FEMALE TRIAXIAL
KEITHLEY 7078-TRX-TBC
3
BANANA JACK
KEITHLEY BJ-9-2
4
TRIAXIAL CABLE
KEITHLEY 237-ALG-2
5
TRIAXIAL CABLE
KEITHLEY 7078-TRX-3
Figure 2-16
Shielded fixture construction
Summary of Contents for 6512
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