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SECTION III
Model 4277A
The internal de bias voltage is monitored by a
DVM or an oscilloscope at the EXT INPUT/INT
MONITOR connector on the rear panel.
Note
The dc bias voltage monitored at the
EXT INPUT/INT MONITOR connector
may contain a small ae component.
When the BUT impedance is higher than lOOkH,
the monitored voltage is equal to the de voltage
source voltage, and to the voltage applied to the
BUT.
These voltages, however, are different
when the BUT impedance is less than lOOkfi.
The
following
paragraph
describes
how
to
measure the actual bias voltage across the BUT.
1, R
1
/R
2
/RL. Detection
(a)
Set the TEST SIG LEVEL to LOW.
(b)
Set the LC I Z ! range so that the range
resistor value will be lOOIl.
Refer to
Figure 3-5.
(c)
Set the DC BIAS voltage to +5V on the
FREQUENCY/DC BIAS display.
(e)
Set the DC BIAS switch on the front
panel to ON.
(f)
Measure the monitor voltage (Vo) at the
EXT INPUT/INT MONITOR connector.
(g)
Connect a reference resistor (Ro) (e.g.,
100Q±l%)to the test fixture.
(h)
Measure the dc voltages at the HIGH and
LOW terminals of the test fixture and at
the
EXT
INPUT/INT
MONITOR
connector
(Vh
,
V
i.,
and V
k).
Note
Connect the LOW terminal of the
DVM or the oscilloscope to the
GUARD
terminal
of
the
instrument.
(i)
Calculate the resistances, Ri, R
2
, and
Rl, using the following equations;
Ri ={Vo "Vk)*Ro/(Vh -Vl)
R2 =(Vk = Vh)-Ro/(Vh -Vl)
Rl = Vl-Ro/(Vh - Vl)
(d)
Connect nothing to the test fixture.
Figure 3-32. Internal DC Bias Voltage Monitor (Sheet 1 of 2),
3-74
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