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SECTION 3
Operation
3.6
FREQUENCY
SELECTION
The impedance
of a device under test changes with fre-
quency. Therefore,
it is advisable
to measure the device
at its operating frequency.
Use the following
guidelines
to select a measurement
frequency:
120Hz: Use to measure large values of C
1kHz:
Use to measure intermediate
values of R, L
and C
1OkHz and 1OOkHz: Use to measure small values of
LandC
To select
one of the four measurement
frequencies
labeled
on
the
front
panel
(120Hz,
lkHz,
lOk.Hz,
lOOkHz), press and release the appropriate
FREQ key.
The
selected
frequency
is denoted
by the enabled
annunciator.
3.7
SIGNAL LEVEL SELECTION
The instrument
measures
devices by applying
an rms
signal (at the selected
frequency)
and then measuring
the subsequent
current.
The signal levels that can be
selected are 50mV rms and 1V rms.
The IV signal level can be used for most measurements.
For semiconductors
that have non-linear
characteristics
that are affected by signal magnitude,
use the 50mV
level.
The front panel LEVEL annunciator
denotes the current
signal level. To select the alternate
level, simple press
the 50mV or 1V key.
The specified
signal levels are maintained
for devices
that have
an impedance
of lm
or more. At lower
impedances,
loading lowers the level of selected signal.
Note: After a signal level is selected,
the displays
will
blank until the next measurement
is completed.
3.8
DC BIAS
When
measuring
capacitance
(C), a DC bias can be
applied.
This
b ias
allows
capacitance
changes
in
semiconductor
junctions
due to applied voltage
to be
measured.
The instrument
has a built in 2V bias that can be used,
or an external
bias voltage
from 0 to *35V
can be
applied via a BNC connector on the rear panel. The bias
voltage (and drive signal) can be monitored
on the H
CUR terminal.
DC bias cannot be used to measure L, R or Z. Pressing
the BIAS key while in one of these functions will result
in "Err 14".
Bias
Stabilization
Time
-
When
bias
is abruptly
changed
by turning
BIAS on to its peak value, some
time is required
to allow the bias voltage
to stabilize
before
an accurate
capacitance
measurement
can be
made. Use the following equation to determine the time
required to achieve stability:
Stabilization
time (in seconds) = 4 + 0.015C
where; C is the capacitance
in PF
Perform the following steps use DC bias:
1.
2.
3.
4.
If using an external
supply, connect it to the BIAS
EXT IN BNC connector on the rear panel. Make sure
the supply is in standby and is set for OV.
CAUTION
To prevent
damage
to the instrument,
do not
apply voltage that is not within the OV to f35V
range. Also, adhere to the precautions
and con-
siderations
explained affer this procedure.
Set the rear panel BIAS switch. If using the internal
supply, set the toggle switch to the INT 2V position.
If using an external bias supply, set the toggle switch
to the EXT position.
Using the (A( DISPLAY keys, select the C function.
Note that AUTO function must be disabled. DC bias
cannot be enabled in AUTO function.
Connect the device to be measured.
CAUTION
When checking a polarized device, make sure to
observe
polarity. Also, make sure to discharge
capacitors before connecting
them to the input.
3-15
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