Keithley 2002, User Manual

Page: 49 / 366

Front Panel Operation
2-32
2.4.4 Frequency
The Model 2002 can make frequency measurements from
1Hz to 15MHz through its INPUT HI and INPUT LO termi-
nals, and from 1Hz to 1MHz through its AMPS and INPUT
LO terminals. The basic measurement procedure for FREQ
is contained in the Getting Started manual.
Trigger level
The frequency function has an adjustable trigger level. An
appropriate trigger level is needed for the frequency counter
to operate properly. The instrument only counts cycles with
peak amplitudes that reach the trigger level. For example, if
the trigger level is set for 5V, cycles with peak amplitudes
less than 5V are not counted.
If using AC+DC coupling, make sure the trigger level
accounts for the DC bias level. For example, if a 1V peak-to-
peak input signal is riding on a 5V DC bias level, a trigger
level of 5.5V is appropriate.
While the display is showing frequency measurements, the
RANGE
▲
and
▼
keys can be used to change the trigger lev-
el of the measurement.
Each press of a RANGE
▲
or
▼
key adjusts the trigger level
by 0.5% of the presently selected maximum signal level to a
maximum of 60% of the range. The AUTO RANGE key
returns the trigger level to 0V or 0mA. After pressing one of
the range keys, the present trigger level is momentarily
displayed. The trigger level is also available as a multiple
display.
See Table 2-21 for the trigger level ranges and increments.
Frequency configuration
The following information explains the various configura-
tion options for frequency measurements. The configuration
menu is summarized in Table 2-22. This menu is accessed by
pressing CONFIG and then FREQ. Paragraph 2.3.5 summa-
rizes the rules for navigating through the menus.
Note that a function does not have to be selected in order to
be configured. When the function is selected, it will assume
the programmed status.
MAX-SIGNAL-LEVEL
The maximum signal level is used to specify the maximum
expected peak input voltage or current level for frequency
measurements.
The following MAX-SIGNAL-LEVEL selections depend on
which input terminals are presently selected (see INPUT-
TERMINALS):
1V, 10V, 100V, 1000V or TTL: Sets the maximum signal
level for the voltage input terminals. Selecting the TTL
parameter configures the maximum signal level to 10V and
the trigger level to +0.8V.
1mA, 10mA, 100mA or 1A: Sets the maximum signal level
for the AMPS input terminals.
The MAX-SIGNAL-LEVEL setting is critical to the accu-
racy of frequency measurements. If the selected level is too
large, the input signal is not large enough to trigger the
instrument’s zero-crossing circuitry, and a 0mHz reading
results.
To verify the proper level, use the AC voltage or AC current
function to measure the rms value of the input signal. The
resultant reading indicates the best level to use. For example,
if the rms value is 2V, a maximum signal level setting of 10V
is appropriate. Note that the coupling used in the AC
measurement should match that used in the frequency
measurement.
RESOLUTION
The available display resolutions are 4 digits and 5 digits.
There is no auto resolution parameter on the frequency func-
tion.
4-DIGITS: Sets display resolution for frequency measure-
ments to 4-digits.
5-DIGITS: Sets display resolution for frequency measure-
ments to 5-digits.
Table 2-21
Trigger level range and increments
Maximum
signal level
Trigger level
Range Increment
1V
10V
100V
1000V -0.600 to +0.600V
-6.00 to +6.00V
-60.0 to +60.0V
-600 to +600V 0.005V
0.05V
0.5V
5V
1mA
10mA
100mA
1A -0.600mA to +0.600mA
-6.00mA to +6.00mA
-60.0mA to +60.0mA
-600mA to +600mA 0.005mA
0.05mA
0.5mA
5mA