Keithley 2790, Reference Manual

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Model 2790 SourceMeter
®
Switch System Reference Manual 3-27
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While in the normal measurement state, you can use the and keys to close
channels. In general, each key press will open the presently closed channel and
then close the next higher or lower channel.
6. Observe the displayed reading.
7. To measure other switching channels, repeat steps 5 and 6.
8. When finished, press OPEN if there is a channel closed.
Frequency and period measurements
The Model 2790 can make frequency measurements from 3Hz to 500kHz on voltage
ranges of 100mV, 1V, 10V, 100V, and 750V. Period (1/ frequency) measurements can be
taken from 2µs to 333ms on the same voltage ranges as the frequency.
Input impedance: 1M
Ω
|| <100pF, AC coupled.
The instrument uses the volts input to measure frequency. The AC voltage range can be
changed with the RANGE
and keys. The signal voltage must be greater than 10% of
the full-scale range.
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The voltage limit is subject to the 8
×
10
7
VHz product.
Trigger level
Frequency and period use a zero-crossing trigger, meaning that a count is taken when the
frequency crosses the zero level. The Model 2790 uses a reciprocal counting technique to
measure frequency and period. This method generates constant measurement resolution
for any input frequency. The multimeter’s AC voltage measurement section performs input
signal conditioning.
Gate time
The gate time is the amount of time the Model 2790 uses to sample frequency or period
readings. Use the RATE key to set the gate time; SLOW sets the gate time to 1.0 sec, MED
sets it to 0.1 sec, and FAST sets it to 0.01 sec. For remote programming, the gate time can
be set from 0.01 to 1.0 sec using the FREQuency:APERture and PERiod:APERture com-
mands (
Table 3-5 ). Note however, that if you set a gate time other than 1.0, 0.1, or 0.01
sec, the SLOW, MED, and FAST annunciators will be off.
The Model 2790 completes a reading when it receives its first zero-crossing after the gate
time expires. In other words, the reading is completed 1/2 cycle after the gate time has
expired. For example, to sample a 3Hz frequency, you may wait up to three seconds before
the Model 2790 returns a reading.