Measuring Frequency And Period; Trigger Level - Keithley 2000 User Manual

2-26 Basic Measurements

Measuring frequency and period

The Model 2000 can make frequency measurements from 3Hz to 500kHz on voltage ranges
of 100mV, 1V, 10V, 100V, and 750V. Period measurements can be taken from 2µs to 333ms on
the same voltage ranges as the frequency.
The instrument uses the volts input terminals to measure frequency. The AC voltage range can
be changed with the RANGE
the full-scale range.
CAUTION

Trigger level

Frequency and Period use a zero-crossing trigger, meaning that a count is taken when the fre-
quency crosses the zero level. The Model 2000 uses a reciprocal counting technique to measure
frequency and period. This method generates constant measurement resolution for any input fre-
quency. The multimeter's AC voltage measurement section performs input signal conditioning.
Gate time
The gate time is the amount of time the Model 2000 uses to sample frequency or period read-
ings. All settings of the RATE key (FAST, MEDium, SLOW) yield a gate time of one second.
The Model 2000 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, with a 1sec gate time to sample a 3Hz frequency, you may wait up to 3 seconds
before the Model 2000 returns a reading.
and keys. The signal voltage must be greater than 10% of
The voltage limit is subject to the 8 10
7
V•Hz product.