Frequency And Period Measurements; Trigger Level; Gate Time - Keithley 2701 User Manual

3-46 Basic DMM Operation

Frequency and period measurements

The Model 2701 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:1MΩ || <100pF, AC coupled.
The instrument uses the volts input 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
frequency crosses the zero level. The Model 2701 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 2701 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
commands (Table 3-7). 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 2701 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 2701 returns a reading.
and keys. The signal voltage must be greater than 10% of
The voltage limit is subject to the 8 × 10
Model 2701 User's Manual
7
VHz product.