User`s guide

Remarks
Frequency is measured on ac voltage signals. Thus, the proper ac voltage
range should be set first using the
[SENSe:]FREQuency:VOLTage:RANGe[:UPPer] command. The voltage
range should be 0.1 volts or greater for more accurate frequency
measurements.
The <range> parameter is required only to specify the measurement
resolution parameter. Therefore, it is not necessary to send a new
command for each new frequency to be measured.
If the input signal voltage is greater than can be measured on the selected
voltage range (manual ranging), the instrument gives an overload
indication: "VAC OVERLOAD" from the front panel or "+9.9E+37" from the
remote interface. Autoranging can be enabled for the input voltage.
The frequency measurement does not autorange. For frequency
measurements, the range and resolution parameters affect the aperture
(gate time) as follows:
Resolution Aperture
100 ppm x
<range>
1 ms
10 ppm x
<range>
10 ms
1 ppm x <range>
100 ms
(default)
0.1 ppm x
<range>
1 s
With no signal applied "0" is returned.