User`s guide

Remarks
This command sets the aperture for both frequency and period
measurements.
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
The instrument sets the aperture time to 100 ms after a Factory Reset
(*RST command) or an Instrument Preset (SYSTem:PRESet command).