F113 – J3 Input (Freq Meas)
cycle with 240 picosecond resolution. The number of zero crossings between successive
measurement intervals is also recorded.
When the measurement interval elapses, the previous measurement timestamp is
subtracted from the current one and the difference is divided by the number of zero crossings
between the two timestamps. The result is the average period of the external frequency over
the interval. The reciprocal of this period is compared to the nominal frequency to determine
the fractional frequency offset. The timestamp reported with the resulting measurement is
the ending timestamp of the two phase readings used to make the measurement. Since this
ending timestamp is now the beginning timestamp for the next measurement, there is no
“dead time” in the measurements.
The reported timestamp resolution is sufficient to allow integrating the fractional frequency
offset measurements to fully recover the relative phase of the external frequency source
being measured versus the disciplined XL-GPS internal or external oscillator.
Display: Freq Meas appears as follows in the front panel display/keypad:
FREQ OFFSET | COUNT
+x.xxxxxx-xx | xxxxxx
Where FREQ OFFSET +x.xxxxxx-xx is the fractional frequency offset measurement divided
by the COUNTxxxxxxx measurement interval. These measurements are displayed until a new
F113 configuration is selected, or another function performed.
Command Line
To display the J3 Input Configuration, enter:
F113<CR>
XL-GPS responds as follows if the Freq Meas option has not been installed:
F113<S>DISABLE<CR><LF>
When the Frequency Measurement option is enabled, F113 responds:
F113<S>FREQ MEAS<S><FREQ><S><IMP><S><INT><CR><LF>
Where the F113 entry and request formats are defined as:
F
= ASCII character F.
113
= function number.
SHOW
= ASCII character string "SHOW" for displaying frequency measurements.
098-00116-000 Rev. A............................... Page 139 of 221
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