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997-01530-01, Rev. C-25, Jan 2008 101
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Theory of Operation: Freq Meas uses a heterodyne phase error multiplier to achieve high resolution at
short sample periods. Using its internal disciplined frequency, the XL-GPS records or timestamps the
zero crossing of the J3 input frequency once per measurement 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:
Where
FREQ OFFSET +x.xxxxxx-xx is the fractional frequency offset measurement divided by the
COUNT xxxxxxx 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.
DISABLE = ASCII character string "DISABLE" to disable J3 as input port
FREQ OFFSET COUNT
+x.xxxxxx-xx xxxxxx