XLi Time & Frequency System
147
XLi-man, Issue 8, 6/17/2008, Rev. H
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For example, with a single GPS receiver card configured as PRIMARY in F119, and F74 Clock
Reference set to PRI,
When the XLi reacquires a time reference source and is steering its own internal oscillator, it stops using
Aux Ref as its frequency reference.
For Aux Ref to work:
•
The Aux Ref frequency source must be connected to the J3 input.
•
F113 Input Configuration must be set to Aux Ref
•
F113 Input Frequency must be set to the correct frequency
•
F113 Input Impedance must be set to the correct impedance
Frequency Measurement (Freq Meas) Input
The Frequency Measurement (Freq Meas) option measures an external frequency applied to the J3
input relative to the disciplined XLi oscillator.
Operation:
Measurements are displayed on the front panel display and on the command line at the
specified measurement interval. The front panel displays the measured frequency offset (FREQ
OFFSET) and a countdown (COUNT) to completion of the measurement interval. The command line
states the measurement interval (e.g.,“Interval is 1 seconds”) on the first line, and then starts displaying
each measurements; each one on a new line.
Limitations:
Note that the accuracy of the frequency measurement is based on the accuracy and stability
of the disciplined XLi oscillator over the measurement interval. Short interval measurements of external
very high stability oscillators (e.g., Cesium) tend to measure the XLi oscillator instead. Refer to
“System
Time & Frequency Accuracy” on page 6
, and to
“P7: Oscillators” on page 223
for more information for
the specifications of disciplined oscillators (while locked to a GPS reference source).
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 XLi 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 XLi internal or external oscillator.
Display:
Freq Meas appears as follows in the front panel display/keypad:
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