X
X
L
L
D
D
C
C
M
M
A
A
N
N
.
.
d
d
o
o
c
c
3
3
-
-
5
5
4
4
R
R
E
E
V
V
E
E
Function 29 offers three modes of operation:
OFF
-- No measurements are reported in this mode.
QUERY
-- Measurements are reported only when requested in this mode.
PERIODIC
--
Measurements are continuously output at the specified interval in this mode until
a CTRL-C is sent.
The measurement technique uses a heterodyne phase error multiplier approach to achieve high
resolution at short sample periods. An important feature of this implementation is that "zero dead time"
frequency measurements are performed. In essence the position in XL-DC internal time of a zero
crossing of the externally applied frequency being measured is recorded, or "timestamped", once per
measurement interval with 60 picosecond resolution. (In the QUERY mode of operation, the reception
of the measurement request string causes a timestamp to be performed, rather than at a fixed repetitive
rate as in the PERIODIC mode of operation). In addition, the number of zero crossings between
successive timestamps is also recorded.
When it is time to perform a measurement, either because the selected interval in PERIODIC mode has
elapsed or a QUERY mode measurement request has been received, the previous measurement
timestamp is subtracted from the current one and the difference is divided by the number of zero
crossings between these two timestamps. This result is the average period of the external frequency
being measured over the interval. The reciprocal of this period is then 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" present 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-DC internal or external oscillator.
To view the current settings, send a character string of the form:
F29<CR>
where:
F
=
ASCII character F
29
=
function number
<CR>
=
ASCII carriage return character, Hex 0D
The Serial Port will respond with:
F29<SEP><MODE><SEP><FREQ><SEP><INTERVAL><CR><LF>
where:
<SEP>
=
separator character, either space, comma or tab
<MODE>
=
either OFF, QUERY or PERIODIC
<FREQ>
=
Input frequency in MHz, either 1, 5 or 10 (this field is not sent if
the mode is OFF)
Содержание 151-600
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