[time-nuts] Symmetricom CSAC is Cs acting like a Rb unit
brooke at pacific.net
Wed Jan 19 15:17:58 UTC 2011
Here's a response I got from Symmetricom:
· The SA.45s actually uses cesium, not rubidium. But it is a vapor-cell
clock just like our Rb clocks.
· The Tx and Rx pins are for an RS-232 interface. Using the interface,
the clock can be programmed, and the clock’s functions can be monitored.
I have attached a User’s Guide that describes all the commands in detail.
· The SA.45s does not attempt to correct the 1 PPS input, nor does it
use a sawtooth to correct itself to the 1 PPS input. Instead, it can be
commanded (see User’s Guide) to discipline itself to the 1 PPS input.
How fast it does this is a function of user-selected bandwidth inputs.
In the manual (CSACUsersGuide rev 004.pdf) they talk about the
evaluation board and it's associated software. The main screen shot
shows the following parameters for the physics package:
Laser Current: 0.86 ma
Heater Power 11.01 mW
DC Signal 0.996 V
TCXO Tuning 1.518 V
Contrast 4415 (no units)
A block diagram shows a 10 MHz TCXO driving a 4596 MHz Synthesizer which
drives the physics package.
The error signal from the physics package drives the loop filter which
tunes the TCXO.
The BITE output is a lock indicator.
I get the feeling that a vapor cell is not an absolute standard but
rather works like a Rb vapor cell, i.e. the frequency has some drift and
the starting frequency depends on how the cell was made.
The 1 PPS output can be synchronized (using the 1 PPS input) to the
nearest 10 MHz zero crossing (i.e. to within 50 ns). With a continuously
applied 1 PPS input after some time it will adjust both the frequency
and 1 PPS output to about 5E-13.
"5.7.2 The "Art" of Disciplining
Implemented correctly, disciplining can be utilized to calibrate the
CSAC frequency in the field, even if a reference source is only
occasionally or sporadically available, thereby improving the long-term
performance (phase and frequency drift) of the CSAC. Alternatively, the
disciplined CSAC may be used to "clean-up" the short-term stability of
an accurate, but noisy,, reference source, such as GPS.
Implemented incorrectly, however, disciplining may degrade the
performance of the CSAC if, for example, the CSAC is disciplined with a
short time constant to a source which is itself noisier than the CSAC,
such as GPS.
Implementing a successful discipling strategy involves understanding the
noise properties of the CSAC, the reference source, and the phase meter
itself, and selecting the appropriate time constant that makes the best
use of the available timing information."
Fig 16 is an Allan plot showing: CSAC, Phase Meter noise floor, Lab
grade Cesium standard & GPS
At 1 second:
CSAC is 1E-10, Phase Meter is 4E-10, GPS is 2E-8, lab Cs is 1E-11
GPS gets better than the CSAC at about 5,000 seconds
The Phase meter gets better than the CSAC in about 20 seconds
So, for example, when using GPS to disipline the CSAC the time constant
should be set to >= 5,000 seconds. Or when using a lab grade Cs standard
the time constant should be set to >= 20 seconds.
R. Lutwak et. al., "The Chip-Scale atomic Clock - Low-Power Physics
Package", Proceedings of the 36th Annual PTTI Dec 7-9, 2004
R. Lutwak, et. al., "The MAC - A Minature Atomic Clock", Proceedings of
the 2005 joint IEE Freq Ctrl Sym & PTTI
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