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The Fe-5680A Rb frequency standard is 3.5x5.1
inches, a design suited to PC board slot mounted applications.
A simplified functional block diagram is shown in Figure 3.
Many commercial applications for precision
frequency standards demand performance over an expended temperature
range. Often frequency variation with temperature is the performance
range. Often frequency variation with temperature is the performance-limiting
feature of these devices for CDMA holdover applications. The
two products described in this paper have both been designed
to operate over extended temperature ranges while guaranteeing
a frequency variation of less than 1E-10. The FE-5680A operates
at any temperature between -10 and +60 deg. C, a temperature
range required for indoor rack-mounted telecommunication hardware
with limited air flow. The FE-5652A operates at any temperature
between -40 and +85 deg. C, a temperature range required for
outdoor, limited access, low maintenance telecommunication
hardware.
FREQUENCY vs. TEMPERATURE
Frequency variations due to ambient temperature variations
can easily swamp other sources of instability for a Rb frequency
standard operating in a real life environment. Therefore,
special attention was given to frequency stability vs. temperature
in these designs. Figure 5 and 5 show typical performance
data for FE-5680A and FE-5652A. Rb standards over the ambient
temperature range. In addition to the information provided
by these plots, it is useful to understand the distribution
of performance associated with a production lot of these devices.
Table 2 provides this information.
The information in this table shows that the
mean total frequency deviation over temperature is very small.
What this really means is that the frequency deviation is
equally likely to be positive or negative. The standard deviation
values give an indication of the range of deviations to be
expected amongst production devices.
FREQUENCY STABILITY
Allan deviation stability data for the FE 5680A Rb standards
are shown in Figure 7. The measurements used to derive the
Allan deviation were all made at 25 deg. C. It is of interest
to note that the result for SN 4005 is significantly improved
by comparing it to another Rb standard (SN 5359) rather than
a Cs reference. This indicates that the result is limited
by the stability of the Cs reference.
SUMMARY
Performance results for two new Rubidium frequency
standard products have been reported. These products continue
the trend toward small, robust, high-performance atomic frequency
standards capable of operating in harsh non-laboratory environments,
yet providing stabilities approaching those measured under
controlled laboratory conditions. These devices are well suited
to the demands of the telecommunications industry for the
performance-limiting feature of these devices for CDMA increasingly
precise time and frequency references capable of operating
unattended in harsh environments.
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