Condition 
 Used

Features 
 Extremely low silhouette: less than 1.0 inch 
 Stability over temperature: 5x10-11 
 Digitally programmable to 1x10-13 
 Frequency: 10MHz sinewave 
 Fast Warm-up: <5 min.
 Stability: To 5x10-12/ to 2x10-10/year

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.