How come the display shows “UNCAL” below 1Hz?
The DDS system resolution below 2MHz output frequencies is about 1.7764µHz. The output frequency of the FireFox can be set by the user with 10µHz resolution. This step size when combined with the DDS internal resolution of 1.7764µHz creates an average residual output error of about +-0.8882µHz. This error becomes more pronounced as the output frequency goes below 1Hz, with a worst-case performance when selecting an output frequency of 10µHz, which is actually generated as 10.6584µHz. The worst-case error at 10µHz output is thus a significant 6.58%. With an output frequency of say 100Hz, the error due to the DDS resolution is about +-9×10-9, which is typically neglectable, but still above the FireFox OCXO accuracy when the system is locked to GPS. An external CMOS, TTL, or PLD divider is recommended if extreme accuracy for frequencies below 100Hz is required. This divider will allow the FireFox system to run the DDS at a frequency of 100Hz or higher, preventing the residual DDS error to
