Jackson Labs FireFox, User Manual, Page: 50 / 68

FireFox Synthesizer User Manual
44 © 2006 Jackson-Labs
3.3.5 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 +-9x10-9, which
is typically negligible, 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 from becoming significant.
3.3.6 What do the LED’s in the front panel indicate?
The front panel outputs have frequency range limits at which their outputs are valid. The green
LED’s indicate when a particular output is valid and usable. The CMOS and RF outputs are disabled
outside of their operating ranges. The LVDS outputs are active below their respective lower
operating limit (10MHz) in an under-scan mode. Operation of the LVDS outputs in under-scan mode
cannot be guaranteed, and the LVDS LED is thus turned off. The CMOS output is driven in over-scan
mode between 350MHz and 390390MHz, and the CMOS LED is thus turned off in this frequency
range.
3.3.7 What do I have to do to move the FireFox unit to a
different location?
The FireFox unit is location sensitive due to the GPS receiver operating in location-hold mode. When
factory new, the receiver averages ten thousand GPS readings to establish its permanent new
location. This operation is done only once for every new location, or whenever the receiver has been
turned-off long enough to discharge its internal backup batteries. It uses this location as a reference
point for receiving the timing solution. Establishing a permanent position is called Auto-Survey
mode, and operating with this location data is called position-hold mode.
For the FireFox unit to operate properly and within specification, the unit should be placed into Auto
Survey mode any time its antenna is moved more than 30 feet (10m). This has to be done only once
every time the receiver antenna is moved.
Placing the unit into Auto Survey mode also requires the antenna to have an unobstructed view of the
sky for as long as it takes to receive the ten thousand 3D readings that are required to establish the
final position data. After the receiver has established its permanent position, only one GPS satellite is
required for synchronizing the internal OCXO, thus greatly relaxing the placement requirements of
the GPS antenna (please note that the receiver requires at least 4 satellites to be operational when the
T-RAIM timing mode is turned on).