Frequently Asked Questions - Application/Usage
- What is the difference between the FireFox Desktop unit and the FireFox OEM PCB?
- What applications is the FireFox Synthesizer designed for?
- Can the FireFox system keypad be locked?
- How come the Frequency Display doesn’t let me select the exact Frequency I need?
- How come the display shows “UNCAL” below 1Hz?
- What do the LED’s in the front panel indicate?
- What do I have to do to move the FireFox unit to a different location?
- What happens to the FireFox settings when the power is turned off?
- How accurate is the output power on the RF output connector?
- 1. What is the difference between the FireFox Desktop unit and the FireFox OEM PCB?
The FireFox Desktop unit uses the same FireFox OEM board that is also sold separately. The desktop unit includes a Swiss-made enclosure, a power supply, an LCD Display, a Keypad, and external connectors for all the I/O connections available on the FireFox OEM board. The FireFox OEM board is typically installed into a user enclosure, and statically set to a fixed frequency, or controlled via the built-in RS-232 serial port.
- 2. What applications is the FireFox Synthesizer designed for?
The FireFox synthesizer has five independent outputs to allow it to be used in almost any application that requires a highly stable and accurate analog sine-wave signal, or digital clock signal. It combines a function generator, signal generator, and GPS Disciplined frequency reference into one small unit. As a bonus, the unit also includes a generic, non-dedicated 2GHz mixer usable for building receivers, or up/down-converters.
The units’ outputs are optimized for the following uses:
High-power HCMOS 3.3V/5Vpp digital output signal, DC to 398MHz. This signal is a typical CMOS output signal that can drive an un-terminated 50 Ohm transmission line at either 5V or 3.3V peak (selectable via switch on the back panel). The output driver is strong enough to also drive 50 Ohm resistor end-terminated transmission lines up to 2.4V (5Vpp setting), which is enough signal-level to switch typical 3.3V CMOS logic or TTL inputs. The rise and fall times of this output are typically less than 1ns. The output can typically provide 32mA of drive current. This signal can be used in any digital logic application up to 398MHz, far outperforming competitive solutions allowing it to be used in SDR, DDR, QDR memory interfaces, or as CPU master clock drivers.
50 Ohm AC-coupled spectrally pure RF sine-wave output from 2MHz to 1640MHz. Used in any type of communication applications that require a low phase-noise, spectrally pure output signal. Adjustable from typically less than -40dbm to up to +18dbm depending on the frequency (in relative output mode, when setting 0% to 100% output power).
LVDS digital differential, 50 Ohm signal on two SMA connectors. Typically terminated by a 100 Ohm resistor between the two signals. This signal is used in modern high-speed interfaces such as LCD panels, FPGA’s, memories etc. These signals can also be used as standard RF signals when connected via an AC coupler (output at ca. -3 to -5dbm), or as PECL differential signals when terminated to ground via two 50 Ohm resistors. The signal is being driven from 10MHz to 1640MHz, with an under-range down to 2MHz.
BNC connector with a 10MHz very low phase-noise Reference clock output, digital CMOS 3.3V level. This signal in combination with a CMOS 3.3V level 1PPS output is similar to other frequency references on the market, and may be used as an in-house frequency standard to drive other instruments for better than 10-12 long term performance. The 1PPS signal has a better than <10ns 1-Sigma offset to the UTC clock (<20ns 6-Sigma). This signal may also drive 50 Ohm end-terminated transmission lines with typically 1.6Vpp, and less than 2ns rise/fall times.
Broadband 2GHz analog mixer. A general-purpose broadband mixer with over 350MHz bandwidth is available on the front panel. This can be used in receiver, transmitter, or transponder applications. Any RF signal up to 2GHz may be mixed with an LO signal up to 2GHz, and produce a 350MHz bandwidth IF signal. The RF signal input can be driven by a -20dbm to +10dbm signal. The LO input can typically be driven by the Synthesizers’ own RF-type output (see above) at 0dbm. To avoid damage, a signal amplitude maximum of +10dbm should not be exceeded on the RF and LO inputs.
- 3. Can the FireFox system keypad be locked?
Yes, the FireFox keypad can be locked with jumper JP11.
- 4. How come the Frequency Display doesn’t let me select the exact Frequency I need?
You probably have turned off Fractional-N mode (see also “Resolution” FAQ under Performance section). With fractional-N mode disabled, the frequency granularity that can be generated is 1/65536 of the granularity when the fractional-N mode is enabled.
A warning message in the LCD display shows “FractOff” to indicate that the fractional-N mode is turned off.
When the fractional-N mode is turned off, the frequency will still be increased or decreased in 10µHz steps when pressing the UP/DOWN buttons, but the display will show the actual frequency that can be generated by the DDS (closest match to the desired frequency) once the button is released.
The advantage of turning off the fractional-N mode is that the system generates an exact mathematically deterministic frequency without residual, fractional errors that are present in fractional-N mode.
- 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 +-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 become significant.
- 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.
- 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).
- 8. What happens to the FireFox settings when the power is turned off?
The FireFox units have non-volatile memory that stores settings such as the frequency, amplitude, and menu options. This non-volatile memory does not require any battery backup. The unit will thus power-up with the last frequency and amplitude setting used prior to power-down.
Several important settings are stored in the GPS receiver and are backed up by a rechargeable battery that has a life span of typically several days when the unit is powered off. These settings are the position established by the Auto-Survey function and used for position-hold mode, the time offset from GMT, and the receiver operating-mode.
- 9. How accurate is the output power on the RF output connector?
The FireFox units offer two output modes on the RF BNC connectors, relative and absolute output mode. The operating mode is selected in the “SETUP” menu. Relative mode allows controlling the output power from 0% to 100% of the units’ capability, with the actual output power level varying with the frequency. This mode allows the user to harvest the maximum possible output power the unit can generate at any given frequency. Maximum power levels in relative mode typically range from +8dBm to +18dBm depending on the output frequency.
Absolute mode is used when highly accurate power levels are required. Absolute mode sets the output power to factory-calibrated levels ranging from -40dBm to +10dBm. The output power at the RF BNC connector is accurate to within -1dBm of the value displayed in the LCD over the units’ frequency range (-0.15dbm typical accuracy).