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FireFox Synthesizer User Manual
estimate of the overall time base drift over the last 30 minutes can be established. This drift is
typically less than +5x10^-11 over a thirty-minute period when locked to the GPS signal.
As an example, let's assume the Frequency Estimate display indicates 1.6 x10^-11. At 100MHz
output frequency we can calculate the estimated accuracy of the output signal to be:
100MHz * 1.6 x10^-11 = 0.0016Hz
Thus the absolute frequency error of the output (to the UTC atomic reference) is estimated to be
1.6mHz, and the output frequency is thus estimated as 100,000,000.0016Hz.
3.6.5 There is a Resolution number displayed in the menus.
What is this number?
This value shows the minimum step resolution that the DDS can generate in the particular frequency
band it is operating in.
The DDS system has 32bit integer accuracy with an additional 16bit fractional component. The
resolution is applied to three frequency ranges (DC-390, 390-770, 770-1640MHz). The effective
frequency resolution is thus over 48bits.
This resolution determines the smallest frequency step size that can be generated by the DDS system,
and thus determines the residual mathematical error of the system. The smallest fractional frequency
step that can be generated in Fractional-N mode is smaller than the 10µHz step size of the FireFox
system, and the value displayed in the LCD can thus be used to estimate the frequency error at any
particular frequency.
See also the Fractional-N mode FAQ.
3.6.6 There is an option to turn Fractional-N mode on/off. What
is this used for?
This setting option allows exact, cycle-accurate coarse frequencies to be generated (32bit integer
mode), or frequencies with a much finer resolution (48bit fractional mode) but with a very small
fractional residual uncertainty.
Consider the following explanation:
the DDS has a 32bit integer component. This integer component alongside the 1Gs/s sample rate
allows 2^32 individual frequencies to be generated without any residual error according to the
following formula:
Fout = (32bit Tuning Word * 1GHz)/ 2^32
This formula is accurate for output signals from 2MHz to 390MHz. Use the following formulas to
determine the resolution of the output signal for the remaining frequency bands:
DC to 2MHz: Fout = (32bit Tuning Word * 0.5GHz)/ 2^32
390 to 770MHz: Fout = (32bit Tuning Word * 2GHz)/ 2^32
770MHz to 1640MHz: Fout = (32bit Tuning Word * 4GHz)/ 2^32
The frequencies generated with fractional-N mode turned off are thus cycle-accurate without any
slipped cycles or residual frequency error. As an example, consider the following example of
generating a 10MHz frequency with a tuning word of 4294967296:
Summary of Contents for FireFox
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Page 2: ...FireFox Synthesizer User Manual Copyright 2008 Jackson Labs...
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