Design Integration Considerations
X72 Susceptibility to Input Noise
40 X72 Desiger’s Reference and User’s Guide 097-10603-01 Rev. A – November 2004
X72 Susceptibility to Input Noise
If you have an application where the output spectrum phase noise and spur integrity
is crucial, the X72 must have a comparatively clean source of dc power (free of
spurious current or voltage noise). Connecting fans, heaters, and other switching
devices to the dc supply powering the X72 can result in degraded phase noise and
spur performance. Best performance is achieved with only one output turned on and
the other frequency outputs turned off.
The Rb atomic frequency source uses a modulation/demodulation lock-in amplifier
scheme with a modulation frequency of approximately 156 Hz. Inherent in this
approach is sensitivity to noise at multiples of the modulation frequency. This noise
is coupled through both the heater and electronic power lines to cause modulation
spurs on the output frequency.
The X72 has an internal dc to dc converter supplying power to the critical
electronics, including the crystal oscillator, which is the source for the outputs.
Input Filtering
If filtering is added at the input connector of the X72, it cannot have resonance
points greater than the specified impedance. This would result in peaking the
potential for detrimental oscillations. The optimum filtering is less than 1 Ω from dc
to 100 kHz.
In addition, the input operating voltage range specified for the X72 during turn-on
must continue to be met during operation of the unit. For example, using a 1 Ω dc
source resistance for the input supply line would not be appropriate. The voltage
drop resulting from this resistance (caused by the turn-on current or quiescent
operating current) would cause the input voltage to drop below the allowable value.
Note: Avoid the modulation frequency and its lower harmonics
(roughly up to the tenth harmonic).
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