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The PRS10 is an ultra-low phase noise, 10 MHz rubidium-disciplined
crystal oscillator. The device fulfills a variety of communication,
synchronization and instrumentation requirements. The phase noise
of the 10 MHz output is low enough to be used as the reference source
for synthesizers. The unit's short-term stability and low environmental
coefficients make it an ideal component for network synchronization.
Its low aging rate makes it an excellent timebase for precision
frequency measurements.
The PRS10 can time-tag an external 1 pps input with 1 ns resolution.
These values may be reported back via RS-232, or used to phase-lock
the unit to an external reference (such as GPS) with time constants
of several hours. This feature can provide Stratum 1 performance
at a very low cost.
The PRS10 establishes a new level of features and performance in
atomic frequency standards. Its design provides the lowest phase
noise, greatest versatility, and easiest path to system integration
of any rubidium frequency standard available.
PRS10 Operation and Design
All commercial rubidium frequency standards operate by disciplining
a crystal oscillator to the hyperfine transition at 6.834,682,612
GHz in rubidium. The amount of light from a rubidium discharge lamp
that reaches a photodetector through a resonance cell will drop
by about 0.1 % when the rubidium vapor in the resonance cell is
exposed to microwave power near the transition frequency. The crystal
oscillator is stabilized to the rubidium transition by detecting
the light dip while sweeping an RF frequency synthesizer (referenced
to the crystal) through the transition frequency.
The PRS10 uses a microcontroller, clocked at 10 MHz, to control
all aspects of operation and to allow diagnostics, measurement,
and closed case calibration via an RS-232 interface. The processor
sweeps the RF synthesizer, synchronously detects the optical signal
from the physics package, and servos the 10 MHz crystal oscillator
to the rubidium transition via a 22-bit DAC and a varactor.
When turned "on", the processor applies a voltage to
the varactor corresponding to the last locked value. The frequency-lock
servo is disabled until a useful resonance signal is detected from
the physics package, providing a smooth transition to the final
frequency as the unit warms up. In the case of a problem with the
physics package, the unit will suspend the frequency servo and hold
the varactor voltage at the last locked value.
Manufacturers of rubidium frequency standards sometimes use a crystal
frequency that is an exact sub-multiple of the hyperfine transition
frequency in order to simplify the design of the RF frequency synthesizer.
Some designs use a DDS synthesizer, clocked by the crystal, to generate
the 10 MHz output. Often, the crystal frequency is modulated in
order to sweep the synthesizer through the transition frequency.
The crystals are usually operated in the fundamental mode and not
temperature stabilized. While such approaches are simpler to design,
the phase noise, short-term stability, and spur content of their
outputs suffer.
In contrast, the 10 MHz output from a PRS10 comes directly from
a 3rd overtone, stress-compensated
(SC-cut) crystal oscillator operated in an oven at its plateau temperature.
A dual-loop RF synthesizer, with a crystal IF at 22.482 MHz, is
used to generate 359.72 MHz and make 6.834 GHz via a step recovery
diode. There are several advantages to this approach: the phase
noise is very low (<-130 dBc/Hz @ 10 Hz offset), there are no
spurious components, and the output will be well behaved should
the physics package fail to provide a lock signal. (The aging will
be about 5 × 10-10/day
when not locked to rubidium.) The phase noise plot of a PRS10 shows
a 42 dB reduction in phase noise at 10 Hz offset from the carrier
at 10 MHz when compared to a conventional rubidium standard.
Historically, the lifetime of rubidium frequency standards has
been dominated by rubidium depletion in the discharge lamp. To avoid
excess flicker noise, manufacturers would load less than 100 µg
of rubidium into spherical discharge lamps. The PRS10 uses a lamp
with a side arm loaded with 1 mg of rubidium. This design eliminates
rubidium depletion as a failure mechanism, and provides better temperature
control without excess flicker noise.
GPS Tracking
Frequency offsets and long-term aging of the PRS10 can be eliminated
by phase-locking to a source with better long-term stability, such
as the 1 pps from a GPS timing receiver. As shown in the Allan variance
plot, the short-term stability of GPS is poor (about 5,000 ×
10-12) compared to the stability
of the PRS10 (about 5 × 10-12).
However, over several hours, GPS is more stable, and so the stability
can be improved by phase-locking the PRS10 to GPS with a long time
constant.
The PRS10 can time-tag or phase-lock to a 1 pps input, and provides
a slewable 1 pps output. The input can be time-tagged with 1 ns
resolution, and the result may be read back via the RS-232 interface.
When tracking an external input the time constant can be set from
5 minutes to 18 hours. The 1 pps output may be moved with 1 ns resolution
over the range of 0 to 999,999,999 ns via the RS-232 interface.
When an external 1 pps signal is applied the PRS10 will verify
the integrity of that input and will then align its 1 pps output
with the external input. The processor will continue to track the
1 pps output to the 1 pps input by controlling the frequency of
the rubidium transition with a small magnetic field adjustment inside
the resonance cell.
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Quantity Pricing
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Qty
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Price
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1 to 499
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$1495
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500 to 999
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$995
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>1000
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$895
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Standard Interface Connector
The interface connector and device form-factor are compatible with
Efratom’s Model FRS rubidium frequency standard. In its default
configuration, the PRS10 uses pins #4 and #7 for the RS-232 interface
to provide a complete set of systems diagnostics and control. Internal
hardware jumpers allow these pins to be configured as analog outputs
to monitor the lamp intensity and varactor voltage for complete
compatibility with the FRS.
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