Precise Timing for Small Modules
An Atomic Clock in Miniature Ushers in Precise Timing for Small Modules
It doesn’t receive signals from Fort Collins. It measures time against the element Cesium in a space the size of an IC package and may open up a world of new applications.
BY TOM WILLIAMS, EDITOR-IN-CHIEF
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Imagine a small form factor single-board computer with an actual atomic clock on board. Well, imagine no longer because one is now available from Symmetricom, a specialist in and supplier of precision time instruments and the only supplier of commercial Cesium oscillator-based timing devices, specifically the newest version called the SA.45s Chip Scale Atomic Clock (CSAC). The CSAC comes in a package measuring only 1.6 x 1.39 x 0.45 inches high and consuming less than 115 mW while operating on a 3.3V power supply (Figure 1). According to Symmetricom Director of New Business Development Steve Fossi, the accuracy falls within 1/10 second over an 80-year human life span. It amounts to a stability of <2 x 10-10 @ 1 second.
The Symmetricom SA.45s chip scale atomic clock takes up just 16 cc in volume, runs on less than 115 mW at 3.3V and provides a 10 MHz CMOS-compatible output.
Atomic clocks in considerably larger form factors have been used for many years in communication applications such as network synchronization where somewhat higher accuracy is required. For that, Symmetricom offers its Miniature Atomic Clock (MAC), which operates on 5W and is about three times larger in volume than the CSAC. However, the gains in size, weight and power (SWaP) open a much larger world of applications for mobile and battery operated systems that can take advantage of precise timing—a few of which have already been identified but many of which remain yet to be imagined.
The CSAC is built with a physics package containing Cesium in a resonance cell that is heated so that the element is diffused as a vapor and can be excited by the application of a vertical-cavity surface emitting laser (VCSEL). The laser is tuned to a very precise optical frequency and is controlled by a microwave synthesizer to tune to the resonance of the Cesium. The entire physics package is powered by only 10 mW (Figure 2).
The physics package contains the VCSEL laser, the resonance cell with Cesium vapor, heater and photodetector and mounts on the 1.6 x 1.39 inch circuit board of the CSAC.
As the Cesium absorbs the laser light, its electrons are raised to the next quantum level and then decay to emit photons, basically scattering in all directions. The laser light that gets through is picked up by a photodetector. The photodetector output signal drives a feedback loop that tunes the local oscillator toward the frequency associated with the lowest amount of light detected. That means that the Cesium is at maximum absorption and the output frequency is therefore in resonance with the element.
The output of the tuned local oscillator is also the clock signal, which represents the stability of the atomic resonance. The entire physics package is hermetically sealed within a double-layer magnetic shield and braised to the substrate with connectors that fit onto the small circuit board that contains the other components of the CSAC (Figure 3).
Atomic resonance is inherently more stabile than a local oscillator, so a control loop continuously steers the local oscillator frequency to atomic resonance and the RF output—typically 10 MHz—embodies the stability of atomic resonance.
There is an extended temperature range (-40° to +85°C), which also draws slightly more power (<125 mW) and with a shorter MTBF. However the CSAC can also be programmed to operate in ultra-low-power mode—averaging under 50 mW. In this mode, the physics package shuts down and the unit operates as a free-running oscillator. Periodically, the physics package is turned back on and after a warm-up of less than 120 seconds it “redisciplines” the local oscillator.
CSAC was originally a DARPA program whose goal was to develop an atomic clock that was 100 times smaller and used 100 times less power than existing technologies. As the program progressed, Symmetricom decided to use its own investment to commercialize the technology and accelerate development. Of course, this isn’t a clock in the sense that it will tell you to the nanosecond what time it is in Cleveland, but a highly precise time reference that can be used for control and synchronization of a vast number of possible applications. It can be powered off for considerable lengths of time and still be accurate upon being repowered.
Atomic time sources are finding application in an increasing number of areas, one of the best known of which are GPS satellites where the time reference is required for accurate location. GPS satellites must periodically correct for the effects of relativity since their speed causes their onboard clocks to run at a slower rate relative to ground-based atomic clocks. Such atomic clocks, also designed by Symmetricom, have daunting requirements in terms of ruggedization, radiation hardening and magnetic shielding. All of this experience has, according to Fossi, been of great use in designing the CSAC.
One major advantage of a super precise time signal that is also small and mobile is the ability to maintain synchronization with other devices that are not connected by either wired or wireless links—those which do not share a common clock reference signal as we are accustomed to in many applications. One such application under development for the military is a dismounted backpack jamming system for countering things like IEDs, which are the plague of troops in Iraq and Afghanistan. The roadside devices are typically set off by means of a remote radio signal such as a cell phone. Jamming such signals is vital to many operations.
The problem is that generally jamming radio signals in a given area may well cut off the enemy detonation signals, but it also jams friendly communications. Another problem is that field jammers are mounted on vehicles while some 70 percent of all patrols are on foot. What is needed is a way to a) make jammers portable without unduly loading already heavily burdened individual soldiers and b) effectively jam IEDs and still allow needed radio traffic among friendlies. So how can CSAC address this problem?
The light weight and low power consumption are pretty obvious, but what does it do to address the problem of communicating while jamming enemy signals? The answer is precise time synchronization between portable jamming units. Since the units are not connected, they need to be able to insert precisely coordinated time slots in which the jamming signal is turned off and communication signals can be sent. Synching up the units at the beginning of the mission will mean they are exactly on the same time base throughout and can execute whatever algorithms may be applied to the insertion of communication slots (frequency, duration, coded sequences, etc.) interspersing the jamming operations.
Another commercial application involves undersea seismic analysis searching for oil and gas deposits. On land, this involves distributing sensors on the ground, whose location is known via GPS positioning and which synchronize to the time based on the GPS signal as well. This is not possible under water. Sensors can send data to an underwater antenna on a ship, but cannot interact with devices above water, let alone in space.
If underwater sensors are placed by means of a remote submersible, are exactly synchronized with each other and with the systems on the ship, then the time differentials between the arriving systems can be recorded and post-processed to yield a three-dimensional image of the different densities of material beneath the sea floor. This is basically the same sort of post processing that is done with seismic signals for surface analysis. The difference is the way the relative location of the sensors and the time stamping of the signals is carried out.
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