LXI and the IEEE 1588 Precision Timing Protocol have many advantages that coordinate the timing of instrument systems with accuracy into the nanosecond range.1,2 But as powerful as this arrangement is, there are applications where it is not appropriate, particularly in systems distributed by large distances from thousands of feet to thousands of miles.
In these cases, the jitter that arises when data packets move through various routers can be detrimental even though the latest version, 1588-2008, sends a time-correction packet multiple times per second. As a result, time accuracy might drop to the range of 1 ms, which is fine for some applications but not for others.
A Worldwide Fabric of Time
One way to achieve high time correlation over large distances is for each site in a distributed instrumentation setup to use a GPS, which creates a fabric of time across the planet. A GPS receiver can set each test system’s time to an exact worldwide reference, in this case Coordinated Universal Time (UTC).
Indeed, GPS has revolutionized the way precise time synchronization can be achieved. In effect, it offers the services of precise atomic clocks free to anyone around the world with a suitable receiver and a clear view of the sky.
In addition, many GPS receivers provide a timing pulse known as the one-pulse-per-second (1-pps) signal. This pulse normally has a rising edge aligned with the UTC second, and it can be used to discipline local clocks to maintain synchronization.
Although GPS-based synchronization systems can’t achieve quite the accuracy of an IEEE 1588 PTP system based on a direct connection, GPS still is quite accurate and adequate for many applications. GPS satellites are guaranteed to be within 100 ns of UTC, so the offset between any two GPS-synchronized installations can be as high as 200 ns plus the offset of each device to GPS.
When LXI Class A or Class B instruments tie into a GPS-based grandmaster clock as their timing source, they operate on the basis of UTC. Adding this capability isn’t technically challenging, but it can be expensive. Depending on their accuracy, GPS clocks that connect to test systems can have price tags in excess of $10,000.
While there are no LXI-certified GPS clock units, that’s not a problem: LXI Class A and B systems conform to IEEE 1588, and a handful of companies sell IEEE 1588 GPS clocks. Users can plug such a unit into an existing LXI network, and the test system immediately will recognize it as likely being the most accurate clock in the system and synchronize to it as the grandmaster—without any user intervention.
Clock Sources
One of the most powerful clock sources, with a price of roughly $10,000, is the XLi IEEE 1588 Grandmaster from Symmetricom. This 1U rack-mount box operates as either a grandmaster or a slave clock and features <50-ns timestamp inaccuracy to UTC. It synchronizes to GPS satellites using a 12-channel receiver and comes with an L1 GPS antenna assembly and 50 feet of RG-59 cable. It contains a dedicated 1588 timestamp processor and 256 timestamp packet buffers and can support thousands of 1588 slaves.
The unit also provides a voltage-controlled/temperature-compensated crystal oscillator, and when not tracking satellites, it reverts to the flywheel or holdover mode and specs a stability of 1 ms/hr. For additional accuracy, the unit can be optionally updated to an oven-controlled crystal oscillator for 1 ms/day or a Rubidium oscillator to 25 µs/day.
One thing that makes this unit unique is its time-interval measurement port used to find the difference between 1-pps signals. Having the XLi Grandmaster work as a slave is useful for network time-transfer accuracy measurements involving a 1588 slave separated from the XLi Grandmaster by network elements or topology.
The 1 pps in the remote slave is compared to the 1 pps of the grandmaster; this enables accurate measurements of the network to be made between the GPS-referenced 1588 grandmaster and the remote slave. The unit also can be configured with two 1588 ports that can operate as two independently configured grandmasters or as a grandmaster and slave.
When the test topology physically separates the grandmaster from the slave by an inconvenient distance, two GPS-referenced XLi units work well to make the measurements (Figure 1). One grandmaster operates as a master with timestamps referenced to UTC by way of the GPS receiver. The other grandmaster is configured with the 1588 module operating as a slave. The 1 pps from the slave module is measured using the 1-pps time-interval function of the GPS-referenced XLi clock. The precise UTC available via GPS is the common time reference used to make the measurements of the slave clock’s accuracy.
Another high-end choice is the Meinberg LANTIME M600/PTP Time Server, which synchronizes IEEE 1588-compatible clients and systems and uses a built-in Meinberg GPS radio clock with a six-channel GPS C/A-code receiver as its reference time source. It is equipped with a high-precision oven-controlled crystal-oscillator (OCXO HQ) with a pulse output accuracy of ±100 ns. That oscillator determines holdover characteristics such as when the GPS signal is disturbed or jammed.
The unit comes in several form factors: a 19″ module case 1U high, a 19″ desktop case 3U high, and a 19″ module case also 3U high. The company has most recently introduced this capability in a package that allows more legacy timing outputs such as IRIG and 1 pps; it is called the M900/GPS/PTPv2.
GPS capability also is available at lower prices. For instance, the PXI-6682 Timing and Synchronization Module from National Instruments (NI) costs $1,800. But if you don’t already have a PXI chassis, you’ll have to purchase one. This PXI card uses GPS, IEEE 1588, or IRIG-B signals as the time reference to synchronize other devices.
The GPS receiver on the card can power an active GPS antenna. It generates a 1-pps signal that the card uses to achieve submicrosecond synchronization. Once the unit is synchronized to GPS, it can function as an IEEE 1588 grandmaster. Synchronization accuracy in the GPS mode is speced at ±100 ns, <13-ns standard deviation.
According to NI, with this card it is easy to set up a hybrid system that includes LXI instruments running off the 1588 clock and PXI instruments that share the synchronization signal coming off the PXI trigger backplane. The PXI-6682 can override the reference clock built into PXI chassis, most of which provide a 10-MHz reference clock with 25-ppm accuracy. But that value is improved to 1 ppm with the PXI-6682 thanks to a built-in temperature-compensated crystal oscillator.
Measuring Across Distances
New applications are being developed thanks to GPS-synchronized instrumentation. NI points to several based on its PXI-6682 and data acquisition hardware. One is structural health monitoring to determine the stability, reliability, and livability of multiple large structures throughout China, including Olympic venues such as the Beijing National Stadium. GPS helps synchronize the timing of several independent monitoring stations throughout the stadium. In another case, the U.S. National Renewable Energy Laboratory uses multiple GPS-based instrumentation stations to timestamp data captured to describe the turbulent airflow around wind turbines and combine it with information from the structure of the turbine itself.
A classic example of GPS-enabled test timing, says Conrad Proft, technology product planner at Agilent Technologies, deals with coordinating triggering between a transmitted signal from a ground station over a satellite link to a receiver hundreds of miles away. In one case he is aware of, the transmitter is in California while the receiver is in Hawaii. Here, accurate timing coordination can improve testing efficiency.
The brute force method would be to start the source and receiver/network analyzer at the same time. However, there is a transmission delay before the receiver actually needs to start collecting data. Taking that delay into account and triggering a receiver just before the signal arrives have two main advantages. First, the system needs less storage capacity. Second, the received signal might be deep in noise, so to reduce post-processing, it is advantageous to sample only what you need.
While it’s possible to transfer acquired data through a network, synchronizing timing across such a large distance over Ethernet, even with a dedicated link, would be difficult because of all the jitter introduced by the many hubs or routers that would be required. But here’s where GPS synchronization makes perfect sense.
In a typical far-field antenna test setup (Figure 2), a source generates a signal that is sent line-of-sight to a network analyzer (NA) that works with a stationary reference antenna as well as a moveable test-receiver antenna. The NA performs ratio measurements between these two antennas to compensate for various terrain effects between the transmitter and receiver.
In antenna tests, it is important to have phase coherence between the instruments, which is achieved by using the 10-MHz timing signal coming from a GPS receiver. If the local oscillators at the transmit and receive ends are not in phase, measurement errors will occur. This effect is especially noticeable in triangulation.
Because NA measurement times are highly dependent upon frequency and signal type, using time to synchronize far-field antenna measurements is not practical. Instead, each instrument signals the other when it is ready to advance to the next frequency or measurement. An operation referred to as a list sweep sequences through the suite of signals and measurements, and asynchronous triggering is used to handshake the sequence.
For far-field antenna measurements, LXI Class B instruments can use LAN packets to trigger each other through a dedicated LAN, wireless, or a virtual private network (VPN) through the Internet. Figure 2 illustrates the use of an LXI Class B Trigger box that converts trigger signals from one instrument to LAN packets and regenerates the trigger at the instrument on the other remote end. Triggering is delayed several milliseconds, but many measurements made by the NA are considerably longer. As a result, the overall speed is almost the same as hardwired triggers.
The combination of LXI Class B LAN triggering and GPS to synchronize to 10-MHz references allows an application to be solved at relatively low cost compared to running dedicated trigger lines.
References
1. Schreier, Paul G., “The Killer Bs Are Coming,” LXI ConneXion, September 2008, pp. 50-53.
2. Schreier, Paul G., “IEEE 1588 to Transform Timing Synchronization,” EE-Evaluation Engineering, April 2009, pp. 26-30.
Paul G. Schreier is a technical journalist and marketing consultant working in Zurich, Switzerland. He was the founding editor of Personal Engineering & Instrumentation News, served as chief editor of EDN Magazine, and has written articles for countless technical magazines. Currently, he is the editor for LXI ConneXion at EE-Evaluation Engineering. Mr. Schreier earned a B.S.E.E. and a B.A. in humanities from the University of Notre Dame and an M.S. in engineering management from Northeastern University. e-mail: [email protected]
July 2009