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GPS synchronization enables measurement systems to make synchronized measurements over extremely large areas. GPS provides a method of sharing timing signals without the need of running timing cables to each measurement system. Another benefit to GPS is that your data is always time stamped to a global time standard which allows you to associate your data with data sets from other systems which are also synchronized to GPS with a high level of confidence. Some applications which can take advantage of the benefits of GPS include structural monitoring, large microphone arrays, ground vibration monitoring, and environmental noise monitoring. This article will provide an overview of how to set up a GPS synchronized system with NI Dynamic Signal Acquisition (DSA) devices.
Table of Contents
Hardware Architecture
In order to synchronize DSA devices (such as the PXI-4462 and PXI-4498 ), we need to be able to provide three timing signals. Two of the signals are triggers. These are the Sync Pulse which resets the device’s internal counters and ADCs as well as the Start Trigger which begins an acquisition. The third signal is a 10MHz clock. The DSA device will phase lock loop (PLL) its internal clock to this 10MHz signal.
The PXI-6682 has the ability to generate Future Time Events (triggers based on GPS time). Future Time Events can be used for the Sync Pulse and Start Trigger. For the best measurement synchronization, the 10MHz clock needs to be disciplined to GPS. A GPS disciplined clock is continuously adjusted to stay synchronized to the GPS signal so it does not continuously drift as a free running oscillator would. The Trimble Thunderbolt E (www.trimble.com) is a GPS receiver that provides a 10MHz GPS disciplined clock. This is the receiver used to validate this architecture. With two different GPS receivers (PXI-6682 and Trimble Thunderbolt E), a GPS splitter is needed in order to connect both to the same GPS antenna. We chose a GPS splitter from GPS Networking (www.navtechgps.com), part number ALDCBS1X2-B/5. The figure below shows how these GPS pieces fit together.
Figure 1: GPS Hardware connections needed for GPS Synchronized Measurements for DSA.
While cabling for synchronization is not required for this GPS architecture, there still remains the need to be able to communicate with each of the systems over TCP/IP. If the systems cannot be physically tied into a network communications system, a wireless local area network (WLAN) can be used. Things to consider if using a WLAN are the slower transmission speeds compared to a wired network as well as the range, reliability and even security requirements of your application.
Since no cabling between PXI chassis is required for synchronization, in order to expand a measurement system you only need to duplicate a GPS synchronized node. Figure 2 shows how a system can be expanded.
Figure 2: Expansion of a single GPS synchronized measurement system. Each node may contain a different number of measurement channels.
GPS allows a lot of flexibility in the overall system architecture. Each of the nodes does not need to be identical. For example, in many applications the sensors are not equally distributed over the test area. This means that the measurement devices also need to have a similar physical distribution. With GPS synchronization, one node may contain only 16 channels while another has 400 channels. If a specific location requires many channels, multiple PXI chassis can be synchronized together through a cabled solution while the entire node is synchronized to GPS. This is an example of a hybrid cabled and GPS synchronized system. The examples linked below show how to synchronize a single PXI chassis up to many PXI chassis to a single GPS antenna.
Figure 3: This system implements a hybrid synchronization scheme with both GPS and cabled synchronization. Not all pieces of the system are shown in this figure such as the MXI connections and Trimble Thunderbolt.
Software Coordination
When multiple systems are synchronized together, a coordinator is required to get all of the systems working together. The coordinator is responsible for querying all of the systems for their current times, calculating the best time to send a future time event and relaying the results back to each system. The coordinator software can reside on any PC which can communicate with all of the measurement systems over TCP/IP. It could even reside on one of the measurement systems.
Figure 4: Basic communication flow between coordinator and measurement systems.
Once the data is acquired, the most common processes are to:
- Save the data to the local hard drive for post processing.
- Stream the data to a central server over TCP/IP for live monitoring.
- Perform analysis on the data as it is being acquired and return results to central server.
Typically, not just one of these is performed in an application, rather a combination of them. The GPS examples linked below will save all of the data to the local hard drive as well as stream a single channel over TCP/IP to the “Coordinator”. This allows for some basic monitoring of the data as it is being acquired. Once the acquisition is complete, the data files from each measurement system can be combined in a single location for processing or data storage.
Results
The tightness of synchronization is an important factor to consider when determining which kind of synchronization scheme is required. With a cabled synchronization solution, you can achieve the tightest accuracy possible, but your system is constrained by the length of the cables. With a GPS synchronization scheme, the timing accuracy is reduced due to the error in the GPS signals, but the measurement system can be placed anywhere that a GPS signal can be received.
Below are results which show the typical amount of phase mismatch in time between each of these synchronization schemes. In each case, the same signal from a function generator (1kHz sine wave) was provided to two DSA devices, each in a different PXI chassis. The time difference between the signals was calculated and plotted on a graph. This process was repeated many times.
Figure 5: Mismatch results using cabled synchronization between two PXI chassis with PXI-4462 devices.
Figure 6: Mismatch results using GPS synchronization techniques between two PXI chassis with PXI-4462 devices. These results represent 24 hours of continuous testing.
From these results, you can see that the cabled synchronization method provides very tight synchronization which does not vary from acquisition to acquisition. On the other hand, the GPS synchronization results do have variation in it. This variation is due to the difference between the GPS disciplined 10MHz clock and you can see how the clock is continually corrected over time.
The results are presented as mismatch in terms of time. Typically, phase mismatch requirements are given in degrees. Here’s a chart which shows some common phase mismatch requirements in degrees and how they correspond to a mismatch in time.
Figure 7: Correlation between phase mismatch in degrees and mismatch in time at different input bandwidths.
With a typical synchronization mismatch of ±25ns, GPS synchronization is still tight enough to solve all but the most stringent synchronization requirements.
Links
GPS Links:
Example Program: GPS Synchronization of a Single PXI Chassis of DSA Devices
Example Program: GPS Synchronization of Two PXI Chassis of DSA Devices
Example Program: GPS Synchronization of Three or More PXI Chassis of DSA Devices
Cabled Synchronization Links:
Tutorial: High Performance High Channel Count Multi-Chassis System
Tutorial: High Channel Medium Performance Multi-Chassis System
Tutorial: Medium Channel Low Cost Multi-Chassis System
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