The PXI-6683 Series is capable of achieving tight synchronization with various other devices using GPS, IRIG-B, PPS, IEEE 802.1AS, or IEEE 1588. When GPS or IRIG-B are selected as the synchronization source, the PXI-6683 Series module can also serve as an IEEE 1588 grandmaster. The following sections describe the synchronization capabilities of the PXI-6683 Series.

GPS

GPS stands for Global Positioning System, and it is a system of over two dozen satellites in medium Earth orbit that are constantly transmitting signals down to Earth. GPS receivers are able to detect these signals and determine location, speed, direction, and time very precisely. GPS satellites are fitted with atomic clocks, and the signals they transmit to Earth contain timing information. This makes the GPS system a precise timing and synchronization source.

The PXI-6683 Series has a GPS receiver which powers an active GPS antenna and receives and processes the RF signals (1.575 GHz) from the satellites. The GPS receiver then generates a very precise pulse-per-second (PPS) signal that the PXI-6683 Series uses to achieve sub-microsecond synchronization.

GPS enables the PXI-6683 Series to synchronize PXI systems located far away from each other, as long as GPS satellites are visible to the antenna from each location. Furthermore, once the PXI-6683 Series is synchronized to GPS, it can function as an IEEE 1588 grandmaster to enable synchronization of external 1588 devices.

IRIG-B

IRIG is a standard used to transmit precise timing information between instruments to achieve synchronization. IRIG-B is a particular application of the IRIG standard, in which 100 bits of data are sent every second. Embedded in the data is a seconds' boundary marker that the receiving instrument uses to synchronize its timebase to the IRIG source. The rest of the data contains information such as the time of day, days since the beginning of the year, and optionally, control functions and the number of seconds since the start of the day, encoded as a straight binary number.

The PXI-6683 Series can function as an IRIG-B receiver, supporting synchronization to sources outputting IRIG-B 12X (AM) and IRIG-B 00X (DC), compliant with the IRIG 200-04 standard.

When configured to synchronize to an IRIG-B AM source, the PXI-6683 Series will be able to accept a 1 kHz AM modulated IRIG-B 12X signal on its PFI0 input. When configured to synchronize to an IRIG-B DC source, the PXI-6683 Series will be able to accept an IRIG-B 00X DC encoded signal on its PFI0 input.
Caution Do not connect an AM signal to PFI0 when the PFI line is configured for digital operations. This could cause damage to the digital circuitry, the device driving the AM signal, or both. Always ensure the line is configured for IRIG-B AM operation before connecting an IRIG-B AM signal.

Furthermore, once the PXI-6683 Series is synchronized to IRIG-B, it can function as an IEEE 1588 grandmaster to synchronize external 1588 devices.

The following assumptions are made regarding the received IRIG-B signal. All conditions must be met for the PXI-6683 Series to be able synchronize accurately:
  • Seconds begin every minute at 0, increment to 59, and then roll over to 0.
  • Minutes begin every hour at 0, increment to 59, and then roll over to 0.
  • Hours begin every day at 0, increment to 23, and then roll over to 0.
  • Days begin every year at 1. Days increment to 365 in non-leap years, or to 366 in leap years, and then roll over to 1. Leap years must be supported. Valid values for year are 01 to 99, inclusive. Years are assumed to be in the 21st century. For instance, year 19 represents 2019. If the year is not supplied (sent as 00), the OS system time is read and the year is derived from it.

To achieve proper synchronization of the PXI-6683 Series, ensure that the IRIG-B source used conforms to the requirements listed above. Note that most IRIG-B sources conform to these requirements.

802.1AS

The IEEE 802.1AS generalized precision time protocol (gPTP) is a standard based on IEEE 1588 that is better optimized for time-sensitive applications. Like 1588, 802.1AS controls synchronization across a series of nodes connected by Ethernet cables, switches, and bridges by automatically designating a grandmaster clock using the Best Master Clock algorithm (BMCA). The grandmaster clock then serves as the source of time to slave devices (devices not elected as the 1588 grandmaster) across the network.

Each distributed device runs the BMCA to determine the highest ranking, most accurate device on the network, which automatically becomes the grandmaster. If the grandmaster clock leaves the network or is no longer the most accurate clock on the network, the BMCA selects a new grandmaster. The BMCA defines a standard set of clock characteristics—such as the origin of a clock's time source and the stability of the clock's frequency—and then assigns a value to each clock in order to determine the grandmaster. The BMCA also takes into account user-defined priority values, which you can set using the NI-Sync driver software.

One of the main differences between 802.1AS and 1588 is that 802.1AS requires 802.1AS-capable network switches and network interfaces providing hardware-assisted timestamping in order to operate. You can determine whether or not a device is capable of running 802.1AS using the AS Capable property, accessible with NI-Sync.

IEEE 1588

The PXI-6683 Series is capable of performing synchronization over Ethernet using IEEE 1588. It is possible to configure the PXI-6683 Series to synchronize to GPS or IRIG-B and then function as an IEEE 1588 grandmaster. It is also possible to configure the PXI-6683 Series to synchronize to IEEE 1588, in which case, the standard defines how the master will be selected. If the PXI-6683 Series is selected as the IEEE 1588 master, and it is not configured to synchronize to GPS or IRIG-B, it will use its internal free-running timebase, which will be updated to the host computer's system time during power up.

Pulse Per Second (PPS)

The PXI-6683 Series is capable of using a PPS signal for synchronization. Any PFI, PXI_Trigger, or PXI_Star line can be configured as the PPS input terminal. When synchronizing based on a PPS, the first pulse received will set the PXI-6683 Series internal timebase to either an arbitrary time supplied by the user, or the host computer's system time. Each subsequent pulse received will be interpreted as a second's boundary (the pulse occurring exactly one second after the previous pulse). As each pulse is recieved, the PXI-6683 Series will adjust its internal timebase to match the frequency of the PPS source.

For best results when using a PPS time reference, ensure that the device supplying the PPS signal is capable of providing a stable, consistent 1 Hz signal. Error can be induced into the system if the reference signal contains significant jitter, or if the reference frequency strays from 1 Hz.

Synchronization Best Practices

The PXI-6683 Series can achieve sub-microsecond synchronization. The following sections describe some guidelines for achieving the best possible performance from the PXI-6683 Series. While the PXI-6683 Series will function properly if you follow the specifications, the following guidelines may increase the synchronization performance.

Operating Environment

In order to achieve the best synchronization performance, refer to the following guidelines to provide a thermally stable environment. Also, ensure you remain within the specified operating temperature limits:
  • Place the PXI or PXI Express chassis containing the PXI-6683 Series module in an environment free of rapid temperature transitions.
  • Ensure that PXI filler panels are properly installed for unused PXI or PXI Express slots, since consistent airflow can degrade the PXI-6683 Series performance.
  • Perform the same steps listed above for any other synchronization partners and/or systems.

Timing System Performance

The PXI-6683 Series can generate or receive a 1 Hz pulse per second signal on any PFI or PXI trigger terminal. You can set up this signal to transition on the seconds boundary of the synchronized system time. You can then use this signal to analyze system performance by connecting two or more pulse per second signals to an oscilloscope and measuring the latency between them. Adjustments can be made to account for deterministic latency; refer to NI-Sync documentation for more information. The PXI-6683 Series can also timestamp an incoming pulse per second signal. The PXI-6683 Series will timestamp the externally-generated pulse per second with its internal timebase. By comparing this timestamp with the nearest seconds boundary, you can quickly determine the synchronization performance.

IEEE 1588 Synchronization Best Practices

Network Topology

To obtain the best PXI-6683 Series performance, follow these guidelines to set up the Ethernet network topology:
  • Use short cabling when possible. Ethernet cabling is inherently asymmetric; the longer the cabling, the higher the asymmetry. This impacts synchronization performance, because the IEEE 1588 protocol assumes a symmetrical network path.
  • If several 1588 devices need to be synchronized on the same network, use a 1588-enabled switch. 1588-enabled switches are specifically designed to compensate for the varying latency of packets passing through them; thus enhancing synchronization performance.
  • If a 1588-enabled switch is not available, use hubs when connecting to multiple IEEE 1588devices. Unlike standard switches, hubs offer low latency and close to deterministic performance for Ethernet traffic. Standard Ethernet switches can have Ethernet packet latencies vary by hundreds of nanoseconds. This latency uncertainty degrades synchronization performance significantly.
  • Ensure that the network is running at 1 Gbps by noting the Ethernet Speed LED status. Synchronization performance is degraded when running at 10 or 100 Mbps.
    Note If it is impossible to use a 1000 or 100 Mbps network and you must run IEEE 1588 synchronization using a 10 Mbps network, ensure the network interface of the PXI-6683 Series is explicitly configured for 10 Mbps full duplex operation using the Windows configuration panels.

GPS Synchronization Best Practices

The PXI-6683 Series module has one SMB female connector on its front panel for a GPS active antenna. The connector provides a DC voltage to power the antenna and alos serves as input for the GPS RF signal.

Antenna Installation

Caution National Instruments recommends using a lightning arrester in line with the GPS antenna installation to protect the PXI-6683 Series module and the PXI system from possible damage and operators from injury in the event of lightning.

The embedded GPS receiver in the PXI-6683 Series module requires signals from several satellites to compute accurate timing and location. The more satellites available to the receiver, the more accurately it can determine time and location. Therefore, the antenna location should be such that it receives signals from the greatest number of satellites possible. As the number of satellites visible to the antenna decreases, the synchronization performance may also decrease. Choose the antenna location so that the antenna has a clear view of the sky. There is no strict definition for a clear view of the sky, but a suitable guideline is that the GPS antenna should have a straight line of sight to the sky in all directions (360°), down to an imaginary line making a 30° angle with the ground. Locations far from trees and tall buildings that could block or reflect GPS satellite signals are best.

Maximum Cable Length

Maximum cable length depends on the GPS antenna gain and the cable's loss of per unit of distance. National Instruments recommends a GPS signal strength of between -135 dBm and -120 dBm at the PXI-6683 Series module's SMB input. GPS signal strength on the Earth's surface is typically -130 dBm. Targeting a signal strength of -125 dBm at the SMB input, you can compute the maximum cable length as: 

        
          Max_cable_loss = -130 dBm + antenna_gain - (-125 dBm)
Max_cable_length = Max_cable_loss / (loss_per_unit_of_distance)

For example, if you use an active antenna with gain of 28 dB and RG-58 cable, which has a rated loss at 1.5 GHz of about 0.8 dB/m (24.5 dB/100 ft), the maximum cable length you could use is: 

        
          Max_cable_loss = -130 dBm + 28 DB - (-125 dBm) = 23 dB
Max_cable_length = 23 dB / (.08 dB/m) ≈ 29 m
Note The GPS antenna kit offered by National Instruments comes with a 30 m cable, which has a loss of 15 dB/100 ft, making the total loss in the cable approximately 14.8 dB.