Timing and synchronization are an integral part of measurement and control systems. We will explore common strategies for achieving performant timing and synchronization. We will examine how NI PXI provides valuable hardware and software advantages to make it easier to design flexible systems that can meet today’s timing and synchronization requirements.
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NI PXI Express hardware is the industry’s best-in-class way to build out flexible, multichannel, and multi-instrument systems, and nowhere is that more evident than in timing and synchronization capabilities. Generating high-quality clocks, and sharing sample clocks, time-bases, and low-skew triggers are critical for many measurements and applications.
Clock quality directly affects the following factors:
Tight synchronization from shared clocks, timebases, and triggers is needed for the following purposes:
While the complexity of managing timing and synchronization in systems can often scale exponentially with system size and mix, the NI PXI Express platform has numerous built-in features that make it easy to synchronize varied I/O with each other within a single PXI Express chassis to a high level of performance. With an assortment of PXI Express timing and synchronization add-on hardware, you can expand synchronization to build multichassis instruments and interface with common standardized time protocols even at large scale.
The built-in features of PXI Express chassis and controllers have many uses:
For the next level of accuracy or extendibility, you can augment your PXI Express system with timing and synchronization upgrades to accomplish the following tasks:
While PXI Gen 1 was a huge step forward in standardizing complex and flexible system design, PXI Express, as the latest evolution of the PXI platform, delivers a higher degree of synchronization to measurement I/O devices while remaining backward compatible with PXI Gen 1. PXI Express maintains the 10 MHz backplane clock as well as the single-ended PXI trigger bus and length-matched PXI star trigger signal provided by the original PXI specification. PXI Express also adds a 100 MHz differential clock and differential star triggers to the backplane to offer increased noise immunity and industry-leading synchronization accuracy (100 ps and 150 ps of module-to-module skew, respectively). The higher frequency 100 MHz clock can be especially helpful in reducing phase noise of higher sampling rate devices such as oscilloscopes that steer their sample clocks based on the backplane clock. These clock and trigger signals (PXI_CLK10, PXI_TRIG, PXI_STAR, and PXI_DSTARA/B/C) can be imported and exported to other PXIe chassis and third-party devices using NI PXI Express timing devices that can control the PXI Express system timing slot. Additionally, PXI Express systems can synchronize their clock and trigger signals to time-aware protocols such as IEEE , IEEE 802.1AS, GPS, IRIG-B, and PPS through specialized NI time-protocol interfaces. For more details and supported NI modules, see Table 1 later in this paper.
Figure 1: PXIe Chassis Timing and Synchronization Features
The phase noise and stability of the backplane system reference clocks are important characteristics of the PXI chassis, as they indicate how reliably you can expect to synchronize modules within the system. With NI PXI Express’s choice of components and backplane design, phase noise performance of the PXI Express 100 MHz differential system clock on an 18-slot chassis is best-in-class compared to other chassis available on the market.
You can phase-lock-loop (PLL) the 10 MHz and 100 MHz system reference clocks to a higher stability clock source than the one provided on the chassis backplane. This helps higher sample rate PXI modules to better align their samples across multiple instruments. The PLL circuitry of the NI PXI chassis can suppress more noise when locking to an external reference, thus permitting cleaner transmission of the higher stability clock source. With other vendors’ chassis, depending on the system clock source phase noise required by the application, you may need to phase-lock the external reference clock to each module individually, rather than at a system-level to the chassis backplane, resulting in an increase in system complexity and cost.
NI PXI Express timing and synchronization features work with two different types of synchronization architectures, signal-based and time-based, so you can achieve the flexibility and customization you need when developing applications that require different levels of performance.
Figure 2: Signal-Based and Time-Based Synchronization Architecture Performance
With an NI PXI Express signal-based timing and synchronization architecture, clocks and triggers are physically connected between modules and chassis. This typically generates the highest precision of synchronization. However, with longer than certain subsystem-connecting cable distances (approximately 200 m), this performance is unachievable because of clock skew and noise.
If the required distance between synchronized chassis is too great for cables to reliably transmit clock and trigger signals, you must use a time-based synchronization architecture. Timebase synchronization can also be an important aspect of integrating with other systems or DUTs that rely on time protocols. The NI PXI Express timing and synchronization solution allows you to use absolute time reference protocols such as IEEE , IEEE 802.1AS, GPS, and IRIG-B to achieve synchronization over large distances.
No oscillator perfectly generates the specified frequency. Clock error is commonly described by three major components: absolute accuracy, long-term stability, and jitter. While synchronizing devices together can eliminate long-term stability as a clock error source between components of a system, and while PLLs can help reduce jitter when sharing clocks, you cannot improve clock accuracy without having a better clock. If clock accuracy matters, a high-quality clock source is necessary.
For an NI PXI Express chassis, the clock source is an oscillator that is accurate to 25 parts per million (ppm). By inserting an NI PXI timing and synchronization module, such as the PXIe-T with a higher-accuracy and stability oven-controlled crystal oscillator (OCXO), into the system timing slot of the chassis, you can lock the backplane reference clock to an oscillator frequency source that is accurate to 50 parts per billion (ppb) or import an external clock source. For special cases where 50 ppb is insufficient for long-term stability (such as systems that run offline for days but still need to correlate time with other systems and devices that they aren’t connected to), the PXIe-T can be combined with a best-in-industry rubidium-based PXIe- clock source or through a GPS-disciplined-oscillator (GPSDO) such as the PXI-H, PXIe-, or CDA-.
Figure 3: NI PXIe-T—The Industry’s Highest Performance PXI Timing and Synchronization Module with OCXO
NI PXI timing and synchronization modules can improve the overall system clock error by generating and sharing a precision clock source among multiple PXI devices.
The PXI timing triggers and clocks are configurable at a low level through NI I/O instrument drivers as well as through the NI-Sync driver, which exposes all the timing features of the chassis for direct utilization by the I/O devices. Intelligent signal aliasing makes it easier and more flexible to set up dynamic routes for triggering that rely on device name rather than on slot number, making it easier to migrate an application to a different chassis or slightly different set of instruments. This modularity makes NI PXI Express hardware well suited to system designs that are subject to changing requirements and last-minute device substitutions, since the complex work of synchronization architecture and routing can usually be established programmatically.
For more information on synchronization in NI IO instrument drivers, refer to Timing and Synchronization Features of NI-DAQmx.
For applications requiring the synchronization of high-speed PXI modular instruments, distributing the necessary clocks and triggers to achieve this synchronization can be challenging because of the latencies and timing uncertainties caused by skew and jitter. The complexity is further compounded when attempting to synchronize heterogeneous instruments running at different sample rates.
Precise synchronization in these cases often involves attempting to share a low-skew reference clock between all the instruments, employing phase-lock loops (PLLs) and multipliers attached to that reference clock to generate each device’s unique sample clock, and distributing a low skew trigger to all the instruments, some sort of finely tunable time delay compensation mechanism, and some sort of calibration measurement upon which to base adjustments. Not only is this complicated to do both in the hardware and the software implementations, it’s also fragile since even a slight instrument model swap can cause the whole architecture to have to be reworked and all the calibration to be redone.
NI PXI is the best way to build mixed, flexible, high-speed, and expandable IO systems. To help reduce the inherent complexity of high-speed instrument synchronization, NI developed a patented method for synchronization, called NI-TClk, that greatly simplifies adding additional high speed instruments and keeping them synchronous down to 1 ns accuracy out-of-the-box or to tens of ps with manual calibration.
NI-TClk technology can help you in the following ways:
The flexible NI-TClk technology is applicable to the following use cases:
NI-TClk technology delivers PXI modular instrument synchronization performance improvements right out of the box. The key software components comprise three LabVIEW VIs/C functions that require no parameters to be set. NI-TClk architectures can deliver synchronized devices with skews of at worst 1 ns between each device. Typical skews observed are in the range of 200 ps to 500 ps. Manual calibration of the sample clock on each device can lower skews to less than 30 ps between devices.
Figure 4: Required NI-TClk Functions
In situations where high speed and heterogeneous device synchronization must be highly accurate, NI-TClk reduces a mountain of complexity into just three VIs. Within a single PXI chassis, as large as 18 slots, all the NI-TClk-capable devices can synchronize with those three VIs using built-in features of the NI PXI chassis. No additional cables. No external PLLs. No manual routing. No special software. It’s all handled automatically by NI-TClk. Neither swapping models out for other models, nor changing slots of the instruments, nor changing samples rates will result in having to rework this synchronization code. NI-TClk automatically adapts to all of those changes and keeps the test writer working on the logic of the test steps rather than on solving complex triggering architectures. NI Modules and Devices Supported by NI-TClk provides a complete list of I/O devices that support NI-TClk.
Nowadays, the NTP protocol is available for distributing time via Ethernet. NTP provide an accurate and reliable time across local or global networks for various systems and applications.
Benefits
The NTP protocol provide a synchronization within 1 to 10 milliseconds. UTC is used as a unique time reference for distributed systems and is delivered across local or global networks. The NTP protocol is easy to deploy as no specific network requirements are needed.
Applications
Data Centers, Power network nodes, Power plants, Finance, Telecoms, Air Traffic Control, Railways, Hospitals, universities and public buildings. All sectors where an accurate and reliable time reference is required.
Nowadays, different protocols are available for distributing time via Ethernet. PTP is used to provide an highly accurate and reliable time across local or global networks for various systems and applications. PTP is defined in IEEE
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Benefits
These Highly accurate protocol provide the most precise time and sync in submicroseconds. The TAI is used as a unique time reference for distributed systems and delivering this level of performance across local or global networks is a challenge and depending on requirements needs special network equipment.
Applications
Data Centers, Power network nodes, Power plants, Finance, Telecoms, Air Traffic Control, Railways. All sectors where a Highly accurate and reliable time reference is required.
The Synchronous Ethernet “SyncE” provide a frequency synchronization signal to Ethernet Equipment Slave clocks. ITU-T G. defines Synchronous Ethernet clocks compatible with SDH clocks.
Benefits
The Synchronous Ethernet means that the Ethernet physical layer is synchronized by a stable frequency source and should be traceable to an external clock, ideally a master and unique clock “PRC” for the whole network. As SyncE is not carry clock synchronization information it is often used together with Precision Time Protocol PTP (IEEE ).
Applications
Applications include cellular networks, access technologies such as Telecom (packet based backbones) and applications such as IPTV or VoIP.
Different technical outputs for a highly accurate synchronization. PRC (Primary Reference Clocks) with E1/2.048 MHz (G811) signal for SDH network synchronization. Other frequency outputs such as: 10Mhz, PPS, serial ToD or IRIG-B for device synchronization.
Benefits
These Highly accurate Time/Servers/Grandmasters/PRCs provides also the most stable time and frequency outputs are phase locked to an ultra-stable oscillator e.g. atomic Rubidium oscillator.
Applications
Power network nodes, Power plants, Telecoms, Air Traffic Control, Railways. All sectors where a Highly accurate and reliable frequency and time reference is required.
TSN is a set of standards under development by the Time-Sensitive Networking task group of the IEEE 802.1 working group. TSN is an extensions to normal Ethernet.
Benefits
Not driven by one large company, unlike e.g. Profinet. Best effort and critical data over one network. Vertical integration from the cloud to the processing. High bandwidth, low latency, high availability and determinism are key requirements which are all addressed by TSN
Applications
TSN/OPC-UA will be the field communication of the future in automation, automotive, utilities.
Electrical impulses are sent from the master clock to the slave clocks every minute. The slave clockworks are equipped with incremental motors that move the hand one minute forward with each impulse.
Benefits
A simple two-wire installation is sufficient. The topology can be freely selected. The clockworks and the simple installation are inexpensive. The conventional version was the standard technology for a long time (before microprocessor-controlled movements became available).
Applications
Still used to enhance older systems but no longer recommended for new installations.
The master clock sends all the time and date information to the slave clocks, while our renowned MOBALine technology secures the power supply. The range also includes DCF active.
Benefits
The two-wire installation is simple and cost-effective, with a free topology. It is also easy to put the system into operation: simply connect the clock to MOBALine and it then automatically sets itself. The clocks do not have to be initialized manually (this is necessary when using conventional impulse technology with cables, and can result in significant added expense).
Applications
Standard installation for all clock systems.
Nowadays, NTP (Network Time Protocol) is the prevalent technology for distributing time via Ethernet/LAN. In this case, the master clock acts as the NTP time server and the slave clocks act as NTP clients.
Benefits
The technology is contemporary. The clocks automatically set themselves to the correct time and can also be monitored. A Windows PC serves as the operating terminal, which allows you to identify malfunctions remotely. Power can be supplied via Ethernet cable (PoE), any may have to be in some systems.
Applications
New installations. New installations are often integrated into extensive IT infrastructures.
These clocks are synchronized through the on-site time code transmitter. Power is supplied via the mains or battery.
Benefits
A time distribution installation is not required and an unlimited number of clocks can be controlled by a single transmitter.
Applications
Suitable for clocks in buildings and spaces in which a cable installation is not possible or too expensive.
Also a good solution for stand-alone clocks (see above) that cannot receive DCF77 or GPS.
As with our LAN clocks, NTP is used for synchronization. Power is supplied via the mains or battery.
Benefits
Place your clock where ever you like. The installation is easy and not limited by cabling. The WiFi clocks are portable and can be mounted easily.
Applications
It is useful in environments such as schools or hospitals, where the hole cabling job was already made or the power supply can’t be reached.
Read more about our WiFi-Clocks
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