Optical Time and Frequency Distribution

Application Note – 53

Optical Time and Frequency Distribution.

Introduction

There are a number of situations where the traditional approach to providing precise time and frequency references is not suitable for the local environmental requirements.

The attached paper describes an architecture designed to provide precision timing and frequency references inside a secure area that has no direct access to any external references.

The type of facility where this critical capability is required is, for example, a secure laboratory, or testing facility which is fully enclosed (possibly underground, in a bunker etc.) and where there is no external access permitted.

In this case the traditional approach (typically utilizing satellite receivers receiving signals by means of an antenna with a clear view of the sky) will not suffice, and a more comprehensive approach is required.

Precise Time and Frequency, LLC (ptf), is always looking for new ways to enhance the value and utility of our product offerings to our customers.

Our latest development is a suite of options for distribution units and time and frequency generators that enable precision time and frequency distribution through optical fiber in a fully closed network using time code protocol.

This is of particular interest to Government facilities or private Companies that need to operate in a highly secure environment, yet require precision frequency control and/or timestamping of transactions or other information.

In these circumstances it is not unusual to require complete electrical isolation from the outside world, which can make obtaining the necessary time and frequency references a significant challenge. Transmitting through optical fiber with a synchronized precision time code provides complete electrical isolation to the outside world whilst still providing the necessary precision frequency and timing signals necessary to drive sensitive equipment securely.

Implementation

In theory, almost any consistent protocol/data stream could be used to provide frequency and time synchronization, however from a cost and compatibility perspective, it makes sense to use an already established protocol to do this.

In this respect it makes sense to use a standard time code format. The most common time codes that are still widely used today, were originally developed back in the 1950’s by the US Military’s Inter-Range Instrumentation Group (IRIG). There are a number of IRIG time codes with different resolution/precision designated by different letters of the alphabet, namely IRIG A, IRIG B, IRIG D, IRIG E, IRIG G, and IRIG H,  the most popular being IRIG B, which is a precision code repeating every second, with a data rate of 100 pulses per second (the ‘B’ defines the 100 pulses per second.)

The IRIG B code is typically generated as a phase width modulated pulse train (technically referred to as IRIG B 0XX  *note1) however the most popular format is an amplitude modulated 1kHz sine wave version (IRIG B 1XX) as this is more amenable to transmission over longer distances (several hundred feet) through regular coaxial cable. The downside of using the amplitude modulated version is loss of accuracy due to porly defined zero crossover, and typical accuracies attainable are of the order of a few microseconds, versus the <100ns attainable with the unmodulated version.

The system described below offers the “best of both worlds” by taking the highly accurate unmodulated version of time code and transmitting it optically, thus preserving the accuracy whilst allowing transmission over thousands of feet.

In addition, the system uses IRIG A which is a higher speed, more accurate form of time code transmitted at one thousand pulses per second, ten times the pulse rate of IRIG B .

Note 1 :

Different versions of IRIG time code contain various information in addition to the actual time. This additional information may be the Year, the signal Quality different representations of the time such as a straight binary code, SBS (the basic time signal is transmitted in binary coded decimal, BCD, form.) The suffix after the initial IRIG is a three digit number, where the initial 0 represents an unmodulated version (1 represents modulated) and the next two digits (XX) define what additional information is contained in the code. XX represents two numbers that define the specific information in addition to time that the code contains.

System Elements

The system is comprised of a number of “building blocks” as follws;

  1. ptf 3207A time and frequency generator
  2. ptf 1208A optical transmitter / distribution unit
  3. IRIG A DCLS (pulse width modulated) to IRIG A optical signal
  4. IRIG A Optical to IRIG A DCLS (pulse width modulated) signal
  5. IRIG A DCLS (pulse width modulated) to IRIG A AM (amplitude modulated) convertor module
  6. IRIG A DCLS (pulse width modulated) to IRIG B AM (amplitude modulated) convertor module
  7. ptf 3207A units with IRIG A receiver modules (two), NTP Server / Time code generator

In addition to these individual modules, the generator unit and receivers act as Network Time Protocol (NTP) servers to generate a local NTP signal for distribution to NTP clients within the secure network. This happens to be very convenient as  many commercially available products act as NTP clients, which enables them to use NTP to internally synchronize their time to the NTP transmitted over the secure network.

The system is expandable by adding receivers as required to provide local frequency and timing outputs. For long distances (several miles). the company also provides time offset delay compensation to compensate for the physical time it takes for the signals to travel through the fiber interconnections.

Not only is the system described fully redundant, but it has built-in redundancy at all levels such that once the slave units are locked to the distributed time code signals, if the reference link is for some reason lost, each individual receiver will run in flywheel, or “holdover”, mode to maintain the system precision for many hours, or until the reference is re-established

For more information please contact : info@ptf-llc.com

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