Bryant Labs

Precise Time and Frequency Systems

Ten years ago as I began thinking about monitoring and data collection, I realized that it was important to have a common time reference for all of my gear. I built a box to receive WWV on 10.00MHz, decode the subcarrier time code, and distribute it on a simple RS-485 network that I had built. I credit that simple effort with landing me a job at TrueTime, now a division of Symmetricom, where I learned what Precise Time and Frequency Systems were all about.

Luckily, a lot of the necessary hardware to have a world class system can be found on eBay for pennies on the dollar.

Here's a block diagram of my Precise Time and Frequency System:

Block Diagram

In the Precise Time world, time references are refered to as Stratum Levels. A Stratum 0 reference is the best you can get. As you get farther and farther removed from the original reference, the stratum level goes up. Because my system is referenced to GPS satellites in the sky, I put Stratum 0 at the top.

My Stratum 0 reference is a TrueTime XL-DC GPS receiver with a Rubidium Oscillator. I paid about 2% of the original sales price by buying it on eBay. The XL-DC has the following outputs:

  • 1 pulse per second, on-time to UTC within less than 50ns
  • 10.000000000MHz
  • IRIG-B Time Code
  • Serial Time-of-Day

To better use all of these, I built a distribution box which buffers the outputs at least four ways. In addition, I take the 10MHz Square wave and turn it into a sine wave with a crystal filter. This is also buffered four ways.

The 10MHz references are sent to my frequency counter and my HP3586C selective level meter to give them the best possible frequency accuracy. The Serial Time-of-Day is sent to a PC running Debian Linux with NTP on it. The NTP is configured to use the XL-DC serial data as a reference clock. This machine is called "LAB3" and becomes a Stratum 1 Network Time Protocol server for the other computers. Each of them is considered to have Stratum 2 time, since they are two steps away from the XL-DC. On a small network like mine, staying off of the public internet, the time on all of the computers is within 10 milliseconds. For more details of how to set up NTP on your own network, check out my NTP How-To Page.

Here are some of my favorite links to Time and Frequency information:

  • The Time and Frequency Division of the US National Institute for Standards and Technology
  • A REALLY dedicated Amateur
  • Home of the Network Time Protocol
  • GPS System Information from the US Coast Guard
  • Back to Bryant Labs Radio Astronomy Page
    Back to Bryant Labs Seismology Page
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    Last Revised: 30-December-2005