Friday, November 18, 2011

Sharing Reference and Timing Signals Over Long Distances

In this post we look at easy way to share reference, timing, and trigger signals between test equipment separated by long distances. This is useful if you want to share a 10 MHz precision timebase signal with multiple instruments in various laboratories across your company's site or maybe you need to route a trigger signal among multiple instruments in a distributive test setup. Common short distance signal mediums like coax cables is often not a viable solution for long distance needs because of issues like high signal attenuation, group delay, and outside electromagnetic interfernce. A good viable solution for routing signals over long distances is by employing fiber optic equipment.
In the following example a pulsed timing signal was sent over 10 Km. This was accomplished using a fiber optic transmitter and receiver link as well as 10 Km of fiber optic cable. Below is a scope screen capture of the signal before modulation and transmission (in yellow) and after it was received and demodulated (in green)

For this example the optic transmitter / receiver pair that I used was the DiLink 4 GHz microwave link modules by Linear Photonics (link below). The conversion and transmission process of the optic link is not perfect but it's close, with performance specs that are orders of magnitude better then using coax or any metal based transmission method. Fiber optic cables have low signal attenuation and they do not suffer from external signal degradation factors such as electromagnetic interference and cross talk.
In a vacuum light travels at about 3.3 us per kilometer (1000 m / 299,792,458 m/s).  Because the index of refraction of most fiber optic cables is about 1.5, light travels about 1.5 times as fast in a vacuum as it does in the cable. This works out to about 4.9 us of latency for every kilometer and for our example with 10 Km of fiber optic cabling we would expect the delay to be 49 us. If we zoom in a bit on our input signal and 10 Km later output signal we can see the expected signal path delay in the below scope screen capture.

When you need to share a reference, timing, or trigger signal across long distances fiber optic links are a great solution for signal transmission. Compared to using a metal based cabling, like coax, fiber optic links provide better signal integrity, less attenuation, and are less susceptible to outside interference. If you have used fiber optic links for test and measurement applications please share your experiences in a comment below.

Click here to check out the Linear Photonics webpage