OPTICAL TRIGGER FOR SCOPE REDUCES RF INTERFERENCE.

A simple, inexpensive modification of the external input triggering-circuit of an oscilloscope eliminates electromagnetic interference from the triggering circuit. The modification doesn't interfere with any of the oscilloscope's other triggering abilities, and the instrument can be restored easily to the conventional external triggering when desired. The modification consists mainly of substituting a fiber-optic cable for the conventional coaxial cable and terminating it at a photodetector inside the scope. This provides a light-activated, electrically isolated trigger. In addition to an easily constructed assembly to hold the photodiode and provide a receptacle for the fiber-optic cable, a couple of minor wiring changes are all the modifications that are necessary in the instrument. No external components or voltages are required.