External pulse generator

The design is the visual features of an EXTERNAL PULSE GENERATOR shown in solid lines in the drawings, whether those features are features of one of shape, configuration, ornament, or pattern, or are a combination of any of these features. The broken lines are provided for illustrative purposes only, and form no part of the claimed design. FIG. 1 is a front, top, right side perspective view of an EXTERNAL PULSE GENERATOR, illustrating the design; FIG. 2 is a rear, top, left side perspective view thereof; FIG. 3 is a front view thereof; FIG. 4 is a rear view thereof; FIG. 5 is a left side view thereof; FIG. 6 is a right side view thereof; FIG. 7 is a top view thereof; FIG. 8 is a bottom view thereof; Drawings of the design are included.