HP RF test equipment uses a small 3 pin connector to provide power to external probes and accessories. HP doesn't provide these easily and they are not a standard Digi-Key type of buy. If you want to make accessories for HP Network or Spectrum Analyzers- custom probes, pre-amps, fixtures, etc- Its nice to be be able to power them from this internal power source rather than a power brick or batteries. This is a simple design that I've used with several cables and gadgets- I generally use a 7805 and 7905 regulator in my designs to regulate the power down to +-5V. The connector body has the correct geometry to line up with the pins of the HP female connector. IC socket pins, discarded DB-25 pins, etc can be soldered and friction fit in the hole. Some scrounging required. A groove that will enhance the grip of heat shrink at the transition from the cable to the connector is provided. Comments welcome.