Air taxi Captain reported that the flight descended below ATC clearance altitude on approach. Flight crew climbed and continued the approach.

Narrative

Austin approach cleared us directly to the initial approach fix; to cross it at or above 2;500ft and cleared the RNAV 13 approach to EDC. I saw the FO (pilot flying press the APPR button) as I was looking down to turn the lights on for the approach; and to present the tower along with the ground frequencies into the radio unit. While my head was down looking for the frequencies; I heard the FO say disconnecting auto pilot; we descended below 2;500. I looked up at the altitude we were at 2;340ft. He forgot to arm VNAV and had turn the altitude pre-selector to the minimum for the approach. as he was climbing back to 2;500; ATC said that their radar had us at 2;300 and advised us to climb to 2 500. I apologized to him; and told him that we were correcting. Cause: The Pilot flying awareness of the flight guidance computer buttons that were selected. My awareness of what the pilot flying did and didn't select. I was distracted with looking up the upcoming frequencies. Suggestions: Verify the modes that were selected prior to starting another task.