This looks like a good beginner's GPSDO on eBay: 172148560746 "This is GPS Disciplined Clock made with trimble GPSDO Board.Full tested by Agilent 53132A with US-012 option and Ex-ref from trimble thunderbolt GPSDO." The seller says it has a sine wave output and is accurate from 10e-11 to 10e-12. Any thoughts? Richard