This paper presents internal calibration methods which are especially suited for implementation in software-defined radio hardware platforms which integrate a large number of transceivers. The procedures provide reciprocity calibration of the system for e.g. large scale MU-MIMO TDD communications and full absolute phase and amplitude coherence between transmitters and receivers for e.g. MIMO channel sounding, direction of arrival estimation or other smart antenna algorithms. The advantage of completely internal calibration includes increased robustness because no interference or reflections are picked up during calibration, the steady repeatable calibration performance independent from the connected antennas, and much more reliable and numerically stable estimated calibration coefficients than other techniques based on over-the-air pilot exchange. The proposed calibration methods can be realized via an internal calibration interconnect network which reduces transceiver interconnection effort and clutter to a minimum and is thus very well suited for implementation in highly compact large scale SDR systems with hundreds of transceivers.