  The Event Horizon Telescope (EHT) is a very-long-baseline interferometry
(VLBI) experiment that aims to observe supermassive black holes with an angular
resolution that is comparable to the event horizon scale. The South Pole
occupies an important position in the array, greatly increasing its north-south
extent and therefore its resolution.
  The South Pole Telescope (SPT) is a 10-meter diameter, millimeter-wavelength
telescope equipped for bolometric observations of the cosmic microwave
background. To enable VLBI observations with the SPT we have constructed a
coherent signal chain suitable for the South Pole environment. The
dual-frequency receiver incorporates state-of-the-art SIS mixers and is
installed in the SPT receiver cabin. The VLBI signal chain also includes a
recording system and reference frequency generator tied to a hydrogen maser.
Here we describe the SPT VLBI system design in detail and present both the lab
measurements and on-sky results.
