  The digital phase lock loop described in this paper has been in use on the
Submillimeter Array (SMA) front-end receivers for over a decade and has been a
key element in obtaining all of the high quality images that have been
published from this telescope over the years. The technical achievements
enabled by these devices include the first phase closure observations in the
690 GHz band, the first attempts at band-to-band phase transfer at
submillimeter wavelengths, and the first successful demonstration of
interferometry using a fully photonic millimeter-wave local oscillator. Copies
of these devices are also in place at the Caltech Submillimeter Observatory and
the James Clerk Maxwell Telescope in support of the eSMA project and
submillimeter VLBI experiments. Additional units of this design were used by
the Princeton Millimeter Interferometer and the Microwave Anisotropy Telescope.
In total, over three dozen units have been constructed and used in astronomical
studies. In this paper, we briefly describe the background theory, design,
performance, and calibration steps, and provide useful testing and repair
information.
