  The use of precision timing measurements will be a major tool at the HL-LHC,
where it will be used to suppress pile-up and to search for long-lived
particles. To control a reference clock with sub-picosecond accuracy, we have
fabricated in the TSMC 65 nm process a digitally controlled phase shifter. It
is composed of a chain of 66 cells, each with a digitally controlled planar
wave guide with either a short or long delay. With this a reference clock's
phase can be controlled to a precision of 200 fs with dynamic range of 12 ps.
