VLBA observers using more than 1 BBC will want to sum over the BBCs to reduce noise levels. This should not be done with the raw signals delivered by the BBCs: the independent local oscillators in each BBC introduce an unknown phase offset from one BBC to the next, so such a summation of the raw signals would be incoherent. A so-called ``phase cal'' or ``pulse cal'' system (Thompson 1995) is available at VLBA antennas to overcome this problem. This system, in conjuction with the LO cable length measuring system, is also used to measure changes in the delays through the cables and electronics which must be removed for accurate geodetic and astrometric observations. The pulse cal system consists of a pulse generator and a sine-wave detector. The interval between the pulses can be either 0.2 or 1 microsecond. They are injected into the signal path at the receivers and serve to define the delay reference point for astrometry. The weak pulses appear in the spectrum as a ``comb'' of very narrow, weak spectral lines at intervals of 1 MHz (or, optionally, 5 MHz). The detector measures the phase of one or more of these lines, and their relative offsets can be used to correct the phases of data from different BBCs. The detector is in the correlator for the Mark III system and at the antenna in the VLBA. The VLBA pulse cal data are logged as a function of time during observations with VLBA antennas, and included in the calibration information delivered to the user in tabular form. Although AIPS software can be used to load and apply the pulse cal data, this path is still new; VLBA users may still want to observe a strong compact source (see Section 15.1) so they can do a ``manual'' pulse cal if necessary (Diamond 1995).