  The Atacama Large millimeter/submillimeter Array (ALMA) obtains spatial
resolutions of 15 to 5 milli-arcsecond (mas) at 275-950GHz (0.87-0.32mm) with
16km baselines. Calibration at higher-frequencies is challenging as ALMA
sensitivity and quasar density decrease. The Band-to-Band (B2B) technique
observes a detectable quasar at lower frequency that is closer to the target,
compared to one at the target high-frequency. Calibration involves a nearly
constant instrumental phase offset between the frequencies and the conversion
of the temporal phases to the target frequency. The instrumental offsets are
solved with a differential-gain-calibration (DGC) sequence, consisting of
alternating low and high frequency scans of strong quasar. Here we compare B2B
and in-band phase referencing for high-frequencies ($>$289GHz) using 2-15km
baselines and calibrator separation angles between $\sim$0.68 and
$\sim$11.65$^{\circ}$. The analysis shows that: (1) DGC for B2B produces a
coherence loss $<$7% for DGC phase RMS residuals $<$30$^{\circ}$. (2) B2B
images using close calibrators ( $<$1.67$^{\circ}$ ) are superior to in-band
images using distant ones ( $>$2.42$^{\circ}$ ). (3) For more distant
calibrators, B2B is preferred if it provides a calibrator $\sim$2$^{\circ}$
closer than the best in-band calibrator. (4) Decreasing image coherence and
poorer image quality occur with increasing phase calibrator separation angle
because of uncertainties in the antenna positions and sub-optimal phase
referencing. (5) To achieve $>$70% coherence for long-baseline (16 km) band 7
(289GHz) observations, calibrators should be within $\sim$4$^{\circ}$ of the
target.
