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The Atacama Large millimeter/submillimeter Array (ALMA) obtains spatialresolutions of 15 to 5 milli-arcsecond (mas) at 275-950GHz (0.87-0.32mm) with16km baselines. Calibration at higher-frequencies is challenging as ALMAsensitivity and quasar density decrease. The Band-to-Band (B2B) techniqueobserves a detectable quasar at lower frequency that is closer to the target,compared to one at the target high-frequency. Calibration involves a nearlyconstant instrumental phase offset between the frequencies and the conversionof the temporal phases to the target frequency. The instrumental offsets aresolved with a differential-gain-calibration (DGC) sequence, consisting ofalternating low and high frequency scans of strong quasar. Here we compare B2Band in-band phase referencing for high-frequencies ($>$289GHz) using 2-15kmbaselines and calibrator separation angles between $\sim$0.68 and$\sim$11.65$^{\circ}$. The analysis shows that: (1) DGC for B2B produces acoherence loss $<$7% for DGC phase RMS residuals $<$30$^{\circ}$. (2) B2Bimages using close calibrators ( $<$1.67$^{\circ}$ ) are superior to in-bandimages using distant ones ( $>$2.42$^{\circ}$ ). (3) For more distantcalibrators, B2B is preferred if it provides a calibrator $\sim$2$^{\circ}$closer than the best in-band calibrator. (4) Decreasing image coherence andpoorer image quality occur with increasing phase calibrator separation anglebecause of uncertainties in the antenna positions and sub-optimal phasereferencing. (5) To achieve $>$70% coherence for long-baseline (16 km) band 7(289GHz) observations, calibrators should be within $\sim$4$^{\circ}$ of thetarget.

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ALMA High-frequency Long-baseline Campaign in 2017: A Comparison of the  Band-to-band and In-band Phase Calibration Techniques and Phase-calibrator  Separation Angles