  We implement an ensemble quantum counting algorithm on three NMR
spectrometers with 1H resonance frequencies of 500, 600 and 750 MHz. At higher
frequencies, the results deviate markedly from naive theoretical predictions.
These systematic errors can be attributed almost entirely to off-resonance
effects, which can be substantially corrected for using fully-compensating
composite rotation pulse sequences originally developed by Tycko. We also
derive an analytic expression for generating such sequences with arbitrary
rotation angles.
