A sub-miliimeter-wave heterodyne receiver designed for a D2O laser Thomson scattering ion temperature diagnostic has been used to measure the synchrotron emission from the Alcator A tokamak at 762 ± 11 GHz down to levels of 10−20 W·Hz−1. A comparison with predicted Thomson-scattered signal levels indicates that the measured background will be much lower except near the third electron cyclotron harmonic for fields near 9 T.