The bipolar output of the rf phase detector of the receiver in a nuclear magnetic resonance pulse experiment is fed through a 4‐diode sampling switch to an RC integrator, whose output is read by a recording electrometer. The switch (a ``transmission gate'') includes two type 7266 ceramic thermionic diodes, whose back resistance is extremely high (> 1012&OHgr; at −50 v). The Mylar integrating capacitor, therefore, discharges so slowly between successive sampling gates that the typical holding time with a 0.01‐&mgr;f capacitor is ∼10 min. A simple method is given for using the ``boxcar'' as a sampling oscillograph for slowly and automatically plotting the entire boxcar input signal as it is seen on a cathode ray oscilloscope, but with the noise integrated out. There is a detailed discussion of circuitry, components, and performance.