The Tektronix 067-0596-00 Chopped Voltage Reference Calibration Fixture is a signal generator that produces an approximately square waveform that alternates between two precisely controlled voltages, "V1" and "V2".
The unit consists of three subsystems − a variable DC voltage source for V1, a variable DC voltage source for V2, and an astable multivibrator that drives the coil of a relay that switches the output between V1 and V2. Each of the three subsystems has its own floating power supply (isolated secondary) coming off a power transformer that they share.
The 067-0596-00 uses an Airpax A175 chopper to switch the output between V1 and V2.
The 067-0596-00 is specified as part of calibration procedure for the 141A.