The Agilent 35653A is a Source Calibration Module. HP 35653A Specifications: 35653A: 51.2kHz bandwidth, Provides the following excitation signals for frequency response measurements: Continuous sine wave, Band-limited random noise, or Burst random noise of variable duration; Also includes reference signal for system calibration of input modules; The 35653A is part of 3565S Multichannel Measurement System.