Voltage source behind an impedance is a term widely used in grid-codes requirements for grid forming (GFM) power electronic converters. However, the exact meaning of this requirement is hard to quantify and verify, and its necessity is not adequately justified. This paper identifies key characteristics that GFM controls should possess and evaluates how GFM controls, with and without a current controller, meet these characteristics. The study utilizes the dq-frame admittance and the frequency-domain power flow Jacobian to analyze an ideal voltage source behind an RL impedance, a synchronous generator, and various GFM and grid-following controls implemented in modular multilevel converters (MMCs). The research identifies several essential voltage source characteristics, which can be mathematically calculated and used to define the frequency ranges where the requirements are met. The paper demonstrates that the GFM structures meet the essential voltage source characteristics in a limited frequency range. The GFM with the current controller is particularly affected due to the nonpassivities of the current controller and virtual impedance. The requirements calculated from the essential voltage source characteristics can serve as a screening method to assess the effectiveness of converter controls, and the insights gained can be utilized to enhance the design thereof.