**ABSTRACT**

The present paper deals with ТЕМ propagation of waves along lossless transmission lines terminated by a crystal oscillator circuit. The crystal oscillator circuit generates nonlinear boundary conditions derived by the Kirchhoff’s law. Then the mixed problem for the hyperbolic transmission line system to an initial value problem for neutral system on the boundary is reduced. The main purpose of the present paper is to show an existence-uniqueness of a periodic solution of the neutral system. This is achieved by introducing a suitable operator acting on a space of periodic functions. Its fixed point is a periodic solution of the neutral system. The advantages of the presented method on numerical example are demonstrated. The solution can be obtained by successive approximations.

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