This contribution focuses on phase-noise added during frequency conversion in low-level radio frequency (LLRF) control systems. The stability of beams' parameters in linear accelerators depends on the stability of amplitude and phase of the accelerating field. A LLRF control system regulates the electromagnetic field inside accelerating modules based on the input RF signals. Typically active mixers down-convert those signals, which are later sampled and digitized by ADCs. This field detection scheme necessitates synthesis of a heterodyne/local oscillator (LO) signal which is often generated using a passive mixer and a frequency divider. Additive close-to-carrier phase noise can be observed in the aforementioned circuits. The phase noise of a passive mixer's output signal is typically calculated using a small-signal model based on modulation theory. Experimental results indicate that the power level of the input signals has a nonlinear effect on phase noise beyond the noise floor. The frequency dividers' output phase noise was measured as a function of input power, input frequency, and division ratio. The influence of the LO signal power level on the active mixers' output signal phase noise was measured and two hypotheses were made. Further measurements of the AM-PM and PM-AM conversion were made to verify one of the hypotheses. The fidelity of the LO signal is partially determined by the phase noise of the IF signal.