Considering the loop response based on these regions can provide rapid insight to noise contributions from various parts of the loop.Įach noise source can be considered in terms of a traditional feedback system as shown in Figure 6. Note that K F is different in each region, but is a constant that can be used in evaluating the PLL transfer function for each region. The constant rolloff regions are integrators and can be written as K F/s. The constant gain region can be written as K F. These regions have either a constant gain or a constant roll-off. An additional pole after loop bandwidth provides additional filtering of the reference noise and forms the third region of additional filtering. In order to ensure stability, a zero is typically applied in the loop filter forming the middle region of nearly constant gain. Loop gain in this region provides the mechanism for the VCO to track the reference oscillator. The Loop Filter response can be considered in three general regions. R LP and C LP form a low pass filter that is typically set above the PLL loop bandwidth.Ī typical response of this filter is shown in Figure 5. C F forms an integrator below the zero set by C F and R F. This form is typical of many used in low noise phase locked loop design. For this analysis, a loop filter of the form shown in Figure 4 is assumed. Many forms of loop filters exist and have been demonstrated. Noise transfer functions will be derived for each contributor to overall output phase noise.įigure 2. Noise sources can be added to the control model as shown in Figure 3. Every component in the loop adds noise to the circuit. Again, references provide thorough derivations of this method.
The phase locked loop circuit of Figure 1 can be constructed in a control system block diagram form as shown in Figure 2. References provide detailed descriptions of the phase locked loop process. The integrator adjusts the VCO tuning voltage to minimize the output of the phase detector and thus phase locks the VCO to a reference input signal. The phase detector produces a signal proportional to the phase difference of the two input signals. Derivation of noise transfer functions and some key points for phase locked loop noise analysis is provided along with a simulation and measured example.Ī basic phase locked loop block diagram is shown in Figure 1. This area seems to be less understood and not explicitly stated in much of the literature. A critical aspect of phase locked loop design for low noise applications is a clear and intuitive understanding of the noise contributions of components in various parts of the loop.
Much literature exists on design and simulation methods. Phase Locked Loops are a fundamental building block in Frequency Synthesizer Design and routinely used in many applications.