I am trying to design a PLL and want to test it first in Matlab.
I have seen that the phase of the input and output are linearized, but I don't understand what is defined by the input/output phase.
Usually, a transfer function is the ratio of the input's voltage/current to the output voltage/current. However, this Wikipedia article gives the transfer function of a PLL as the ratio of two phases:
#\$ \frac{\theta_O}{\theta_I} = \frac{K_P K_v F(s) } { s + K_P K_v F(s)} \$
where
\$\theta_O\$ is the output phase in radians
\$\theta_I\$ is the input phase in radians
\$K_P\$ is the phase detector gain in volts per radian
\$K_V\$ is the VCO gain in radians per volt-second
\$F(s)\$ is the loop filter transfer function (dimensionless)