FM Modulator with PLL

[jmpac]

Hello all,

in a FM modulator with PLL, I wonder if someone knows the ratio which is usually used between the PFD (phase frequency detector) and the DC modulating frequency. I think PFD must be greater than the DC modulating frequency, otherwise PLL will not work. Is that true? Does anyone know what's the recommended ratio? Any application note to take into account? When I say "DC modulating frequency" i am thinking of two varactors in the VCO: one is tuned by the charge pump of the PLL, after the low pass filter, and the other one is tuned by a voltage wave at a fixed frequency.

Thank you.

 

[chuckey]

Unless your have a problem with the time the PLL takes to lock up , slower is better. As it gets faster it starts to try to compensate for the "DC modulation frequency" (call it modulation).
Frank