Help me combine coarse voltage and fine voltage to control VCO

[dushan1]

i would combine a coarse voltage and fine voltage to control a VCO.anybody can give me some circuits or documents about these function,thanks!

 

[biff44]

Re: need help about pll

It is common to use an op amp summing junction, like a unity gain inverter circuit. You sum in a DAC coarse tune voltage there, and its output goes to the VCO tune port. The output of your integrating loop filter is also summed there (ie the integrator comes first, then the summing junction, then the VCO). You need to invert the polarity of the circuit somewhere, to take into account the -1 gain of the new summing junction. +1 gain "summing" junctions do not work as well.

 

[dushan1]

need help about pll

thanks for your help.