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Design >> RF Design >> questions about PLL's LPF design and simulation？
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Message started by lwzunique on Aug 22^nd, 2017, 7:25pm

Title: questions about PLL's LPF design and simulation？
Post by lwzunique on Aug 22^nd, 2017, 7:25pm

hi everyone
I am a beginner to PLL, something about PLL'S LPF design and simulation bothers me so much.
firstly, speaking of the simulation problem, looking at the v-tune curve of PLL below, in this axes scale, the line will be flat enough( let us ignore the ripple here, we will discuss the ripple later.) but when you zoom in to the detail, we can see that there is some declination , and the delta is about 4mv.
so the questions are:
simulation:
1. is the declination the nature behavior of CP_PLL? or it's because of the simulator's accuracy?
2. there is an approximation formulas to determine the locking the of PLL, Tc=4/fc, the fc is the bandwidth of PLL, when the simulation time reaches 4/fc, how accurate is the frequency of PLL output compared to the frequency we want. for example, in some datasheets, we can see these kinds of specs：±100KHz , 100ppm. when doing simulation, can we achieve this accuracy when the simulation time get close to 4/fc, and from this time point, V-tune will be flat or declination still exists?
ripple:
1.in the v-tune curve below, because VCO is veriloga model, so there is no varactor coupling here, so the ripple is caused by charge pump's charging and discharging behavior, and the fluctuation can't be attenuated by third order LPF.
if we want this ripple much smaller, C1 should be much bigger and R1 smaller, but when doing this the bandwidth of PLL will be smaller and the locking time will be much longer, so how to trade off here, or if there are some ways that we can design an LPF make the ripple much smaller without increase the locking time?
2. there is also some ripple on V-tune caused by the coupling effect of varactor in VCO, what we should do and what we can do to this kind of ripple?

thanks very much！

Title: Re: questions about PLL's LPF design and simulation？
Post by lwzunique on Aug 22^nd, 2017, 7:26pm

v-tune's detail