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LPF in PLL -- Reasons explored??

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gayu

Junior Member level 3
Actually if we use a phase detector in the PLL, then we use LPF only to cut the transients that comes out from the detector along with a DC voltage that denotes the phase error.Because only when the control voltage to the VCO is stable, it can work properly.

If we make use of a Frequency detector instead of Phase detector, what would be the reason for using the LPF.

GD

in fact a frequency detector multiplies its input. therefor in the out put we have 2 terms with 2 different frequencies f1+f2 and f1-f2 where f1 and f2 are the frequencies of input signals of detector. the LPF elleminates the term of f1+f2 frequency.

There will be poles in all systems. You always need to consider about stability when you have a feedback loop. Without the LPF, you will have parasitics that dominates your frequency responses. Since the parasitics are not controlled, your frequency response of the PLL will be hard to determine. Therefore, a LPF defines the dominate pole, and therefore, help preventing oscillations.

MSGHP,

Thats right. I accept. But I have one more question. According to this concept of using LPF to cut the component f1+f2, how can we justify the use of PLL as frequency multiplier. That is...Let us assume that locking to f1 is done. Now at the feedback we divide it by 2( to have 2*f1 at the output). Now what happens before LPF.. We have f1/2 and 3*f1/2 ..Right LPF allows f1/2.. Then what happens to the frequency multiplication..It behaves like a frequency division.Alas!!!

GD

gayu said:
MSGHP,

Thats right. I accept. But I have one more question. According to this concept of using LPF to cut the component f1+f2, how can we justify the use of PLL as frequency multiplier. That is...Let us assume that locking to f1 is done. Now at the feedback we divide it by 2( to have 2*f1 at the output). Now what happens before LPF.. We have f1/2 and 3*f1/2 ..Right LPF allows f1/2.. Then what happens to the frequency multiplication..It behaves like a frequency division.Alas!!!

GD