Question about the Loop filter of PLL

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ericnguyen

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hi,

I got this from a book: "The Loop filter is to provide a memory for the loop in case lock is momentarily lost due to a large interfering transient", which I dont quite understand.

Could anyone help me with this?

Thanks
 

Low Pass Filter is actually an averaging function.
To do averaging function, at least you need some historical data.
Therefore, acting like a memory that remember historical data and perform averaging function.
 

Thank for helping, Sengyee.

You meant that because the Loop filter could memorize the last data so in case of losing lock due to external cause, the loop filter could pull the PLL back to being locked, didnt you?

However, Once losing lock, the VCO's frequency is actually changed! This will result in the change in the output voltage of the Phase detector ...hence affects the Loop Filter. Actually, It forces the output voltage of the Loopfilter to change, too. The "historical data" is then overwritten!!

Could you explain me how the Loop Filter can act like a memory?

Thank you so much.
 

Phase = ∫frequency dt
To change the phase, you cant change immediately, due to the integration function. You need some time to get to the correct phase.

Assuming there is a phase step, that cause VCO control voltage to change, hence VCO freq changed. You must wait for some time, for integration to perform over time, until Output_phase - Input_phase = constant, where we define as the phase has locked.

The loop filter is doing this integration function. It is not a real memory that remember exactly all historical data, but it able to give a representative value that represents all historical data.

Sorry, I tried my best to explain per my knowledge.
 

thank you very much, Sengyee.

I still dont quite understand.

Why does PLL need this "historical data" memory?

I know that PLL helps filter out the high frequency component. Other than that, the integration carried out by the PLL is not a good thing!! It actually slows down your PLL, right?

I am very confused with this Loop filter. Sorry for asking many questions. I still hope to have you guys answered my question though!!

Thanks a lot!
 

I know that PLL helps filter out the high frequency component. Other than that, the integration carried out by the PLL is not a good thing!! It actually slows down your PLL, right?
Click to expand...
Purpose of PLL is not to filter high freq component. The purpose is to have constant phase error. To achieve this, integration of frequency over time is a MUST. This is the theory of PLL. Whether it slows down or not, we have no choice, all PLL behave like this. Unless you have something like FLL, frequency locked loop.
 

Hello,
u can think of as the PLL response to the changes in the input signal according to the PLL's BW (related also to LPF BW) , so fast signals affecting the input faster than the PLL wont really affect the output , where fast signals in time are high frequency which u could say are averaged by the PLL as the PLL is after all has a closed loop charecteristics of a LPF
 

Once losing lock, the VCO's frequency is actually changed! T
 

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