[SOLVED] Low-pass characteristic of PLL

Hello.

I studied PLL for the first time, so i have confused about some characteristics of PLL.

I know that PLL has LPF after the PD and this LPF has transfer funtion of 1/(1+s/w_lpf) for type 1 and (I_p/2*pi)*(R_p+1/sC_p) for type 2.

What i want to know is the meaning of this two LPF's bandwidth, w_lpf.

I think that this bandwidth is about input frequency before. But considering that LPF's input is PD's output, it is wrong.

So i think that this bandwidth is about the frequency of input-output varying amounts, is it all right?

Hi,

It looks that in both case you have a first order RC filter. In general you need this filter to convert noisy PD Charge pump output into DC input to the VCO (unless you want do some FM modulation). If you increase the order of the filter your far noise floor performance will be better. The BW of the filter will change the PLL lock time and range.
Above is "very" basic ex,plain, you can find plenty material on the web, e.g.:
**broken link removed**
https://www.analog.com/media/en/training-seminars/tutorials/MT-086.pdf

RS

rlagusrb24 said:

What i want to know is the meaning of this two LPF's bandwidth, w_lpf.

I think that this bandwidth is about input frequency before. But considering that LPF's input is PD's output, it is wrong.

So i think that this bandwidth is about the frequency of input-output varying amounts, is it all right?

Click to expand...

1.) Speaking about filtering and transfer functions, we must assume a LINEAR system.
That means: We consider the PLL under locked conditions only.
2.) Because the locked PLL reacts upon phase changes, the input and output quantities for the corresponding PLL transfer function are signal phase relations (and its variations).
For a first-order loop filter, the loop transfer function is of 2nd-order because the VCO acts as an integrator as far as the phase is concerned.
3.) As a result, the PLL`s phase-transfer function can be seen as a second-oder lowpass. The frequency variable is w=d(phi/dt).
The pole frequency wp of this 2nd-oder lowpass is mainly determined by the pole frequency wp of the loop filter (you call it w_lpf).
4.) This lowpass function (and the corresponding cut-off frequency wc~wp) determines the slope of the maximum phase deviations d(phi)dt which can be tolerated by the PLL without loosing lock.
Hence, wp it is called "loop bandwidth".