Why the filter bandwidth should be about tenth of the reference frequency in the PLL?

i think u mean the loop bandwidth is tenth the reference frequency , this to hold that continouse time approximation of the PLL , coz the PLL is a combination between discrete time compoenents like the divider and the PFD and analog cmpaonets like VCO

so u need to make sure that the loop BW is 1/10 of the reference so the linear analysis based on S domain is valid other wise u need to consider the discrete time analysis based on Z domain

khouly

So this is just a practical number, right?

and, I also noticed that there are papers dealing with the analysis of PLL in Z-domain. why it is still recommended to use continuous approximation?

for simplicty and practical reasons like u need to reduce the bandwisth to filter all the noise in the control voltage and so on . we still use the S domain , but if u want to deeply analyze the PLL u need to use the Z domain

i think the value is more significant than justified