PLL loop filter [hlp]
I want to use an active loop filter in my PLL. Which parameters I must consider in selecting the OpAmp? How should I design the filter? Should the OpAmp be a high frequency opamp? Please explain.
frist of all design the loop filter from the system spec's of the PLL and in this stage consider the OPAMP ideal
after u finish the filter design , and be sure that the PLL is locking and working ok ,
take the filter and begin to design the OPAMP to satisfy the spec's of the filter
khouly
It, as usual, depneds on the performance you are trying to meet.
But some general rules:
If you want very low phase noise PLL, you will want a low noise op amp, certainly one with less than 3 nv/√hz input noise, and around 1 nv/√hz would be better.
Also, if you need to have a wide PLL filter bandwidth, say 1 MHz, for fast settling time or other reason, you will need a much wider op amp unity gain bandwidth, say 50 MHz. The reason is that when you use an op amp, you set it up as a negative feedback device with resistors and capacitors so that the gain is appropriate as the frequency increases. In this case, the op amp only behaves as advertised IF the op amps open loop gain is high when the PLL filter open loop gain crosses unity. If you choose and op amp that does not have high frequency gain, and you try to make a wide bandwith PLL filter out of it, you end up with excessive phase shift in the loop filter itself (much more than you would predict from simple equeations that calculate the R and C values for the loop filter). So, you end up with a control loop that oscillates somewhere near the highest open loop bandwidth, since all your phase margin was taken up in a junky op amp.
Hello;
Khouly wrote:
How I can understand whether the PLL is locking and working ok?
Please explain.
use a system level simulator like matlab simulink , and use the transfere function of the filter , this is the ideal filter with ideal OPAMP , this will make u have some insight about the PLL is working with this filter or not
after that from the spec's design ur OPAMP ,
khouly
noise and BW should be considered, firstly.