decoupling pll supply
Probably somewhere connected to that resistor you have a grounded capacitor, which together makes an RC filter to suppress the supply noise.
maybe for a choke as i think
PLL's are very sensitive to noise pickup. A very small unwanted voltage on the tuning port of a VCO can easily cause out of spec. phase noise.
Op amps have inherent power supply noise rejection, but only at lower frequencies. As the frequency of the noise gets higher, the rejection gets poorer. Also, voltage regulator chips have very little noise rejection capability for noise above around 200 KHz.
So, for a lot of applications you must provide passive filtering for DC power supply connections to the PLL. In the 1 to 20 MHz region, shunt filtering capacitors work but are limited by internal parasitic resistance and inductance. By adding the series resistor, you are forming an actual RC lowpass filter that provides better noise rejection in spite of the capacitor's parasitic problems.
The only trouble is that some op amps can become unstable with an RC network between the power supply and the IC (even if you are using a really big C). You might have to reduce the resistor's value to be pretty small to get those op amps to work (possibly only a few ohms allowed).
The value of the resistor is usually defined by acceptable VCC voltage drop so the pll analog portion (VCO, CP, filter) still work well and minimal spec. VCC.
Ie drop of 0.5V with 18Ohms means the device has about 28mA consumption.
I believe you would not want bigger drop....
This resistor and internal/external decoupling is creating a filter to clean up noise on analog power.
I have seen a nice explanation on NBC12429 pll page 11/12 -www.onsemi.com/pub/Collateral/NBC12429-D.PDF
you mean series small resister and paral big cap can make a lowpass filter but not a paral big cap?
Added after 2 minutes:
you mean series small resister and paral big cap can make a lowpass filter but not a paral big cap?