Datasheet

Phase

Detector

C3 C4

R3 R4

LMK0300xx

Internal Loop Filter

External Loop Filter

LMK03000, LMK03000C, LMK03000D, LMK03001

LMK03001C, LMK03001D, LMK03033, LMK03033C

www.ti.com

SNAS381O –NOVEMBER 2006–REVISED MARCH 2013

8.4 LOOP FILTER

Figure 8-2. Loop Filter

The internal charge pump is directly connected to the integrated loop filter components. The first and

second pole of the loop filter are externally attached as shown in Figure 8-2. When the loop filter is

designed, it must be stable over the entire frequency band, meaning that the changes in K

Vtune

from the

low to high band specification will not make the loop filter unstable. The design of the loop filter is

application specific and can be rather involved, but is discussed in depth in the Clock Conditioner Owner's

Manual provided by Texas Instruments. When designing with the integrated loop filter of the LMK03000

family, considerations for minimum resistor thermal noise often lead one to the decision to design for the

minimum value for integrated resistors, R3 and R4. Both the integrated loop filter resistors and capacitors

(C3 and C4) also restrict how wide the loop bandwidth the PLL can have. However, these integrated

components do have the advantage that they are closer to the VCO and can therefore filter out some

noise and spurs better than external components. For this reason, a common strategy is to minimize the

internal loop filter resistors and then design for the largest internal capacitor values that permit a wide

enough loop bandwidth. In some situations where spurs requirements are very stringent and there is

margin on phase noise, it might make sense to design for a loop filter with integrated resistor values that

are larger than their minimum value.

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LMK03033C