LTC6948 datasheet(34/38 Pages) AD

34 / 38 page

LTC6948

6948fa

For more information www.linear.com/LTC6948

REFERENCE SIGNAL ROUTING, SPURIOUS, AND

PHASE NOISE

The charge pump operates at the PFD’s comparison

frequency fPFD. The resultant output spurious energy is

small and is further reduced by the loop filter before it

modulates the VCO frequency.

However,improperPCBlayoutcandegradetheLTC6948’s

inherent spurious performance. Care must be taken to

prevent the reference signal fREF from coupling onto the

VCO’s tune line, or into other loop filter signals. Example

suggestions are the following.

applicaTions inForMaTion

1) Do not share power supply decoupling capacitors

between same-voltage power supply pins.

2) Use separate ground vias for each power supply

decoupling capacitor, especially those connected to

VREF+, VD+, LDO, VCP+, and VVCO+.

3) Physically separate the reference frequency signal

from the loop filter and VCO.

4) Do not place a trace between the CMA, CMB, and CMC

pads underneath the package, as worse phase noise

could result.

LTC6948 Datasheet(PDF) 34 Page - Analog Devices