LTC6948-4 View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC6948-4

Ultralow Noise 0.37GHz to 6.39GHz Fractional-N Synthesizer with Integrated VCO

Linear Technology 

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LTC6948

Applications Information

The package’s exposed pad is a ground connection, and

must be soldered directly to the PCB land pattern. The

PCB land pattern should have multiple thermal vias to the

ground plane for both low ground inductance and also low

thermal resistance (see Figure 20 for an example). See

QFN Package Users Guide, page 8, on Linear Technology

website’s Packaging Information page for specific recom-

mendations concerning land patterns and land via solder

masks. A link is provided below.

http://www.linear.com/designtools/packaging

6948 F20

Figure 20. Example Exposed Pad Land Pattern

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, improper PCB layout can degrade the LTC6948’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.

1) Do not share power supply decoupling capacitors

between same-voltage power supply pins.

2) Use separate ground vias for each power supply decou-

pling 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.

For more information www.linear.com/LTC6948

6948f

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