LTC6946 datasheet(26/30 Pages) LINER

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LTC6946

6946fa

Figure 18. Example SMT Balun Connection

Figure 19. Example TL Balun Connection

APPLICATIONS INFORMATION

SUPPLY BYPASSING AND PCB LAYOUT GUIDELINES

Care must be taken when creating a PCB layout to mini-

mize power supply decoupling and ground inductances.

All power supply V+ pins should be bypassed directly to

the ground plane using a 0.1μF ceramic capacitor as close

to the pin as possible. Multiple vias to the ground plane

should be used for all ground connections, including to

the power supply decoupling capacitors.

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

must be soldered directly to the PCB land. 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 Pack-

age Users Guide, page 8, on Linear Technology website’s

Packaging Information page for specific recommendations

concerning land patterns and land via solder masks. A link

is provided below.

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

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 LTC6946’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 de-

coupling capacitor, especially those connected to VREF+,

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.

LTC6946

VRF

RF–

RF+

TO 50Ω

LOAD

6946 F18

BALUN

BALUN PIN CONFIGURATION

UNBALANCED PORT

GND OR DC FEED

BALANCED PORT

GND

LTC6946

VRF

RF–

RF+

TO 50Ω

LOAD

PRI

SEC

6946 F19

6946 F20

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