ADF4351 datasheet(27/28 Pages) AD

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Data Sheet

ADF4351

Rev. 0 | Page 27 of 28

OUTPUT MATCHING

For optimum operation, the output of the ADF4351 can be

matched in a number of ways; the most basic method is to con-

nect a 50 Ω resistor to VVCO. A dc bypass capacitor of 100 pF is

connected in series, as shown in Figure 37. Because the resistor

is not frequency dependent, this method provides a good broad-

band match. When connected to a 50 Ω load, this circuit typically

gives a differential output power equal to the value selected by

Bits[DB4:DB3] in Register 4 (R4).

100pF

RFOUT

VVCO

50Ω

Figure 37. Simple Output Stage

A better solution is to use a shunt inductor (acting as an RF choke)

to VVCO. This solution gives a better match and, therefore, more

output power.

Experiments have shown that the circuit shown in Figure 38

provides an excellent match to 50 Ω for the W-CDMA UMTS

Band 1 (2110 MHz to 2170 MHz). The maximum output power

in this case is approximately 5 dBm. Both single-ended archi-

tectures can be examined using the EVAL-ADF4351EB1Z

evaluation board.

3.9nF

1nF

RFOUT

VVCO

50Ω

Figure 38. Optimum Output Stage

If differential outputs are not needed, the unused output can be

terminated, or both outputs can be combined using a balun.

A balun using discrete inductors and capacitors can be imple-

mented with the architecture shown in Figure 39. The LC balun

comprises Component L1 and Component C1. L2 provides a dc

path for RFOUTA−, and Capacitor C2 is used for dc blocking.

50Ω

RFOUTA+

RFOUTA–

VVCO

Figure 39. LC Balun for the ADF4351

Table 8. LC Balun Components

Frequency

Range (MHz)

Inductor L1 (nH)

Capacitor C1 (pF)

RF Choke

Inductor L2 (nH)

DC Blocking

Capacitor C2 (pF)

Measured Output

Power (dBm)

137 to 300

100

390

1000

300 to 460

5.6

180

120

400 to 600

5.6

120

600 to 900

120

860 to 1240

2.2

1200 to 1600

5.6

1.2

1600 to 3600

3.3

0.7

2800 to 3800

2.2

0.5

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