ADF4350 datasheet(27/28 Pages) AD

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ADF4350

Rev. 0 | Page 27 of 28

3.9nH

1nF

RFOUT

VVCO

50Ω

OUTPUT MATCHING

There are a number of ways to match the output of the ADF4350

for optimum operation; the most basic is to use 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 provides a good broadband match. Placing

the output power in this circuit into a 50 Ω load typically

gives values chosen by Bit D2 and Bit D1 in Register 4 (R4).

Figure 38.Optimum ADF4350 Output Stage

100pF

RFOUT

VVCO

50Ω

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

terminated or combined with both outputs using a balun.

50Ω

RFOUTA+

RFOUTA–

VVCO

Figure 37. Simple ADF4350 Output Stage

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

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

output power.

Experiments have shown 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 that case is about 5 dBm. Both single-ended architectures can

be examined using the EVAL-ADF4350EB1Z evaluation board.

Figure 39. ADF4350 LC Balun

A balun using discrete inductors and capacitors may be

implemented with the architecture in Figure 39.

Component L1 and Component C1 comprise the LC balun, L2

provides a dc path for RFOUTA−, and Capacitor C2 is used for dc

blocking.

Table 7. LC Balun Components

Frequency

Range (MHz)

Inductor L1 (nH)

Capacitor C1 (pF)

RF Choke

Inductor (nH)

DC Blocking

Capacitor (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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