Datasheet

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 V
VCO
. 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
09800-037
RF
OUT
V
VCO
50Ω
50Ω
Figure 37. Simple Output Stage
A better solution is to use a shunt inductor (acting as an RF choke)
to V
VCO
. 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
09800-038
RF
OUT
V
VCO
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 RF
OUT
A−, and Capacitor C2 is used for dc blocking.
L1
L1
C1
C1
50Ω
RF
OUT
A+
RF
OUT
A–
V
VCO
C2
L2
09800-039
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 10 390 1000 9
300 to 460 51 5.6 180 120 10
400 to 600 30 5.6 120 120 10
600 to 900 18 4 68 120 10
860 to 1240 12 2.2 39 10 9
1200 to 1600 5.6 1.2 15 10 9
1600 to 3600 3.3 0.7 10 10 8
2800 to 3800 2.2 0.5 10 10 8