adf4351 Analog Devices, Inc., adf4351 Datasheet - Page 27

• Manufacturer Part Number: adf4351
• Description: Wideband Synthesizer With Integrated Vco Preliminary Technical Data Adf4351
• Manufacturer: Analog Devices, Inc.

Preliminary Technical Data
OUTPUT MATCHING
There are a number of ways to match the output of the ADF4351
for optimum operation; the most basic is to use a 50 Ω resistor to
V
shown in Figure . 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).
A better solution is to use a shunt inductor (acting as an RF
choke) to V
output power.
Experiments have shown the circuit shown in Figure 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-ADF4351EB1Z evaluation board.
Table 7. LC Balun Components
Frequency
Range (MHz)
137 to 300
300 to 460
400 to 600
600 to 900
860 to 1240
1200 to 1600
1600 to 3600
2800 to 3800
VCO
. A dc bypass capacitor of 100 pF is connected in series as
VCO
. This gives a better match and, therefore, more
Figure 37. Simple ADF4351 Output Stage
RF
V
VCO
OUT
Inductor L1 (nH)
100
51
30
18
12
5.6
3.3
2.2
50Ω
100pF
50Ω
Capacitor C1 (pF)
10
5.6
5.6
4
2.2
1.2
0.7
0.5
Rev. PrC | Page 27 of 28
RF Choke
Inductor (nH)
390
180
120
68
39
15
10
10
If differential outputs are not needed, the unused output can be
terminated or combined with both outputs using a balun.
A balun using discrete inductors and capacitors may be
implemented with the architecture in Figure .
Component L1 and Component C1 comprise the LC balun, L2
provides a dc path for RF
blocking.
Figure 38.Optimum ADF4351 Output Stage
RF
RF
RF
V
OUT
OUT
VCO
OUT
V
VCO
DC Blocking
Capacitor (pF)
1000
120
120
120
10
10
10
10
A+
A–
Figure 39. ADF4351 LC Balun
OUT
3.9nH
A−, and Capacitor C2 is used for dc
L2
1nF
L1
C1
L1
C1
C2
Measured Output
Power (dBm)
9
10
10
10
9
9
8
8
50Ω
50Ω
ADF4351