Circuit Note

CN-0134

Rev. B | Page 3 of 5

Table 2. ADF4350 RF Output Filter Component Values (DNI = Do Not Insert)

Frequency Range

(MHz)

L1

(nH)

L2

(nH)

C1a

(pF)

C1c

(pF)

C2a

(pF)

C2c

(pF)

C3a

(pF)

C3c

(pF)

a.

500–1300

27 nH|| 50 Ω

3.9

3.9

DNI

4.7

DNI

5.6

DNI

3.3

b.

850–2450

19 nH || (100 Ω in position C1c)

2.7

2.7

3.3

100 Ω

4.7

DNI

3.3

DNI

c.

1250–2800

50 Ω

0 Ω

3.6

DNI

DNI

2.2

DNI

1.5

DNI

d.

2800–4400

3.9 nH

0 Ω

0 Ω

DNI

DNI

DNI

DNI

DNI

DNI

a lesser extent, the decoupling capacitors on the supply node. To

get a broadband match it is recommended to use either a

power, depending on the inductor chosen. Note that it is

possible to place the parallel resistor as a differential component

(i.e. 100 Ω) in position C1c to minimize board space. This is

done in filter type c, described in Table 2.

The filter should be designed with a cutoff approximately 1.2 to

1.5 times the highest frequency in the band of interest. This

allows margin in the design, as typically the cutoff will be lower

than designed due to parasitics. The effect of PCB parasitics can

be simulated in an EM simulation tool for improved accuracy.

ADF4350

13

RFOUTA+

12

RFOUTA–

ADL5375

9

LOIP

10

LOIN

L1

L2

1nF

L1

L2

1nF

C1a

C1a

C1c

C2a

C2a

C2c

C3a

C3a

C3c

120pF

120pF

0.1µF

3.3V

Figure 3. ADF4350 RF Output Filter Schematic

As can be seen from Table 2, at lower frequencies below

1250 MHz, a 5th order filter is required. For 1.25 GHz to

2.8 GHz, 3rd order filtering is sufficient. For frequencies above

2.8 GHz, no filtering is required, as the harmonic levels are

sufficiently low to meet sideband suppression specifications.

0

–70

–60

–50

–40

–30

–20

–10

800

1000

1200

1400

1600

1800

2000

2200

2400

FREQUENCY (MHz)

NO FILTERING

FILTER B: 850MHz TO 2450MHz

Figure 4. Sideband Suppression for Filter b, 850 MHz to 2450 MHz

Q

I

ERROR

VECTOR

MEASURED

SIGNAL

PHASE ERROR

(I/Q ERROR PHASE)

MAGNITUDE ERROR

(I/Q ERROR PHASE)

IDEAL SIGNAL

(REFERENCE)

0

Figure 5. EVM Plot

A sweep of sideband suppression vs. frequency is shown in

Figure 4 for the circuit using Filter b (850 MHz to 2450 MHz).

In this sweep, the test conditions were the following:

baseband I/Q amplitude = 1 V p-p differential sine waves in

quadrature with a 500 mV (ADL5375-05) dc bias; baseband I/Q

frequency (fBB) = 1 MHz.