Circuit Note

CN-0285

Rev. 0 | Page 3 of 5

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

get a broadband match, it is recommended to use either a resistive

the inductor chosen. Note that it is possible to place the parallel

resistor as a differential component (that is, 100 Ω) in Position

C1c to minimize board space (see Filter Type B, Table 2).

Design the filter with a cutoff approximately 1.2 times to 1.5 times

the highest frequency in the band of interest. This cutoff allows

margin in the design, because typically the cutoff is lower than

designed due to parasitics. The effect of printed circuit board

(PCB) parasitics can be simulated in an electromagnetic (EM)

simulation tool for improved accuracy.

Figure 3. ADF4351 RF Output Filter Schematic

As can be seen from Table 2, at frequencies lower than 1250 MHz,

a fifth-order filter is required. For 1.25 GHz to2.8 GHz, third-order

filtering is sufficient. For frequencies more than 2.8 GHz, filtering

is not required because the harmonic levels are sufficiently low

to meet the sideband suppression specifications.

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

Figure 5. EVM Plot

A sweep of sideband suppression vs. frequency is shown in Figure 4

for the circuit using Filter Type B (800 MHz to 2400 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

•

EVM is a measure of the quality of the performance of a digital

transmitter or receiver and is a measure of the deviation of the

actual constellation points from their ideal locations, due to

both magnitude and phase errors (see Figure 5).

EVM measurements are given in Table 3 comparing the results

with and without the filter. In this case, the baseband I/Q signals

were generated using 3GPP Test Model 4 using a Rohde & Schwarz

AMIQ I/Q modulation generator with differential I and Q analog

outputs. Filter Type B was also used. A block diagram of the test

setup for the EVM is shown in Figure 6. For comparative purposes,

the ADF4350 is also measured. Lower EVM due to in-band PLL

noise improvements on the ADF4351 can be seen in Table 3. Other

contributing factors to the EVM improvement are the lower

phase frequency detector (PFD) spurious levels on the ADF4351.

ADF4351

12

13

ADL5375

3

LOIP

4

LOIN

L1

L2

1nF

L1

L2

1nF

C1a

C1a

C1c

C2a

C2a

C2c

C3a

C3a

C3c

120pF

120pF

0.1µF

3.3V

–70

–65

–60

–55

–50

–45

–40

–35

–30

–25

–20

800

1000

1200

1400

1600

1800

2000

2200

2400

CARRIER FREQUENCY (MHz)

5dBm

FILTER B: 850MHz TO 2450MHz

Q

I

ERROR

VECTOR

MEASURED

SIGNAL

PHASE ERROR

(I/Q ERROR PHASE)

MAGNITUDE ERROR

(I/Q ERROR PHASE)

IDEAL SIGNAL

(REFERENCE)

0