CN-0134 Datasheet, PDF (3/5 Pages) Analog Devices – Broadband Low Error Vector Magnitude (EVM) Direct Conversion Transmitter

Circuit Note
CN-0134
Table 2. ADF4350 RF Output Filter Component Values (DNI = Do Not Insert)
Frequency Range
(MHz)
ZBIAS
L1 L2
C1a
C1c
C2a
C2c
C3a
C3c
(nH) (nH) (pF)
(pF)
(pF)
(pF)
(pF)
(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
0Ω 0Ω
DNI
DNI
DNI
DNI
DNI
DNI
The ADF4350 output match consists of the ZBIAS pull-up and, to
a lesser extent, the decoupling capacitors on the supply node. To
get a broadband match it is recommended to use either a
resistive load (ZBIAS = 50 Ω) or a resistive in parallel with a
reactive load for ZBIAS. The latter gives slightly higher output
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.
120pF
3.3V
0.1µF
120pF
RFOUTA+ 13
RFOUTA– 12
ADF4350
Z_BIAS
C1a
L1
C2a
L2
Z_BIAS
C1c
L1
C2c
L2
C1a
C2a
C3a
1nF
9 LOIP
C3c
1nF
10 LOIN
C3a
ADL5375
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
NO FILTERING
FILTER B: 850MHz TO 2450MHz
–10
–20
–30
–40
–50
–60
–70
800
1000 1200 1400 1600 1800 2000 2200 2400
FREQUENCY (MHz)
Figure 4. Sideband Suppression for Filter b, 850 MHz to 2450 MHz
Q
MAGNITUDE ERROR
(I/Q ERROR PHASE)
MEASURED
SIGNAL
ERROR
VECTOR
PHASE ERROR
(I/Q ERROR PHASE)
0
IDEAL SIGNAL
(REFERENCE)
I
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.
Rev. B | Page 3 of 5