CN-0285 Datasheet, PDF (3/5 Pages) Analog Devices – Broadband Low Error Vector Magnitude (EVM) Direct Conversion Transmitter

Circuit Note
The ADF4351 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 (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.
3.3V
120pF
0.1µF
120pF
RF_OUTA+ 12
RF_OUTA– 13
Z_BIAS
C1a
L1
C2a
L2
Z_BIAS
C1c
L1
C2c
L2
C3a
1nF
3 LOIP
C3c
1nF
4 LOIN
ADF4351
C1a
C2a
C3a
ADL5375
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.
–20
–25
5dBm
FILTER B: 850MHz TO 2450MHz
–30
–35
–40
–45
–50
–55
–60
–65
–70
800
1000 1200 1400 1600 1800 2000 2200 2400
CARRIER FREQUENCY (MHz)
Figure 4. Sideband Suppression for Filter Type B, 850 MHz to 2450 MHz
Q
MAGNITUDE ERROR
(I/Q ERROR PHASE)
CN-0285
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 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
• Baseband I/Q frequency (fBB) = 1 MHz.
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.
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