CN-0144 Datasheet, PDF (3/4 Pages) Analog Devices – Broadband Low Error Vector Magnitude Direct Conversion Transmitter Using LO Divide-by-2 Modulator

Circuit Note
CN-0144
R&S AMIQ
RF_OUTA+
ADF4350
EVALUATION BOARD
RF_OUTA–
IP IN QP QN
LOIP
AD5385 EVALUATION BOARD
ADAPTED TO ACCEPT
RFOUT
DIFFERENTIAL LO INPUTS
LOIN
5V
SPECTRUM
ANALYZER
POWER SUPPLY
Figure 3. Sideband Suppression Measurement Test Setup (Simplified Diagram)
COMMON VARIATIONS
The PLL-to-modulator interface described in this circuit note is
applicable to all I/Q modulators that contain a 2XLO-based
phase splitter. It is also applicable to 2XLO-based I/Q
demodulators such as the ADL5387.
CIRCUIT EVALUATION AND TEST
The CN-0144 uses the EVAL-ADF4350EB1Z and the
ADL5385-EVALZ boards for evaluation of the described circuit,
allowing for quick setup and evaluation. The EVAL-ADF4350EB1Z
board uses the standard ADF4350 programming software,
contained on the CD that accompanies the evaluation board.
Equipment Needed
Windows® XP, Windows Vista (32-bit), or Windows 7 (32-bit) PC
with USB Port, the EVAL-ADF4350EB1Z, and the ADL5385-
EVALZ circuit evaluation boards, the ADF4350 programming
software, power supplies, I-Q signal source, such as a Rhode &
Schwarz AMIQ, and a spectrum analyzer. See the CN-0144 and the
UG-109 user guide for evaluation board EVAL-ADF4350EB1Z
and the ADF4350 and ADL5385 data sheets.
Getting Started
This circuit note contains a description of the circuit, the schematic,
and a block diagram of the test setup. The user guide UG-109
details the installation and use of the EVAL-ADF4350 evaluation
software. The UG-109 also contains board setup instructions
and the board schematic, layout, and bill of materials. The
ADL5385-EVALZ board schematic, block diagram, bill of
materials, layout, and assembly information is included in the
ADL5385 data sheet. See the ADF4350 and ADL5385 data sheet
for device information.
Functional Block Diagram
The CN-0144 contains the function block diagram of the described
test setup in Figure 3.
Setup and Test
After setting up the equipment, use standard RF test methods to
measure the sideband suppression of the circuit.
FURTHER IMPROVEMENTS WITH FILTERING
The sideband suppression of this circuit can be further improved
by filtering the LO signal before the LOIP and LOIN pins of the
ADL5385. Filtering attenuates harmonic levels so as to minimize
errors in the quadrature generation block of the ADL5385. At
some frequencies, this can result in improvements over 10 dB.
However, using a filter will limit the bandwidth of the circuit.
See Figure 4 for narrowband results.
0
–10
–20
–30
–40
–50
–60
–70
NO FILTER
WITH FILTER
–80
700
800
900
1000 1100 1200 1300
RF_OUT (MHz)
Figure 4. Sideband Suppression Comparison With and Without a Harmonic Filter
The LO signal was passed through a low-pass filter with a 3 dB
point at approximately 2600 MHz. This results in a usable
output frequency up to approximately 1300 MHz.
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