Circuit Note

CN-0144

Rev. C | Page 3 of 4

R&S AMIQ

POWER SUPPLY

AD5385 EVALUATION BOARD

ADAPTED TO ACCEPT

DIFFERENTIAL LO INPUTS

ADF4350

EVALUATION BOARD

SPECTRUM

ANALYZER

IP

LOIP

LOIN

RFOUT

IN

QP

QN

5V

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.

–80

–70

–60

–50

–40

–30

–20

–10

0

700

800

900

1000

1100

1200

1300

NO FILTER

WITH FILTER

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.