Circuit Note CN-0283 Circuits from the Lab™ reference circuits are engineered and tested for quick and easy system integration to help solve today’s analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0283. Devices Connected/Referenced ADL5375 400 MHz to 6 GHz Broadband Quadrature Modulator ADL5320 400 MHz to 2700 MHz ¼ Watt RF Driver Amplifier Providing Fixed Power Gain at the Output of an IQ Modulator EVALUATION AND DESIGN SUPPORT first stage of gain at the output of an IQ modulator will be described. The devices shown in Figure 1 are the ADL5375 IQ modulator and the ADL5320 driver amplifier. They are well matched from a system performance level; that is, they have equivalent performance so neither device contributes to degradation in the overall performance. Because these devices are well matched in terms of their dynamic ranges, a simple direct connection between the IQ modulator and the RF driver amplifier is recommended without any need for attenuation between the devices. Circuit Evaluation Boards ADL5375 Evaluation Board (ADL5375-05-EVALZ) Design and Integration Files Schematics, Layout Files, Bill of Materials CIRCUIT FUNCTION AND BENEFITS Whether an IQ modulator is used in a direct conversion application or as an upconverter to a first intermediate frequency (IF), some gain is generally applied directly after the IQ modulator. How to choose an appropriate driver amplifier to provide the +5V +5V C5 0.1µF C2 100pF C3 100pF +5V C4 0.1µF C9 10µF IP VPS2 VPS1 24 18 IBBP IN LO LOIP 3 RFOUT QUADRATURE PHASE SPLITTER LOIN 4 QN U2 λ1 16 C1 100pF C7 100pF RFIN 22 1 ADL5320 2 RFOUT U1 AD L537 5 IBBN C6 100pF C10 10nF (2) 21 R7 100Ω C100 (C3) 0.5pF C11 22pF L1 15nH 3 λ2 λ3 λ4 C12 22pF AMP_OUT C101 (C7) 1.5pF DSOP 1 QBBN 9 NOTE: SEE ADL5320 DATA SHEET FOR COMPONENT SPACING (λ) VALUES QBBP 10 2 QP COMM 5 8 11 12 17 19 20 14 23 6 7 13 15 NC 10893-001 R12 100Ω Figure 1. Circuit Schematic for IQ Modulator with Output Power Gain Rev. 0 Circuits from the Lab™ circuits from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2012 Analog Devices, Inc. All rights reserved. CN-0283 Circuit Note CIRCUIT DESCRIPTION The ADL5375 is a general-purpose, high performance IQ modulator. It operates at output frequencies from 400 MHz to 6 GHz. Because of its low noise and wide input baseband bandwidth (3 dB) of 750 MHz, it can be driven by signals with a wide variety of modulations and bandwidths. These input signals can be centered at dc or at a complex IF. The LO interface to the ADL5375 is a 1XLO type, that is, the output frequency and LO frequency is equal (when the baseband signal is centered at dc). Circuit Note CN-0134 describes how the ADL5375 can be driven by the ADF4350. System Level Calculations and RF Amplifier Choice Table 1 shows the output-referred IP3 (OIP3) and P1dB (OP1dB) of the ADL5375 IQ modulator along with the input-referred specifications of the ADL5320 driver amplifier at 1900 MHz. In both cases, there is approximately a 3 dB difference between the output-referred specifications of the IQ modulator and the inputreferred specifications of the amplifier. Table 1. IP3 and P1dB Specifications for the ADL5375 IQ Modulator and the ADL5320 Driver Amplifier at 1900 MHz Parameter IP3 P1dB ADL5375 (Output Referred) 24.2 dBm 10 dBm ADL5320 (Input Referred) 28.3 dBm 13 dBm Figure 2 shows the simulated cascaded performance of the IQ modulator and drive amplifier at 2140 MHz. This simulation was done using the ADIsimRF Design Tool. It is notable that the 12.3 dB difference between the OIP3 of the modulator (24.2 dBm) and the composite OIP3 (36.5 dBm) is just slightly less than the gain of the ADL5320 driver amplifier, 13.7 dB. This indicates that the driver amplifier has only a very slight effect on the overall OIP3. 10893-002 In the 1 GHz to 2 GHz frequency range, the ADL5375 has an output compression point (OP1dB) and a third-order compression point (OIP3) of approximately 10 dBm and 25 dBm, respectively. In choosing an RF amplifier to provide gain after the IQ modulator, it is important to choose a device whose input P1dB and input IP3 are equal or a little bit higher than these numbers. Choosing a device with lower specifications results in degraded performance for the cascade while choosing a device whose input P1dB and input IP3 are significantly higher than those of the ADL5375, has little benefit and is likely to needlessly increase the overall supply current of the signal chain. The ADL5320 is a driver amplifier (RF amplifier that requires external tuning components) that is specified for operation from 400 MHz to 2700 MHz. It consumes 104 mA when operating from a 5 V supply (operation down to 3.3 V is possible with reduced power consumption and performance). Figure 2. ADIsimRF Design Tool Screenshot Showing Cascaded Performance of ADL5375 and ADL5320 Rev. 0 | Page 2 of 6 Circuit Note CN-0283 Figure 3 shows a plot of OIP3 vs. output power (POUT) measured at the IQ modulator output and at the output of the composite circuit. The shape of the two OIP3 profiles are quite similar, just shifted in terms of output power and OIP3. This reinforces the idea that the IP3 is only slightly degraded as the signal passes through the RF amplifier 25 POUT ADL5375 AND ADL5320 20 POUT (dBm) 15 50 10 5 45 0 40 –5 –10 0.10 25 1 Figure 4. Transfer Function of Circuit in Terms of Output Power in dBm and Input Level in V p-p Differential 15 10 0 –10 OIP3 ADL5375 AND ADL5320 OIP3 ADL5375 –5 0 5 10 15 COMPOSITE OUTPUT POWER (dBm) 20 10893-003 5 10 VIN (V p-p DIFFERENTIAL) 20 10893-004 30 Figure 3. OIP3 vs. POUT at 2100 MHz for ADL5375 IQ Modulator and for the Composite Circuit (ADL5375 and ADL5320 Driver Amplifier) If it is assumed that the I and Q inputs of the IQ modulator are terminated with 100 Ω as previously discussed, the output power relative to the dBFS drive level of a typical Analog Devices, Inc., DAC can be plotted (see Figure 5). Therefore, a drive level of 0 dBFS corresponds to 1 V p-p, resulting in the same 13 dBm output power previously discussed. Choosing an Output Power Level 20 I AND Q INPUTS UNTERMINATED I AND Q INPUTS TERMINATED WITH 100Ω While the circuit achieves OIP3 levels in the 35 dBm to 40 dBm range for output power levels up to 15 dBm, operation is not practical up to these levels, particularly with nonconstant envelope modulation schemes that tend to have relatively high peak-toaverage ratios. To understand why, look at the volts-in to power-out transfer function of the circuit and consider the typical drive levels that are available at the input to the IQ modulator. Figure 4 shows the transfer function of the circuit in terms of output power (in dBm) and input voltage (in V p-p) with a CW sine wave, drive signal. An IQ modulator, such as the ADL5375, is driven typically by a dual, current-out, digital-to-analog converter (DAC). Normally, the two current outputs (0 mA to 20 mA nominal) of the DAC are terminated to ground with two 50 Ω resistors and two 100 Ω shunt resistors are placed across each of the IQ inputs (for more information on this interface, see Circuit Note CN-0205). With the DAC running at 0 dBFS, this corresponds to a drive level at the IQ modulator of 1 V p-p or 0.353 V rms (this is neglecting the insertion loss of the low-pass filter that is generally placed between the DAC and the IQ modulator). This results in an output power of approximately 13 dBm. 15 POUT (dBm) 10 5 0 –5 –10 –20 –15 –10 –5 dBFS Level (dB) 0 10893-005 OIP3 (dBm) 35 Figure 5. Transfer Function of Circuit in Terms of Output Power vs. DAC Drive Level with IQ Modulator I and Q Inputs Terminated with 100 Ω and with I and Q Inputs Unterminated Figure 5 also shows the transfer function of the circuit when the I and Q inputs are not terminated with 100 Ω resistors. Because the resulting DAC voltage drive level is doubled (2 V p-p maximum), the resulting output power is higher by 6 dB for the same DAC drive level. While operation of the circuit without I and Q termination resistors is possible, it does pose some problems for the filter that is usually placed between the DAC and IQ modulator. Because this filter is generally terminated at both ends, it is desirable to have some resistance across the I and Q inputs of the IQ modulator (the unterminated input resistance of these inputs is approximately 60 kΩ). A value that is in the 100 Ω to 1000 Ω range can be used to increase the resulting DAC voltage drive level and corresponding output power. However, take care to design Rev. 0 | Page 3 of 6 CN-0283 Circuit Note COMMON VARIATIONS the filter between the DAC and IQ modulator so that it can support different source and load impedances. As already noted, from Figure 4 and Figure 5, it can be seen that a 1 V p-p sine wave (0 dBFS) is provided an output power of approximately 13 dBm (the I and Q inputs terminated with 100 Ω). In practice, the DAC drive level must be reduced slightly from 0 dBFS to reduce distortion (typically 1 dB to 2 dB). In addition to this, the rms drive level should be lower again by an amount equal to the peak-to-average ratio of the modulation of the carrier. The ratio of peak envelope power (PEP) to rms power is typically in a range from 5 dB for QPSK-like modulation schemes (0 dB in the special case where the modulation is constant envelope) to around 10 dB for higher order QAM-based modulation. Referring to Figure 6, this suggests that output power levels in the 0 dBm to 10 dBm range are feasible. –50 –52 –54 –56 –58 –60 –62 –64 –66 –68 –70 –72 –74 –76 –78 –80 –82 –84 –86 –88 –90 –92 –8 –4 –2 0 2 4 6 8 OUTPUT POWER (dBm) A number of narrow-band IQ modulators are available that provide higher performance over their operating frequency ranges. Examples are ADL5370/ADL5371/ADL5372/ADL5373/ADL5374. These narrow-band devices provide higher gain and OIP3 compared to ADL5375. When paired with the ADL5320 and ADL5321 driver amplifiers, the net result is overall higher output power with similar composite OIP3. The ADRF6701/ADRF6702/ADRF6703/ADRF6704 families of narrow-band IQ modulators include an integrated phase-locked loop (PLL) and voltage controlled oscillator (VCO). These devices provide similar performance to the ADL5370/ADL5371/ADL5372/ ADL5373/ADL5374 family; however, with a higher level of integration. A number of options exist to drive the I and Q inputs of the IQ modulator. The AD9125 and AD9122 are 16-bit dual DACs that operate at 1 GSPS or 1.2 GSPS, respectively. These devices can be used to generate either a baseband spectrum (centered at 0 Hz) or a complex IF spectrum typically in the 100 MHz to 200 MHz range. ADJACENT CHANNEL POWER RATIO (dB) ALTERNATE CHANNEL POWER RATIO (dB) –6 A broadband internally matched gain block, such as the ADL5601 or the ADL5602, can also be used to provide gain at the output of the IQ modulator. However, because these devices have lower OIP3 (than ADL5320 and ADL5321), they tend to dominate and reduce the overall IP3 of the circuit. 10 10893-006 ADJACENT AND ALTERNATE CHANNEL POWER RATIO (dB) The adjacent channel power ratio (ACPR) of a single carrier, wideband code division multiple access (WCDMA) signal has become a popular metric for assessing the system level distortion of a circuit (that is, as opposed to an assessment that is solely based on IP3 and IMD levels). Figure 6 shows the measured ACPR of the circuit vs. the output power level. In the case of a WCDMA signal, ACPR is defined as the ratio of the power in the carrier (in a bandwidth of 3.84 MHz) to the power in an adjacent channel (channel spacing = 5 MHz), also measured in a 3.84 MHz bandwidth. The plot also shows an alternate channel power ratio that is the same type of measurement; however, at a carrier offset of 10 MHz. The ADL5320 driver amplifier is specified to operate from 400 MHz to 2.7 GHz. This conveniently covers the lower end of the specified frequency range of the ADL5375 IQ modulator. For operation at frequencies in the 2.3 GHz to 4 GHz range, the ADL5321 driver amplifier is recommended. Both the ADL5320 and ADL5321 must be tuned to the frequency at which they will be operating. The data sheets of both devices contain tables that provide recommended values for tuning components at popular operating frequencies. Figure 6. Plot of OIP3 and WDCMA ACPR vs. Output Power In this case, the signal has a PEP-to-rms ratio of approximately 10 dB (the peak-to-average ratio of a WCDMA signal can vary based on how the carrier is configured and loaded). Based on this plot and the desired level of ACPR, select an output power level in the 0 dBm to 10 dBm range. At power levels less than 0 dBm, the ACPR becomes dominated by the degrading signalto-noise ratio of the circuit. Rev. 0 | Page 4 of 6 Circuit Note CN-0283 CIRCUIT EVALUATION AND TEST Setup and Test The circuit was implemented using the ADL5375 evaluation board (ADL5375-05-EVALZ) that includes the ADL5320 driver amplifier. This board can be configured to provide the IQ modulator output signal, or the composite modulator and amplifier signal. The default configuration for this board is the modulator and amplifier composite output with the amplifier tuned for operation in the 1800 MHz to 2200 MHz range. As already noted, the ADL5320 data sheet provides the values and placement locations for tuning capacitors that support other frequencies. Figure 7 shows the test setup that was used for the IP3 testing and for the power sweep testing. The signals from two RF signal generators running at 25 MHz and 26 MHz are passively combined using a 180° phase splitter/combiner that provides good input-toinput isolation. The 2-tone signal is then applied to a 90° phase splitter that is specified to operate from 25 MHz to 50 MHz. These phase splitter outputs are then applied to two 1:2 transformers to create differential output signals (the 0° output of the phase splitter should go towards the IP and IN inputs of the IQ modulator). The differential signals are applied to four bias tees that bias the signals to 0.5 V. The network is terminated by two 100 Ω resistors (pads for these resistors are provided on the ADL5375 evaluation board). Equipment Needed The following equipment is needed: • • • • • • The ADL5375 evaluation board (ADL5375-05-EVALZ) Two RF signal generators: Agilent 8648C or equivalent operating at 25 MHz and 26 MHz A RF signal generator: Agilent 8648C or equivalent operating at approximately 2 GHz A RF spectrum analyzer: Rohde & Schwarz FSIQ, Rohde & Schwarz FSQ, Agilent PSA, or equivalent A ZFSC-2-2-S+ 180° power splitter/combiner, Mini-Circuits A ZMSCQ-2-50+ 90° power splitter, Mini-Circuits Two ADT2-1T 1:2 baluns, Mini-Circuits Four ZFBT-6GW-FT+ bias tees, Mini-Circuits The local oscillator (LO) for the ADL5375 is provided by a third signal generator, generating 0 dBm. The final output frequency is equal to the difference between the input RF signal frequencies and the LO frequency. Therefore, if the 2-tone signals are at 25 MHz and 26 MHz, and the LO is at 2150 MHz, the output spectrum appears at 2124 MHz and 2125 MHz. The circuit can also be implemented using the AD9122 dual DAC evaluation board (AD9122-M5375-EBZ) that includes the ADL5375 IQ modulator. In this case, connect the output of the ADL5375 IQ modulator to a standalone ADL5320 evaluation board (ADL5320-EVALZ). The advantage of this approach is that the DAC generates appropriately biased differential signals without the need for bias tees, phase splitters, and transformers. RF SPECTRUM ANALYZER +0.5V +5V VPOS ZFBT-6GW-FT+ BIAS TEE RF SIG GEN 1 +8 dBm @ 25MHz ADT2-1T 1:2 BALUN ZFSC-2-2-S+ 180 POWER SPLITTER/COMBINER AMP_OUT IBBN ADL5375-05 EVALUATION BOARD (ADL5375-05-EVALZ) ZMSCQ-2-50 90 POWER SPLITTER ZFBT-6GW-FT+ BIAS TEE RF SIG GEN 2 +8 dBm @ 26MHz RF IN R7 100Ω ZFBT-6GW-FT+ BIAS TEE GND IBBP ADT2-1T 1:2 BALUN QBBP R12 100Ω ZFBT-6GW-FT+ BIAS TEE QBBN LOIP RF SIG GEN 2 0 dBm @ 2150MHz Figure 7. Measurement Setup for IP3 Testing and Power Sweep Rev. 0 | Page 5 of 6 10893-007 • • CN-0283 Circuit Note LEARN MORE Circuit Note CN-0134, Broadband Low Error Vector Magnitude (EVM) Direct Conversion Transmitter, Analog Devices. CN0283 Design Support Package: http://www.analog.com/CN0283-DesignSupport Nash, Eamon, Correcting Imperfections in IQ Modulators to Improve RF Signal Fidelity, Application Note AN-1039, Analog Devices ADIsimRF Design Tool Circuit Note CN-0016, Interfacing the ADL5370 I/Q Modulator to the AD9779A Dual-Channel, 1 GSPS High Speed DAC, Analog Devices. Circuit Note CN-0017, Interfacing the ADL5371 I/Q Modulator to the AD9779A Dual-Channel, 1 GSPS High Speed DAC, Analog Devices. Circuit Note CN-0018, Interfacing the ADL5372 I/Q Modulator to the AD9779A Dual-Channel, 1 GSPS High Speed DAC, Analog Devices. Circuit Note CN-0019, Interfacing the ADL5373 I/Q Modulator to the AD9779A Dual-Channel, 1 GSPS High Speed DAC, Analog Devices. Circuit Note CN-0020, Interfacing the ADL5374 I/Q Modulator to the AD9779A Dual-Channel, 1 GSPS High Speed DAC, Analog Devices. Circuit Note CN-0021, Interfacing the ADL5375 I/Q Modulator to the AD9779A Dual-Channel, 1 GSPS High Speed DAC, Analog Devices. Circuit Note CN-0140, High Performance, Dual Channel IF Sampling Receiver, Analog Devices. Circuit Note CN-0144, Broadband Low Error Vector Magnitude (EVM) Direct Conversion Transmitter Using LO Divide-by-2 Modulator, Analog Devices. Circuit Note CN-0205, Interfacing the ADL5375 I/Q Modulator to the AD9122 Dual Channel, 1.2 GSPS High Speed DAC, Analog Devices. Circuit Note CN-0243, High Dynamic Range RF Transmitter Signal Chain using Single External Frequency Reference for DAC Sample Clock and IQ Modulator LO Generation, Analog Devices. Circuit Note CN-0245, Wideband LO PLL Synthesizer with Simple Interface to Quadrature Demodulators, Analog Devices. Data Sheets and Evaluation Boards ADL5375 Evaluation Board, ADL5375-05-EVALZ ADL5320 Evaluation Board, ADL5320-EVALZ AD9122 Evaluation Board, AD9122-M5375-EBZ ADL5375 Data Sheet ADL5320 Data Sheet REVISION HISTORY 9/12—Revision 0: Initial Version Circuit Note CN-0070, Precise Control of I/Q Modulator Output Power Using the ADL5386 Quadrature Modulator and the AD5621 12-Bit DAC, Analog Devices. (Continued from first page) Circuits from the Lab circuits are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may use the Circuits from the Lab circuits in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the Circuits from the Lab circuits. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab circuits are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices reserves the right to change any Circuits from the Lab circuits at any time without notice but is under no obligation to do so. ©2012 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN10893-0-9/12(0) Rev. 0 | Page 6 of 6