Broadband, Low Error Vector Magnitude (EVM) Direct Conversion Transmitter Using LO Divide-by-2 Modulator
To achieve optimum performance, the only requirement is that the LO inputs of the modulator be driven differentially. The ADF4351 provides differential RF outputs and is, therefore, an excellent match. This PLL-to-modulator interface is applicable to all I/Q modulators and I/Q demodulators that contain a 2XLO-based phase splitter. Low noise LDOs ensure that the power management scheme has no adverse impact on phase noise and error vector magnitude (EVM). This combination of components represents an industry-leading direct conversion transmitter performance over a frequency range of 30 MHz to 2.2 GHz. For frequencies above 2.2 GHz, it is recommended to use a divide-by-1 modulator, as described in CN-0285.
The ADF4351 is powered off the ultralow noise 3.3 V ADP150 regulator for optimal LO phase noise performance. The ADL5385 is powered off a 5 V ADP3334 LDO. The ADP150 LDO has an output voltage noise of only 9 μV rms, integrated from 10 Hz to 100 kHz, and helps to optimize VCO phase noise and reduce the impact of VCO pushing (equivalent to power supply rejection). See CN-0147 for more details on powering the ADF4351 with the ADP150 LDO.
The ADL5385 uses a divide-by-2 block to generate the quadrature LO signals. The quadrature accuracy is, thus, dependent on the duty cycle accuracy of the incoming LO signal (as well as the matching of the internal divider flip-flops). Any imbalance in the rise and fall times causes even-order harmonics to appear, as evident on the ADF4351 RF outputs. When driving the modulator LO inputs differentially, even-order cancellation of harmonics is achieved, improving the overall quadrature generation. (See “Wideband A/D Converter Front-End Design Considerations: When to Use a Double Transformer Configuration.” Rob Reeder and Ramya Ramachandran. Analog Dialogue, 40-07.)
Because sideband suppression performance is dependent on the modulator quadrature accuracy, better sideband suppression is achievable when driving the LO input ports differentially vs. single-ended. The ADF4351 has differential RF outputs compared to the single-ended output available on most of the competitor’s PLL devices with integrated VCOs.
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. Use an inductor value of 19 nH or greater for LO operation below 1 GHz. The measured results in this circuit were performed using ZBIAS = 50 Ω and an output power setting of 5 dBm. When using the 50 Ω resistor, this setting gives approximately 0 dBm on each output across the full band, or 3 dBm differentially. The ADL5385 LO input drive level specification is −10 dBm to +5 dBm; therefore, it is possible to reduce the ADF4351 output power to save current.
A sweep of sideband suppression vs. RF output frequency is shown in Figure 2. In this sweep, the test conditions were as follows:
- Baseband I/Q amplitude = 1.4 V p-p differential sine waves in quadrature with a 500 mV dc bias
- Baseband I/Q frequency (fBB) = 1 MHz
- LO = 2 × RFOUT
A simplified diagram of the test setup is shown in Figure 3. A modified ADL5385 evaluation board was used because the standard ADL5385 board does not allow a differential LO input drive.
This circuit achieves comparable or improved sideband suppression performance when compared to driving the ADL5385 with a low noise RF signal generator, as used in the data sheet measurement. Using the differential RF outputs of the ADF4351 provides even-order harmonic cancellation and improves modulator quadrature accuracy. This affects sideband suppression performance and EVM. A single carrier W-CDMA composite EVM of better than 2% was measured with the circuit shown in Figure 1. The solution thus provides a low EVM broad-band solution for frequencies from 30 MHz to 2.2 GHz. For frequencies above 2.2 GHz, use a divide-by-1 modulator block, as described in CN-0285.
The complete design support package can be found at http://www.analog.com/CN0311-DesignSupport.
The following equipment is needed:
- A PC with a USB port that contains Windows® XP, Vista, or Windows 7
- The EVAL-ADF4351EB1Z evaluation board
- The ADL5385-EVALZ evaluation board,
- ADF4351 programming software
- Power supplies (5 V, 500 mA)
- An I-Q signal source, such as a Rohde & Schwarz AMIQ
- A spectrum analyzer
Also, see the UG-435 User Guide for the EVAL-ADF4351EB1Z evaluation board, the ADF4351 data sheet, and the ADL5385 data sheet.
A description of the circuit, the schematic, and a block diagram of the test setup is detailed within the CN-0311 (see Figure 1 and Figure 3). The UG-435 user guide details the installation and use of the EVAL-ADF4351EB1Z evaluation software. The UG-435 also contains the 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 ADF4351 data sheet and ADL5385 data sheet for device information.
Functional Block Diagram
The functional block diagram of the described test setup is shown in Figure 3.
Setup and Test
After setting up the equipment, use standard RF test methods to measure the sideband suppression of the circuit.
|ADL5385||30 MHz TO 2200 MHz Quadrature Modulator||
|ADP3334||High Accuracy Low IQ, 500 mA anyCAP® Adjustable Low Dropout Regulator||
|ADP150||Ultralow Noise, 150 mA CMOS Linear Regulator||
|ADF4351||Wideband Synthesizer with Integrated VCO||