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ADF4351 Scheda tecnica(PDF) 2 Page - Analog Devices |
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ADF4351 Scheda tecnica(HTML) 2 Page - Analog Devices |
2 / 4 page CN-0311 Circuit Note Rev. 0 | Page 2 of 4 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. CIRCUIT DESCRIPTION The circuit shown in Figure 1 uses the ADF4351, a fully integrated fractional-N PLL IC, and the ADL5385 wideband transmit modulator. The ADF4351 provides the local oscillator (the LO is twice the modulator RF output frequency) signal for the ADL5385 transmit quadrature modulator, which upconverts the analog I/Q signals to RF. Taken together, the two devices provide a wideband, baseband I/Q-to-RF transmit solution. 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. Figure 2. Sideband Suppression, RFOUT Swept from 30 MHz to 2200 MHz 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. 0 –70 –60 –50 –40 –30 –20 –10 0 500 1000 1500 2000 FREQUENCY (MHz) DATA SHEET SPECIFICATION |
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