In addition, power supply routing can be challenging. The digital logic includes input/output (I/O) and core logic power supplies, while the RF output network can include as many as four or five additional power supplies. The power domains must be isolated from one another and signal return paths need to be carefully managed to ensure no crosstalk between supply domains. Keeping the power supplies isolated from one another is crucial to low noise performance.

The main DAC clock is one of the most critical signals on the system card. The DAC clock is a differential signal and is isolated from other signals with via fences. In addition, return paths are controlled to ensure no coupling or crosstalk. Any signals coupling onto the clock will directly appear at the output of the DAC. Digital signals corrupting the clock reduce the noise margin in the system. Even the DAC outputs must be prevented from coupling onto the clock, as this will cause second harmonics and potentially higher-order harmonics to appear in the output spectrum.

It is preferable to keep the clock driver as close as possible to the DAC to reduce noise and other coupling concerns. The DAC outputs are connected to their load by transmission lines. The impedance of these transmission lines is carefully controlled to the load to ensure predictable behavior of the DAC output signals. The RF DAC’s output impedance is related to the package as well as the die, so the laminate’s effects must be included in analysis and simulation of the output stage. Matched impedance between the DAC and the load is critical to maximize the power transfer from the DAC to the destination, and also to minimize reflection from the destination back to the DAC. Proper transmission line design improves the signal-to-noise ratio (SNR), which is necessary for a good multiband communication system.

Today, typical multiband communication systems include multiple RF chains that consist of IF DACs, quadrature modulators, bandpass filters, RF power amplifiers and a final filter stage before the antenna. This architecture requires significant board space to fit multiple frequency bands into a single transmitter. This large number of components draws significant amounts of power and generates a fair amount of heat that requires removal via a heat sink or a fan, which adds complexity and cost to the overall system design.

Since RF DACs have enough bandwidth to synthesize multiple RF bands, they can be used to create a single transmitter with a multiple-band output. For example, a triple-band transmitter that may require three pairs of IF DACs, three modulators, and three bandpass filters may be replaced by a single RF DAC and output filter that generate all three bands. As power amplifier designs migrate to wider bandwidths, even greater savings of circuit-board space can be realized as the number of components in distinct RF chains is reduced to those needed only after the power amplifier. Thus, a multiple-band transmitter could be implemented with an RF DAC, an output filter between the DAC and power amplifier, a power amplifier, and output filters between the power amplifier and the antenna.

High-Speed DACs Fuel Multiband Transmitters, Fig. 3(a-c)

High-Speed DACs Fuel Multiband Transmitters, Fig. 3(d-e)

Figure 3 shows the output of the AD9129 RF DAC at a sample rate of 2764.8 MSamples/s using a selectable mode in the DAC that enables use of the second Nyquist zone. It is a 14-b DAC readily capable of operating at sampling rates to 2.8 GSamples/s and beyond. Eight 5-MHz-wide wideband-code-division-multiple-access (WCDMA) channels were synthesized at three different bands. Two channels were created from 1825 to 1835 MHz, two other channels at 1845 to 1855 MHz, and four channels at 2130 to 2150 MHz. The signals were generated in a field-programmable gate array (FPGA) and then directly synthesized by the RF DAC.

High-Speed DACs Fuel Multiband Transmitters, Fig. 4

Figure 4 shows the output of the AD9129 at a sample rate of 2764.8 MSamples/s using a mode that enables synthesis in the first Nyquist zone. Four 5-MHz-wide WCDMA channels with four Long-Term-Evolution (LTE) downstream channels were synthesized at two different bands. Four WCDMA channels at 871 to 891 MHz, and four LTE downstream channels were created at 729 to 749 MHz.

Modern wireless communication networks demand for flexible, easy-to-upgrade multiband, multistandard base stations. The direct-to-RF transmitter architecture provides a cost/performance effective solution for the multi-band, multi-standard radio transmitter design. The advancement of the RF DAC technology, such as the AD9129 from Analog Devices, has helped lower the threshold of a multi-band and multi-standard radio design and has shown a promising trend of having more designs using the direct-to-RF architecture in the future.

Yi Zhang, HCG Application Engineer

Assaf Toledano, HCG Product Engineer

Analog Devices, Inc., One Technology Way, Norwood, MA 02062-9106; (800) 262-5643, (781) 329-4700.

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