[Product Innovation]
High-Speed Wideband DACs Permit Multicarrier Cellular Basestations
Suppliers continue boosting the resolution of high-speed DACs to 16 bits and beyond with the ability to handle faster data rates.
The drive toward multicarrier and multimode basestations and direct-IF transceiver designs is motivating data-converter suppliers to keep propelling the performance envelope of digital-to-analog converters (DACs) to greater heights. These applications demand better resolution, faster update rates, and higher output frequency, in addition to superior ac, noise, and dynamic performance, over a much wider output bandwidth.
Because the power budget is limited as usual, the improved performance must arrive without penalizing the device's power consumption. Furthermore, as these devices are being driven closer to the antenna, designers seek system-level functionality on-chip to cut the component count of the system. While the need is for greater integration on the same piece of silicon, all of the bells and whistles must be packed on a smaller semiconductor die size.
Attaining these objectives was never easy or trivial, but it seems to only get more complex with time. Nevertheless, undaunted by the emerging challenges, developers continue to advance their product lines with clever tweaking methods, better layout, and augmented fabrication processes. Novel architectures and mixed-signal design techniques are being merged with finer CMOS and biCMOS process technologies to meet the end goals.
A major player setting the pace in this arena is Analog Devices Inc. (ADI). This manufacturer continues to refine its architecture and CMOS processes to push the performance bar of its oversampling 14-bit transmit DAC family to new levels. Since the launch of a leading TxDAC family member last summer, ADI has made substantial progress in this line of wideband DACs to handle the broadband and multicarrier needs of multistandard and multimode cellular infrastructure applications (see "Wideband DAC Fosters Multicarrier, Multimode Transmission," Electronic Design, June 14, 1999, p. 37).
By revamping the architecture and tweaking the CMOS process, the supplier has boosted the speed, resolution, and noise performance of oversampling segmented-current-source transmit DACs to achieve 16-bit and better resolution with the ability to handle faster input and output data rates. Additionally, it has successfully set the tone for multichannel DACs required for digital quadrature modulation techniques deployed in these emerging wireless systems. ADI has demonstrated the ability to pack two well-matched high-resolution wideband DACs on a single CMOS chip.
A recent result of this ongoing progress at ADI is the AD9777. Flaunting 16 bits of resolution and 160-Msample/s conversion speed, this newest member of the TxDAC family features selectable 2X/4X/8X interpolating filters, along with dual 16-bit DACs on the same piece of silicon. Because the DAC is designed to handle I and Q data of a quadrature-modulated incoming signal, the converter offers two independent half-band filter channels. Each channel provides up to 8X interpolation (Fig. 1).
In addition to higher integration, the converter's noise and other ac characteristics have undergone refinements. For instance, the noise floor has been lowered, and in-band and out-of-band distortion has been suppressed over a very wide bandwidth. The DAC's inter-modulation distortion (IMD) has been lowered by 3 to 5 dB, and its phase noise further cut by 12 dB in comparison to previous introductions. According to Stephen LaJeunesse, marketing engineer for ADI's high-speed data converters, some of the implemented improvements include reconfiguring the switch cells of the segmented-current-source DAC, bettering capacitive coupling, and resizing the transistors for faster switching characteristics.
Also, LaJeunesse says that the steep transition band and good stop-band rejection of the three user-selectable filters simplifies reconstruction-filter requirements for the output signal. Unlike single-carrier transmission, these on-chip filters enable broadband multicarrier architectures to suppress inband noise and distortion while simplifying the reconstruction analog filter that follows the DAC.
Because the AD9777 is crafted for quadrature modulation, it provides up to 8X interpolation half-band FIR filters for each I and Q data path. Aside from the flexibility afforded by the digital interpolating filters, this converter also incorporates a digital quadrature modulator that lends itself to rejecting image frequencies. "In conjunction with an external analog quadrature modulator, a complex modulation scheme is realized that suppresses the lower image, while the main band is increased in power by 3 dB," LaJeunesse explains. An internal test conducted by ADI engineers indicates that a sideband suppression of 30 to 40 dB can be obtained with this scheme (Fig. 2).
"However," LaJeunesse adds, "local-oscillator leakage then becomes a problem. But it can be filtered out using a single inexpensive SAW filter stage. With a sideband suppression of about 40 dB, cascaded SAW filtering is unnecessary." In this application, the modulation frequency can be programmed via the SPI port for rates of fS/2, fS/4, or fS/8, where fS is the DAC update rate.
Additionally, the interpolation filters enable higher oversampling using the same internal clock that, in turn, further enhances the signal-to-noise ratio (SNR) of the converter. To handle the entire cellular band of the multicarrier basestations, the converter offers a spurious-free dynamic range (SFDR) of 75 dB over a 2- to 35-MHz band. Plus, it achieves 73 dB of adjacent-channel power ratio (ACPR) at an IF of 16.25 MHz. Typical differential nonlinearity for the device is ±1 LSB at 16-bit accuracy.
While the AD9777 is a dual-channel DAC configured to accept digital quadrature-modulation (I and Q) data and generate a quadrature-modulated IF signal along with its orthogonal representatives, it also can be operated in a direct IF mode. The device provides IF transmission frequencies of 70 MHz and higher in this mode and enables designers to cut a mixer stage, thereby lowering the cost for cellular communications systems.
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
Electronic Design UPDATE provides readers with late-breaking news, opinions from industry experts, and timely technology stories. It's a unique opportunity to get your product message in front of engineers, engineering managers, and corporate managers while they're reading about critical information online.
Reprints
