DACs Deliver Multiple Carriers

Nov. 1, 2002
This family of high-resolution, high-speed DACs provides the performance levels needed for DDS sources and generation of multicarrier signals in next-generation cellular base stations.

Digital-to-analog converters (DACs) continue to advance according to the needs of high-performance modern communications systems. As modern systems employ more elaborate modulation and multiple-carrier formats, the demands on system components such as DACs call for higher resolution, wider dynamic range, and improved efficiency. The new 12- to 16-b DACs in the MAX5886-MAX5888 family from Maxim Integrated Products (Sunnyvale, CA) deliver in all three key areas. They support sample rates to 500 MSamples/s. The 14-b MAX5195 converter provides the highest dynamic range of any production DAC at sample rates to 260 MSamples/s. Supplied in compact surface-mount packages, these converters are suitable for the generation of multiple carriers in Universal Mobile Telecommunications System (UMTS), code-division-multiple-access (CDMA), and Global System for Mobile Communications (GSM) systems as well as instrumentation applications.

UMTS requires up to four carriers per Tx. For GSM/EDGE and cdma2000 applications, four to eight carriers may be desired for a single transmitter. The generation of multiple carriers requires substantially more dynamic range in the signal path. Fortunately, high-speed DACs in the MAX5888 family can support up to four UMTS carriers, including appropriate margins with respect to the UMTS standard. As a further refinement, these DACs allow correction of power-amplifier (PA) nonlinearities by introducing digital predistortion to the signals to be amplified. That requirement alone can increase the required signal bandwidth by a factor of three to five. Thus, the signal bandwidth necessary for four UMTS carriers (up to 100 MHz) calls for higher sample rates and higher analog output frequencies. The MAX5888's 500-MSamples/s update rate is designed for these applications.

Transmit waveforms in GSM/EDGE systems demand even more dynamic performance from the DAC. The generation of multicarrier signals pushes spurious-free-dynamic-range (SFDR), intermodulation-distortion (IMD), and signal-to-noise-ratio (SNR) values to extremes. The company's highest-performance DAC for multicarrier applications is the 14-b, 260 MSamples/s MAX5195. It provides outstanding SFDR, SNR, and IMD performance for these systems, with −77-dBc SFDR for a 19.4-MHz output signal when operating from a single +5-VDC supply. The new MAX5888 nearly matches the dynamic performance of the MAX5195 for multicarrier systems, as well as for direct-digital-synthesis (DDS) applications, while dissipating only 235 mW from a single +3.3-VDC supply.

For a 50-MHz output frequency and 400-MSamples/s sample rate, the MAX5888's SFDR exceeds 67 dB. It also provides an SNR level of −155 dBc/Hz and a two-tone IMD of −72 dBc for output frequencies of 80 MHz. The sampling rate for these performance levels is 500 MSamples/s. Digital data to the new DACs is applied through an low-voltage-differential-signalling (LVDS) interface. An LVDS-based logic family supports 500-MSamples/s data rates effectively, and the digital signals' differential input swings help reduce system-level noise at the digital interface.

The SFDR performance of the new MAX5886-MAX5888 family is exceeded only by that of the MAX5195 (Fig. 1). This DAC also delivers industry-leading SNR performance of −160 dB/Hz. Its two-tone IMD performance of −87 dBc for an output frequency of 32 MHz is also outstanding. The digital interface of the 14-b MAX5195 DAC incorporates differential low-voltage positive-emitter-coupled logic (LVPECL), which—similar to LVDS—reduces the system-level noise associated with high-speed digital data transmission.

The MAX5886-MAX5888 family and the MAX5195 DACs are available in miniature QFN packages: the MAX5886-MAX5888 units are supplied in 68-pin housings, while the MAX5195 is supplied in a 48-pin package. The leadless QFN packages combine small physical size (as small as 7 × 7 mm) with excellent thermal and electrical characteristics.

Consider a multicarrier UMTS application that includes digital predistortion techniques. These applications combine demanding dynamic performance with 100-MHz signal bandwidths. The UMTS mask for spurious emissions requires that spurious products within a 1-MHz measurement bandwidth be no greater than −58 dBc. (Figure 2 shows the spectral output for a single tone at 60 MHz and a sample rate of 300 MSamples/s). Clearly, the MAX5888's margin over a desired 100-MHz bandwidth (more than 8 dB greater than the mask requirements) supports relaxed margins elsewhere in the Tx signal chain.

Another important specification to be met in this application is the adjacent-channel leakage power ratio (ACLR). Figure 3 illustrates a single-carrier UMTS spectral response with the carrier centered at 60 MHz. One can see that the ACLR mask levels for first and second adjacent channels (−45 and −50 dBc, respectively) are met with comfortable margins in excess of 25 dB. Figure 4 illustrates ACLR performance for the MAX5888 in a four-carrier UMTS application—probably the most demanding requirement for any ACLR measurement. The MAX5888, offering the highest performance available for this application, meets the -45-dBc and −50-dBc mask requirements with a margin in excess of 20 dB.

CDMA carrier generation requires similar performance measurements. Figure 5 depicts an eight-tone system where the tones are separated by 1 MHz, at an intermediate frequency (IF) centered at 30 MHz. For the most demanding mask combination for the various bands, the spurious-emission mask level is −59 dBc, at an assumed output-power level of 40 W for the Tx. For this worst-case sinusoidal test-simulation case, the MAX5888 meets CDMA mask requirements with a margin of 19 dB.

Among the current wireless-communications protocols, GSM/EDGE-based architectures impose the greatest dynamic-range requirements. Limitations in DAC performance have made multicarrier Txs impractical in the past, but the MAX5195 lifts that restriction—as shown by its IMD performance for four sinusoid tones with 1-MHz spacing between the tones (Fig. 6).

The IMD mask limit of −70 dBc is easily met with an 8-dB margin by the MAX5195. By using a smaller backoff in the output power level (only −15 dB from full scale), the performance can be improved by 6 dB. The MAX5195's SNR of −160 dBc/Hz makes the device ideal for multicarrier applications.

While the MAX5195 offers unrivaled dynamic-range performance when operating from a single +5-VDC supply, the MAX5886-MAX5888 family of devices provide resolutions from 12 to 14 b with low power consumption from lower-voltage, +3.3-VDC supplies. P&A: stock to 30 days. Maxim Integrated Products, Inc., 120 San Gabriel Dr., Sunnyvale, CA 94086; (408) 737-7600, FAX: (408) 737-7194, Internet: www.maxim-ic.com.

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