More basestation manufacturers are moving to software-defined radios (SDRs), which permit multiple bands and standards to be handled with fewer transceivers. SDR receivers use digital signal processing to sort out the signals and perform the demodulation and other signal processing. The signal chain can be reduced dramatically if the analog-to-digital converter (ADC) before the DSP is fast enough.
Recognizing the SDR revolution, Texas Instruments has expanded its line of ADCs to make it simpler to build these radios. The premier device in the family, the ADS5500 14-bit, 125-Msample/s ADC, arrived in the fourth quarter of 2003.
Two other 14-bit converters, the 105-Msample/s ADS5541 and the 80-Msample/s ADS5542, boast a 71-dB signal-to-noise ratio (SNR) and consume about half the power of competitive devices at 710 mW and 670 mW, respectively. Other new family members include a trio of 12-bit devices—the 125-Msample/s ADS5520, the 105-Msample/s ADS6621, and the 80-Msample/s ADS5522—with SNR ranges from 69 to 70 dB.
Each device is designed for multistandard radios, including GSM, CDMA, UMTS 3G, and TD-SCDMA. The ADCs can greatly reduce the number of signal chains in a basestation. (Figure 1.) With high sampling rates, they can digitize a wide range of signals that can be distinguished from one another at the baseband processing level in the DSP. Reducing the number of signal chains halves the power requirement, enabling designers to double the number of channels per board or shrink the board size.
All of the ADS5500 series are sampling now, with volume production in the third quarter. The devices come in a 64-pin TQFP PowerPad package. Evaluation modules are also available. Prices range from $95 for the ADS5500 to $21 for the ADS5522 in 1000-unit volumes.
Texas Instruments Inc.