The critical link in the signal chain of any basestation or other modern digital radio is the analog front end (AFE). The AFE includes all of the analog-to-digital, digital-to-analog, and related functions. Conversions must be fast and accurate, and they must consume minimal power. This is a tough set of specs to meet, but Texas Instruments has introduced a fast, high-performance AFE for femtocell basestations and portable software-defined radio (SDR) applications that easily comply.

The low-power, 12-bit AFE7225 integrates dual 125-Msample/s analog-to-digital converters (ADCs) and dual 250-Msample/s digital-to-analog converters (DACs). Other benefits over equivalent competing products include an improved signal-to-noise ratio (SNR) by 2 dB and up to five times the DAC output current. The lower-speed, 12-bit AFE7222 is available for lower-bandwidth, power-sensitive applications. It integrates a dual 65-Msample/s ADC and dual 130-Msample/s DAC and uses only 398 mW in full-duplex or 212 mW in half-duplex receive mode at full speed.

The AFE7225 and AFE7222 both integrate more digital signal processing capabilities than similar AFEs, which increases design flexibility. They also feature independent transmit and receive, 32-bit numerically controlled oscillators (NCOs), serialized low-voltage differential signaling (LVDS) I/O options, and independent transmit/receive quadrature modulation correction for gain, phase, and offset imbalances.

Additionally, the chips consume only 12 mW of power in deep-sleep mode with a wakeup of only 13 µs, or as little as 120 mW in light-sleep mode with 5-µs wakeup time. They also offer high performance for clearer signals. The dual ADC achieves a 70-dB signal-to-noise ratio (SNR), while the dual DAC achieves an adjacent channel leakage ratio of 75 dB with a 3G W-CDMA cellular signal.

The signal processing reduces FPGA gate count requirements, interface speed, and cost. Both chips feature independent transmit and receive digital block up-conversion and down-conversion with 2x to 4x interpolation, 2x decimation, coarse and 32-bit NCO frequency mixers; digital correction of gain, phase, and offset for quadrature modulation receive and/or transmit; and a peak/rms power meter.

Furthermore, the integration reduces system cost. The dual 12-bit auxiliary DAC and dual-input 12-bit auxiliary ADC eliminate the need for a separate monitoring and control device, reducing board space, power, and cost. And, the chips provide flexibility for designers as the digital DAC inputs and ADC outputs can be configured for serial LVDS to save pin/trace-count and aid long-trace signaling or for multiplexed 12-bit CMOS for a low-power interface, supporting full-duplex over two buses or half-duplex from one or two digital buses.

The AFE7225 and AFE7222 can be used in heterodyne or direct-conversion radio architectures. For heterodyne radios, a single DAC channel in the AFEs can be used to create an intermediate frequency (IF) up to approximately 100 MHz for transmit, and a single ADC channel can be used to capture IFs of more than 300 MHz, with the unused channels shut down to conserve power. For femtocell basestations, the AFEs provide macrocell performance when combined with high-quality RF components, such as the TRF372017 IQ modulator with local oscillator and the TRF371125 IQ modulator.

The AFE7222EVM and AFE7225EVM evaluation modules are available now for $499. They include the TRF370333 IQ modulator and CDCE72010 clock jitter cleaner, as well as TI’s power-management devices like dc-dc converters and low-noise low-dropout regulators (LDOs).

The AFE7225 is available in a 9- by 9-mm quad flat no-lead (QFN) package for a suggested retail price of $29.50 for 1000 units. The AFE7222 is available in a 9- by 9-mm QFN package for a suggested retail price of $19.50 in 1000-unit quantities.

Texas Instruments Inc.
www.ti.com