Base stations for third-generation (3G) and fourthgeneration (4G) wireless communications systems are being installed almost as fast as they can be designed and produced. To speed and simplify the process, Linear Technology has developed a pair of compact integrated receiver modules that not only include RF components, but 14-b analog-to-digital converters (ADCs) as well, so that RF and intermediate- frequency (IF) signals can be converted to digital signals within the same receiver module. Model LTM9004 supports direct-conversion receivers from 800 to 2700 MHz while model LTM9005 is for IF-sampling receivers from 400 to 3800 MHz.
The LTM9004 and LTM9005 receiver modules (see figure) are actually cellular receiver subsystems based on system in a package (SiP) technology. Both are part of the company's μModule family of devices, housed in compact 15 x 22 mm LGA packages. They offer different design approaches for a 3G/4G cellular base-station receiver, such as a Long Term Evolution (LTE) or WiMAX system, by supporting either direct-conversion or IF-sampling designs.
Model LTM9004 is the direct-conversion receiver subsystem, with an in-phase (I) and quadrature (Q) demodulator, differential gain stages, DC-coupled lowpass filters, and a dual-channel 14-b ADC. A number of standard versions are available with lowpass filters for each ADC channel, including 1.92, 4.42, 9.42, and 20.0 MHz. Both the RF and local oscillator (LO) inputs are single-ended, 50-O ports. The LO-to-RF leakage is typically 60 dB at 900 MHz and 64 dB at 1900 MHz. The RF-to-LO isolation is typically 59 dB at 900 MHz and 57 dB at 1900 MHz. The receiver module can handle input signals from 0.8 to 2.7 GHz. It is designed for a local oscillator (LO) operating over the same frequency range and capable of -13 to +5 dBm power.
The LTM9004 receiver module boasts extremely well matched I and Q signal paths, with I/G gain mismatch of typically 0.2 dB and I/Q phase mismatch of typically 1.5. The typical signal-to-noiseratio (SNR) performance of the receiver module is 76 dB while the typical spurious- free dynamic range (SFDR) is 63.5 dB. The I/Q demodulator is powered by a +5-VDC supply while the ADC uses a +3-VDC supply. Total power dissipation for the LTM9004 is about 1.83 W.
Model LTM9005 is the IF-sampling receiver module, covering 400 MHz to 3.8 GHz. It incorporates a downconverting mixer, a 140-MHz surface-acoustic-wave (SAW) IF filter, two gain stages, a variable attenuator, and a single-channel low-power 14-b ADC (versions are available at sampling rates to 125 MSamples/s). It provides a bandwidth of about 60 MHz around the 140-MHz IF (custom frequencies are also available). Broadband transformers on the RF and LO ports provide simple, single-ended 50-O interfaces for ease of integration into 3G/4G cellular receivers. The RF and LO inputs are internally matched to 50 O for signals from 1.6 to 2.3 GHz, but can extend to 3.8 GHz with external matching components. The total power consumption of the LTM9005 is typically 1.2 W.
Both receiver modules employ multilayer substrates to shield analog lines from digital signals. The compact μModule housings include supply and reference bypass capacitors, tightly coupled to the die to save space. Linear Technology Corp., 1630 McCarthy Blvd., Milpitas, CA 95035-7417; (408) 432-1900, FAX: (408) 434-0507, www.linear.com.