Linear Technology has released the LT5557, a downconverting active mixer that covers the 400-MHz to 3.8-GHz bandsAs with all mobile applications, integration will be critical for LTE chips, and chip designers are looking for innovative ways to combine functionality. PicoChip takes a software-defined approach and reports that it has integrated the complete LTE physical layer.
The company recently announced plans for an LTE basestation reference design--the PC8618 picocell and PC8608 femtocell platforms--that are being designed in conjunction with PicoChip's development and research partners, MimoOn and Hong Kong's ASTRI.
James Wong, a product marketing manager for Linear Technology, explains his team's approach to LTE integration, "We design performance to solve some of the most difficult problems facing LTE and basestation designers. For example, RF isolation in RF circuits is difficult to contain, and it's expensive to filter any undesirable RF artifacts. Our mixers (LT5557 and LT5579) have the RF transformers on-chip, facilitating exceptional isolation performance and eliminating external balun transformers."
Selection guide for LTE
For designers that need to select LTE chips, there are some key things to consider during the process. PicoChip's Baines cautions: "It's important to understand that the chips themselves are only a small part of any LTE solution. Just as important is the software to run on them. It is very important to differentiate between a chip you have to program from scratch, a chip with customizable library software, a chip with simple example code that is not actually suitable for a final implementation and a programmable chip with production-ready customizable code."
Brad Bannon, systems applications engineer, high speed converters group at Analog Devices (ADI), suggests that the most desirable specs to look for are high linearity (to meet spectrum quality and out-of-band emissions with the high peak-to-average ratio OFDM signal), low noise (for optimal radio link performance), gain flatness (for limited gain calibration when transmitting OFDMA signals up to maximum channel bandwidth), accurate power measurement (modulation agnostic, for optimum power transmission and high-efficiency design), and low power dissipation/low supply voltage.
To this list, Linear Technology's Wong adds spurious-free dynamic range (this limits the receiver's ability to resolve data integrity at low signal levels) and isolation/LO leakage. This last factor can cause out-of-band emissions exceeding the level required by local regulatory agencies, disqualifying a basestation from use in a given market. Baines points out that it is also important for the LTE device to be able to scale and adjust as the LTE specification changes.
12/01/2008 12:01 AM EST by EE Times by Janine Love
