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Modulator enables cheaper 3G wireless base station transmitters

Posted: 29 Nov 2004 ?? ?Print Version ?Bookmark and Share

Keywords:conversion modulator? linear technology? 3g? wireless basestation? basestation transmitter?

The new direct conversion modulator from Linear Technology Corp. enables the design of next generation, less costly 3G wireless base station transmitters.

The press release said that the LT5528 is a direct i-q modulator with uncompromising performance, enabling more compact base station transmitter designs at lower system cost. The new product is targeted for high performance wireless infrastructure applications, including W-CDMA, TD-SCDMA, GSM and PHS. It is also suitable for broadband fixed wireless access, as well as point-to-point radio links and other high performance radios.

The LT5528 incorporates a matched pair of double-balanced mixers which form a carrier suppressed, image reject I-Q (In phase and Quadrature phase) modulator. The I and Q inputs accept complex modulation signals from the baseband circuitry, and upconvert them directly to RF (1.5GHz to 2.4GHz) using an LO (Local Oscillator) operating at the same frequency. According to Linear Tech, this effectively eliminates one to two stages of frequency upconversion in the radio path, greatly reducing the number of external components and lowering costs.

The device's I/Q gain matching (to within 0.1dB) results in a typical carrier suppression of -47dBm at 2.14GHz. The precision 0/90 phase shift of the on-board LO phase splitter produces 45dBc image rejection at 2.14GHz. The modulated RF I and Q signals are combined in an on-chip balanced RF transformer at the single-ended output. In addition, the LT5528's 50-ohm RF port eliminates the need for external impedance matching components, and all other signal inputs and outputs are also 50-ohm matched to simplify the design task.

The company shared that careful attention is paid to the design and biasing of the mixer to optimize modulator linearity, resulting in an OIP3 (Output Third-Order Intercept) of 20.5dBm at 2.14GHz. Combined with a noise floor of -159dBm/Hz, the LT5528 achieves a best-in-class ACPR (Adjacent Channel Power Ratio) performance of -66dBc and an ACPR of -68dBc, measured with a 4-channel W-CDMA signal. Meanwhile, single channel ACPR performance is -73.5dBc. This ensures ample design margin for base station manufacturers to comply with the standards specifications, Linear Tech added.

The LT5528 operates from a single 5V supply, and comes in a 16-pin 4-by-4mm surface mount QFP package. Available from stock, the product is priced at $5.95 in 1,000-piece quantities.

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