Direct conversion modulator for 3G basestation
November 22, 2004
A new direct conversion modulator from Linear Technology enables the design of next-generation 3G wireless basestation transmitters. The LT5528 is a direct I-Q modulator that enables compact basestation transmitter designs at lower system cost. The LT5528 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 (1.5 to 2.4 GHz) using a LO (local oscillator) operating at the same frequency. 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 LT5528's I/Q gain matching (to within +/- 0.1 dB) results in a typical carrier suppression of -47 dBm at 2.14 GHz. The precision 0 degree/90 degree; phase shift of the on-board LO phase splitter produces 45 dBc image rejection at 2.14 GHz. The modulated RF I and Q signals are combined in an on-chip balanced RF transformer at the single-ended output. The LT5528's 50-Ohm RF port eliminates the need for external impedance matching components. All other signal inputs and outputs are also 50-Ohm matched, thus simplifying the design task.
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 20.5 dBm at 2.14 GHz. Combined with a low noise floor of -159 dBm/Hz, the LT5528 achieves ACPR (adjacent channel power ratio) performance of -66 dBc and an alternate channel power ratio of -68dBc, measured with a 4-channel W-CDMA signal. Single channel ACPR performance extends higher still to -73.5 dBc. This ensures ample design margin for basestation manufacturers to comply with the standards specifications.
The LT5528 operates from a single 5 V supply. The device comes in a 16-pin 4 mm x 4 mm surface mount QFP package.
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