A new direct conversion modulator from Linear Technology enables the design of next-generation, less costly 3G wireless base station transmitters. The LT5528 is a direct IQ modulator that enables more compact base station transmitter designs. It is designed for high-performance wireless infrastructure applications, including WCDMA, TD-SCDMA, GSM and PHS. In addition, it is also suitable for broadband fixed wireless access, as well as point-to-point radio links and other high-performance radios.

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It incorporates a matched pair of double-balanced mixers that form a carrier suppressed, image reject IQ (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 GHz to 2.4 GHz) using a local oscillator (LO) 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 IQ gain matching (to within +/- 0.1 dB) results in a typical carrier suppression of -47 dBm at 2.14 GHz. The precision 0°/90° 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 output third-order intercept (OIP3) of 20.5 dBm at 2.14 GHz. Combined with a low noise floor of -159 dBm/Hz, the LT5528 achieves a best-in-class adjacent channel power ratio (ACPR) performance of -66 dBc and an alternate channel power ratio of -68 dBc, measured with a 4-channel WCDMA signal. Single-channel ACPR performance extends higher still to -73.5 dBc. This ensures ample design margin for base station manufacturers to comply with the standards specifications. Operating from a single 5 V supply, the device comes in a 16-pin 4 mm x 4 mm surface-mount QFP package. It is priced at $5.95 in 1000-piece quantities. The product is available from stock.