Electronic Design

High-Linearity Mixers, Modulators, And Demodulators Improve Basestation Performance

The most critical circuits in any radio design are the mixers and IQ modulators/demodulators as they establish the basic specifications for the entire product. Linear Technology's LT5557 down-converting mixer, LT5571 quadrature modulator, and LT5575 I/Q demodulator step up to tackle these challenges.

The LT5557 targets 3G and WiMAX basestation receivers, power-amplifier (PA) linearization circuits, and wireless local-area network (WLAN) products. It incorporates a double-balanced active mixer core with an on-chip local oscillator (LO) buffer. Differential drive is used internally throughout the mixer to maximize linearity and RF isolation. However, the 50-Ω RF and LO inputs are single-ended for improved compatibility with external circuitry. On-chip RF balun transformers manage the conversion from single-ended to differential (see the figure).

The unusually wide frequency range, 400 MHz to 3.8 GHz, suits the LT5557 for 850- to 965-MHz GSM and other cellular bands, 1.7- to 2.1GHz 3G wireless bands, and 2.5- and 3.5-GHz WiMAX bands in the U.S. and worldwide. Typical third-order intercept (IP3) specifications are 25.6 dBm at 900 MHz, 24.7 dBm at 1950 MHz, and 23.7 dBm at 2.6

GHz. Conversion gain is 3.3 dB at 900 MHz and 2.9 dB at 1950 MHz. Noise figure is 10.6 dB at 900 MHz and 11.7 at 1950 MHz.

The LT5571 suits GSM and cdma2000 basestations in the 850- to 965-MHz range. Other applications include ISM band transceivers and RFID readers. A good choice for zero-IF designs, its two matched high-linearity mixers offer high-impedance differential inputs driven by external digital-to-analog converters (DACs).

The other mixer input comes from the external local oscillator via a 50-Ω single-ended port. That input drives the internal precision 0° and 90° phase shifter, which has an accuracy of better than 0.5°, for an uncalibrated image rejection of –53 dBc. Leakage to the RF output at 900 MHz is an exceptional –42 dBm. Both image rejection and LO leakage can be improved to better than –70 dBc with calibration if needed.

The mixer outputs are combined in an on-chip transformer with a single-ended, 50-Ω output impedance. Typical specs include a 620to 1100-MHz bandwidth, a conversion gain of –4.2 dB, a third-order intercept (OIP3) of 21.7 dBm at 900 MHz, and a low noise floor at 20 MHz with offset of –159 dBm/Hz with no RF and 4-dBm power out of –153 dBm/Hz. The adjacent channel power rejection (ACPR) for three-channel cdma2000 at 900 MHz is –70.4 dBc.

The LT5575 fits into the receive side of designs. This design option for 3G and WiMAX basestations and RFID readers includes two matched high-linearity mixers driven in quadrature by a low-noise, buffered local oscillator with an on-chip precision 0° and 90° phase shifter. The 50-Ω inputs are single-ended thanks to on-chip transformers.

With a frequency range of 800 MHz to 2.7 GHz, the LT5575 can be used in 850-MHz GSM/EDGE bands, 1.9- to 2.1-GHz 3G bands, and cdma2000, WCDMA, UMTS, and TD-SCDMA basestations. It makes a good IF demod for most microwave radios or satellite receivers. Key specifications include IIP3 in the 23.3(2500 MHz) to 28-dBm (900 MHz) range. Noise figure is 12.7 dB at 1900 MHz. The I&Q amplitude mismatch is only 0.04 dB, and the phase mismatch is a low 0.6°.

The LT5557 operates from 3.3 V and draws 81.6 mA. The 16-lead surface-mount quad flat no-lead (QFN) package measures 4 by 4 mm. It costs $5.95. The LT5571 operates from 5 V and draws 97 mA. Housed in a 16-pin 4- by 4-mm QFN, it costs $4.95. Powered by 5 V, the LT5575 draws 132 mA. It comes in a 16-lead 4- by 4-mm QFN and costs $7.75. All prices are for 1000-unit lots.

Linear Technology Corp.
www.linear.com

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