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High Signal Level, Active Down-Converting Mixer

Mon, 03/08/2004 - 11:30am
Linear Technology Corporation announces the LT5522, a new, high linearity, active down-converting mixer with integrated RF transformer. With a frequency range of 600 MHz to 2.7 GHz, the device is well positioned for wireless infrastructure (including base station) applications in the 900 MHz cellular bands, as well as for 1.8 GHz to 2.2 GHz GSM, PHS, CDMA and W-CDMA applications. The mixer's combination of high linearity, low LO leakage and supply current, as well as its matched input characteristics and low external component count makes it a prime product for these applications. The LT5522offers RF designers an innovative tool for reducing system complexity and lowering system cost.

Technical Overview

The LT5522 features an IF output range of 0.1 MHz to 1.0 GHz. The device offers an exceptionally high input third-order intercept (IIP3) among active mixers: +25 dBm at 900 MHz and 21.5 dBm at 1.9 GHz. Its conversion gain is near 0 dB across its RF input frequency range, and its noise figure is 12.5 dB at 900 MHz. It incorporates an on-chip local oscillator buffer that operates with a nominal LO input level of — 5dBm. The LT5522's RF input and LO ports have internal 50W matching, and allow for either differential or single-ended input operation without compromising performance. In the case of the RF input, this performance is enabled by an on-board, integrated RF transformer, which performs impedance transformation and internal single-ended to differential conversion.

The LT5522 offers low port-to-port feedthrough. The LO-to-RF and LO-to-IF leakage is — 49 dBm or less across the operational frequency range. It draws 56 mA of supply current at 5 VDC. The device is available in a small footprint 4 × 4 mm plastic flat package.

Infrastructure Applications

In a typical application, the mixer is used as the first receive mixer in an infrastructure receiver. The output is filtered by an external SAW device. It's differential IF outputs can directly drive a differential input SAW filter. It can also be configured to drive a single-ended filter using either an IF transformer or a narrow-band bridge IF balun using less expensive discrete L's and C's. In any case, IF port output impedance matching is simplified due to the devices low (400W) output impedance.

Other infrastructure applications include frequency translation from PCS to cellular band (1.9 GHz to 900 MHz), and in the feedback path of a power amplifier linearization loop. In this application, the base station PA output is sampled at the transmit antenna and down-converted to baseband using a combination of a down-converting mixer and an ADC. A baseband digital processor measures the distortion in the resulting signal and adjusts the baseband modulation signal to cancel the measured RF distortion. This approach to PA linearization requires very low distortion in the frequency down-conversion feedback path, a specification the device is capable meeting. It is also well suited for cable infrastructure headend transmitter applications, which must operate over an IF frequency range of 50 MHz to 1.0 GHz with very low ± mLO and — nRF distortion products.

The LT5522's combination of performance, matched input characteristics, and low external component count simplifies design and reduces costs for high performance cellular base stations, cable headend equipment, and fixed wireless access radios.

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