Description

The LT5557 active mixer is optimized for high linearity, wide dynamic range downconverter applications. The IC includes a high speed differential LO buffer amplifier driving a double-balanced mixer. Broadband, integrated transformers on the RF and LO inputs provide single-ended 50Ω interfaces. The differential IF output allows convenient interfacing to differential IF filters and amplifiers, or is easily matched to drive a single-ended 50Ω load, with or without an external transformer.

The RF input is internally matched to 50Ω from 1.6GHz to 2.3GHz, and the LO input is internally matched to 50Ω from 1GHz to 5GHz. The frequency range of both ports is easily extended with simple external matching. The IF output is partially matched and usable for IF frequencies up to 600MHz.

The LT5557’s high level of integration minimizes the total solution cost, board space and system-level variation.

 

Features

Wide RF Frequency Range: 400MHz to 3.8GHz*

High Input IP3:  25.6dBm at 900MHz

24.7dBm at 1950MHz

23.7dBm at 2.6GHz

Conversion Gain: 3.3dB at 900MHz

2.9dB at 1950MHz

–3dBm LO Drive Level

Low LO Leakage

Low Noise Figure: 10.6dB at 900MHz

11.7dB at 1950MHz

Low Power: 3.3V/269mW

50Ω Single-Ended RF and LO Ports

Very Few External Components

16-Lead (4mm × 4mm) QFN Package

 

Applications

Cellular, CDMA, WCDMA, TD-SCDMA and UMTS Infrastructure

WiMAX

Wireless Infrastructure Receiver

Wireless Infrastructure PA Linearization

900MHz/2.4GHz/3.5GHz WLAN

 

APPLICATIONS INFORMATION

Introduction

The LT5557 consists of a high linearity double-balanced mixer, RF buffer amplifier, high speed limiting LO buffer amplifier and bias/enable circuits. The RF and LO inputs are both single ended. The IF output is differential. Low side or high side LO injection can be used.

Two evaluation circuits are available. The standard evaluation circuit,incorporates transformerbased IF matching and is intended for applications that require the highest dynamic range and the widest IF bandwidth. The second evaluation circuit, replaces the IF transformer with a discrete IF balun for reduced solution cost and size. The discrete IF balun delivers higher conversion gain, but slightly degraded IIP3 and noise figure, and reduced IF bandwidth.

This series transmission line/shunt capacitor matching topology allows the LT5557 to be used for multiple frequency standards without circuit board layout modifications. The series transmission line can also be replaced with a series chip inductor for a more compact layout.

LO Input Port

The mixer’s LO input, consists of an integrated transformer and high speed limiting differential amplifiers. The amplifiers are designed to precisely drive the mixer for the highest linearity and the lowest noise figure. An internal DC blocking capacitor in series with the transformer’s primary eliminates the need for an external blocking capacitor.

The LO input is internally matched from 1GHz to 5GHz. The input match can be shifted down, as low as 750MHz, with a single shunt capacitor (C4) on Pin 15. One example is plotted in Figure 6 where C4 = 2.7pF produces a 750MHz to 1GHz match.

LO input matching below 750MHz requires the series inductor (L4)/shunt capacitor (C4) network ,where L4 = 2.7nH/C4 = 3.9pF produces a 650MHz to 830MHz match and L4 = 10nH/C4 = 8.2pF produces a 460MHz to 560MHz match. The evaluation boards do not include pads for L4, so the circuit trace needs to be cut near Pin 15 to insert L4. A low cost multilayer chip inductor is adequate for L4.

IF Output Port

The IF outputs, IF+ and IF–, are internally connected to the collectors of the mixer switching transistors. Both pins must be biased at the supply voltage, which can be applied through the center tap of a transformer or through matching inductors. Each IF pin draws 26.6mA of supply current (53.2mA total). For optimum singleended performance, these differential outputs should be combined externally through an IF transformer or a discrete IF balun circuit. The standard evaluation board includes an IF transformer for impedance transformation and differential to single-ended transformation. A second evaluation board realizes the same functionality with a discrete IF balun circuit.