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MAX2680

400MHz to 2.5GHz, Low-Noise, SiGe Downconverter Mixers

Maxim Integrated 

MAX2680/MAX2681/

MAX2682

400MHz to 2.5GHz, Low-Noise,

SiGe Downconverter Mixers

Detailed Description

The MAX2680/MAX2681/MAX2682 are 400MHz to

2.5GHz, silicon-germanium, double-balanced downcon-

verter mixers. They are designed to provide optimum

linearity performance for a specified supply current.

They consist of a double-balanced Gilbert-cell mixer with

single-ended RF, LO, and IF port connections. An on-chip

bias cell provides a low-power shutdown feature. Consult

the Selector Guide for device features and comparison.

Applications Information

Local-Oscillator (LO) Input

The LO input is a single-ended broadband port with a

typical input VSWR of better than 2.0:1 from 400MHz to

2.5GHz. The LO signal is mixed with the RF input sig-

nal, and the resulting downconverted output appears at

IFOUT. AC-couple LO with a capacitor. Drive the LO port

with a signal ranging from -10dBm to 0 (50Ω source).

RF Input

The RF input frequency range is 400MHz to 2.5GHz.

The RF input requires an impedance-matching network

as well as a DC-blocking capacitor that can be part of

the matching network. Consult Tables 1 and 2, as well as

the RF Port Impedance vs. RF Frequency graph in the

Typical Operating Characteristics section for information

on matching.

Table 1. RFIN Port Impedance

PART

MAX2680

MAX2681

MAX2682

400MHz

179-j356

209-j332

206-j306

FREQUENCY

900MHz 1950MHz

54-j179 32-j94

75-j188 34-j108

78-j182 34-j106

2450MHz

33-j73

33-j86

29-j86

IF Output

The IF output frequency range extends from 10MHz to

500MHz. IFOUT is a high-impedance, open-collector output

that requires an external inductor to VCC for proper biasing.

For optimum performance, the IF port requires an imped-

ance-matching network. The configuration and values for

the matching network is dependent upon the frequency and

desired output impedance. For assistance in choosing com-

ponents for optimal performance, see Table 3 and Table 4

as well as the IF Port Impedance vs. IF Frequency graph in

the Typical Operating Characteristics section.

Power-Supply and SHDN Bypassing

Proper attention to voltage supply bypassing is essential

for high-frequency RF circuit stability. Bypass VCC with a

10μF capacitor in parallel with a 1000pF capacitor. Use

separate vias to the ground plane for each of the bypass

capacitors and minimize trace length to reduce induc-

tance. Use separate vias to the ground plane for each

ground pin. Use low-inductance ground connections.

Decouple SHDN with a 1000pF capacitor to ground to

minimize noise on the internal bias cell. Use a series

resistor (typically 100Ω) to reduce coupling of high-fre-

quency signals into the SHDN pin.

Layout Issues

A well-designed PC board is an essential part of an RF

circuit. For best performance, pay attention to power-

supply issues as well as to the layout of the RFIN and

IFOUT impedance-matching network.

Table 2. RF Input Impedance-Matching Component Values

MATCHING

COMPONENTS

Z1

Z2

Z3

400

MHz

86nH

270pF

Open

MAX2680

900

MHz

1950

MHz

270pF 1.5pF

22nH 270pF

Open 1.8nH

2450

MHz

Short

270pF

1.8nH

400

MHz

68nH

270pF

0.5pF

Note: Z1, Z2, and Z3 are found in the Typical Operating Circuit.

FREQUENCY

MAX2681

900

MHz

1950

MHz

270pF 1.5pF

18nH 270pF

Open 1.8nH

2450

MHz

Short

270pF

2.2nH

400

MHz

68nH

270pF

0.5pF

MAX2682

900

MHz

1950

MHz

1.5pF Short

270pF 270pF

10nH 2.2nH

2450

MHz

Short

270pF

1.2nH

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