Detailed Description

The MAX2685 consists of five major components: a

low-noise amplifier (LNA), an LNA bypass switch, a

downconverter mixer, a local-oscillator (LO) buffer, and

a power-management block.

Low-Noise Amplifier (LNA)

The LNA is a wideband, single-ended cascode amplifi-

er that operates over a wide range of frequencies. The

input of the LNA (LNAIN) requires an appropriate

matching network and a DC-blocking capacitor. The

typical operating circuit shown in Figure 1 is optimized

for frequencies around 880MHz, requiring only a 0.1µF

capacitor in series with a 12nH inductor. See Table 1

for the LNA “S” parameters for matching to other fre-

quencies.

The output of the LNA (LNAOUT) is internally biased to

Ω and incorporates

an internal DC-blocking capacitor.

LNA Bypass Switch

and Gain Control

When a large input signal is present, enable the LNA

bypass function to increase linearity and reduce supply

current. Set GAIN low to enable the LNA bypass func-

tion.

Receive Mixer

The downconverter mixer is a wideband, single-bal-

anced design with a low noise figure and high linearity.

The RF signal at the MIXIN port is mixed with the signal

at the LO port, and is downconverted to an IF frequen-

cy at the differential IF port.

RF Input

The MIXIN input requires a simple external matching

network and a series DC-blocking capacitor. See

Figure 1 for a matching network example, optimized for

880MHz operation. Table 2 lists mixer “S” parameters

for matching to other frequencies.

Low-Cost, 900MHz, Low-Noise Amplifier

and Downconverter Mixer

_______________________________________________________________________________________

7

MAG

FREQUENCY

(MHz)

800

0.761

S11

PHASE

(degrees)

-64.5

MAG

S21

PHASE

(degrees)

177.9

4.98

MAG

S12

PHASE

(degrees)

-163.7

MAG

S22

PHASE

(degrees)

-107.3

0.376

0.018

-68.6

167.2

5.06

-167.1

840

0.753

-107.0

0.264

0.022

-73.2

156.6

5.07

-171.3

880

0.747

-94.6

0.172

0.026

-78.0

146.6

4.91

-175.7

920

0.733

-62.9

0.149

0.030

-82.8

137.7

4.68

178.0

960

0.719

-42.4

0.200

0.035

-87.5

130.3

4.40

171.0

1000

0.693

-38.8

0.263

0.039

-45.6

73.0

0.188

71.9

800

0.625

-91.3

0.483

0.191

-48.1

65.5

0.195

64.2

840

0.621

-91.3

0.423

0.198

-50.9

58.1

0.199

56.7

880

0.619

-89.9

0.370

0.201

-53.3

51.6

0.200

50.3

920

0.611

-86.1

0.337

0.202

-55.5

46.1

0.200

44.7

960

0.608

-80.9

0.322

0.201

-57.5

41.2

0.200

40.0

1000

0.607

-76.7

0.317

0.200

High-Gain Mode (GAIN = VCC)

Low-Gain Mode (GAIN = GND)