LT5503(RevA) データシートの表示（PDF） - Linear Technology

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LT5503
(Rev.:RevA)

1.2GHz to 2.7GHz Direct IQ Modulator and Mixer

Linear Technology 

LT5503

Applications Information

The LT5503 consists of a direct quadrature modulator and

a mixer. The mixer operates over the range of 1.7GHz to

2.7GHz, and the modulator operates with an output range

of 1.2GHz to 2.7GHz. The LT5503 is designed specifically

for high accuracy digital modulation with supply voltages

as low as 1.8V. It is suitable for IEEE 802.11b wireless

local area network (WLAN), MMDS and wireless local loop

(WLL) transmitters.

A dual-conversion RF system requires two local oscillators

to convert signals between the baseband and RF domains

(see Figure 2). The LT5503’s double-balanced mixer can be

used to generate the LT5503 modulator’s high frequency

carrier input (MODRFIN) by mixing the systems 1st and

2nd local oscillators (LO1 and LO2). In this case, a band-

pass filter is required to select the desired mixer output

for the modulator input. The mixer’s RF differential output

produces –­ 12dBm typically at 2.45GHz and the modula-

tor MODIN pin requires ≥ –16dBm, driven single-ended.

This allows approximately 4dB margin for bandpass filter

loss. The balanced output from the modulator is applied

to a variable gain amplifier (VGA) that provides a single-

ended output. Note that the modulator can also be used

independently of the mixer, freeing the mixer to be used

anywhere in the system. In this case, MODRFIN will be

driven from an external frequency source.

Modulator Baseband

The baseband I and Q inputs (BI+/BI– and BQ+/BQ–) are

internally biased to 1.4V to maximize the input signal

range at low supply voltage. This bias voltage is stable

over temperature, and increases by approximately 50mV

at the maximum supply voltage. The modulator I and

Q inputs have very wide bandwidth (120MHz typical),

making the LT5503 suitable for even the most wideband

modulation applications. For best carrier suppression and

lowest distortion, differential input drive should be used.

Single-ended drive is possible too, with the unused inputs

AC-coupled to ground.

LT5500

LNA

LT5503

VGA

90°

0°

LT5502

LT5506

90°

÷2

0°

I

A/D

Q

A/D

1ST LO

2ND LO

÷2

÷1

I

D/A

Q

D/A

5503 F02

Figure 2. Example System Block Diagram for a Dual Conversion System

For more information www.linear.com/LT5503

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