LT5503

13

APPLICATIO S I FOR ATIO

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 oscilla-

tors to convert signals between the baseband and RF

domains (see Figure 3). 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 bandpass 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 modulator MODIN pin requires

≥ –16dBm, driven

single-ended. This allows approximately 4dB margin for

Figure 3. Example System Block Diagram for a Dual Conversion System

5503 F03

90

°

0

°

0

°

90

°

÷2

÷1

÷2

1ST LO

2ND LO

D/A

D/A

A/D

A/D

LT5502

LT5503

LNA

I

I

Q

Q

VGA

bandpass filter loss. The balanced output from the modu-

lator 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

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.