LT5575

11

5575f

APPLICATIONS INFORMATION

The LT5575 is a direct I/Q demodulator targeting high

linearity receiver applications, such as RFID readers and

wireless infrastructure. It consists of RF transconductance

amplifiers, I/Q mixers, a quadrature LO phase shifter, and

bias circuitry.

The RF signal is applied to the inputs of the RF

transconductance amplifiers and is then demodulated

into I/Q baseband signals using quadrature LO signals

which are internally generated from an external LO source

by precision 90° phase-shifters. The demodulated I/Q

signals are single-pole low-pass filtered on-chip with a

–3dB bandwidth of 490MHz. The differential outputs of the

I-channel and Q-channel are well matched in amplitude;

their phases are 90° apart.

Broadband transformers are integrated on-chip at both

the RF and LO inputs to enable single-ended RF and LO

interfaces. In the high frequency band (1.5GHz to 2.7GHz),

both RF and LO ports are internally matched to 50Ω. No

external matching components are needed. For the lower

frequency bands (800MHz to 1.5GHz), a simple network

with series and/or shunt capacitors can be used as the

impedance matching network.

RF Input Port

Figure 4 shows the demodulator’s RF input which con-

sists of an integrated transformer and high linearity

transconductance amplifiers. The primary side of the

transformer is connected to the RF input pin. The second-

ary side of the transformer is connected to the differential

inputs of the transconductance amplifiers. Under no cir-

cumstances should an external DC voltage be applied to

the RF input pin. DC current flowing into the primary side

of the transformer may cause damage to the integrated

transformer. A series blocking capacitor should be used

to AC-couple the RF input port to the RF signal source.

The RF input port is internally matched over a wide fre-

quency range from 1.5GHz to 2.7GHz with input return loss

typically better than 10dB. No external matching network is

needed for this frequency range. When the part is operated

at lower frequencies, however, the input return loss can

be improved with the matching network shown in Figure

4. Shunt capacitor C10 and series capacitor C11 can be

selected for optimum input impedance matching at the

desired frequency as illustrated in Figure 5. For lower fre-

quency band operation, the external matching component

C11 can serve as a series DC blocking capacitor.

Figure 5. RF Input Return Loss with External Matching

Figure 4. RF Input Interface

5575 F04

2

3

EXTERNAL

MATCHING

NETWORK FOR

LOW BAND AND

MID BAND

RF

INPUT

RF

C10

C11

TO I-MIXER

TO Q-MIXER

–30

–25

–20

–15

–10

–5

0

0.5

1.0

1.5

2.0

2.5

3.0

FREQUENCY (GHz)

5575 F05

NO EXTERNAL

MATCHING

C11 = 3.9pF;

NO SHUNT CAP

C11 = 5.6pF;

C10 = 4.7pF