LT5516

9

5516f

The LT5516 is a direct I/Q demodulator targeting high

linearity receiver applications, including wireless infra-

structure. It consists of an RF amplifier, I/Q mixers, a

quadrature LO carrier generator and bias circuitry.

The RF signal is applied to the inputs of the RF amplifier

and is then demodulated into I/Q baseband signals using

quadrature LO signals. The quadrature LO signals are

internally generated by precision 90

° phase shifters. The

demodulated I/Q signals are lowpass filtered internally

with a –3dB bandwidth of 265MHz. The differential out-

puts of the I-channel and Q-channel are well matched in

amplitude; their phases are 90

° apart.

RF Input Port

Differential drive is highly recommended for the RF inputs

to minimize the LO feedthrough to the RF port and to

maximize gain. (See Figure 2.) A 1:4 transformer is used

on the demonstration board for wider bandwidth match-

ing. To assure good NF and maximize the demodulator

gain, a low loss transformer is employed. Shunt inductor

L1, with high resonance frequency, is required for proper

impedance matching. Single-ended to differential conver-

sion can also be implemented using narrow band, discrete

L-C circuits to produce the required balanced waveforms

the RF inputs is listed in Table 1.

Table 1. RF Input Differential Impedance

FREQUENCY

DIFFERENTIAL INPUT

DIFFERENTIAL S11

(MHz)

IMPEDANCE (

Ω)

MAG

ANGLE (˚)

800

258.7-j195.2

0.779

–16.9

900

239.9-j181.8

0.766

–18.3

1000

224.1-j170.0

0.753

–19.6

1100

210.9-j160.0

0.740

–20.9

1200

200.7-j152.1

0.729

–21.9

1300

191.4-j144.7

0.718

–23.0

1400

183.2-j138.3

0.707

–24.0

1500

176.5-j133.1

0.698

–24.9

2.44V. These two pins should be DC blocked when con-

nected to ground or other matching components. The RF

input equivalent circuit is shown in Figure 5.

APPLICATIO S I FOR ATIO

the optimum DC current for I/Q mixer linearity. The IIP3 can

be improved with a smaller R1 at a price of slightly higher

R1 are shown in the Typical Performance Characteristics.

LO Input Port

The LO inputs (Pins 10,11) should be driven differentially

to minimize LO feedthrough to the RF port. This can be

accomplished by means of a single-ended to differential

conversion as shown in Figure 2. L4, the 27nH shunt

inductor, serves to tune out the capacitive component of

the LO differential input. The resonance frequency of the

inductor should be greater than the operating frequency.

A 1:4 transformer is used on the demo board to match the

200

Ω on-chip resistance to a 50Ω source. Figure 6 shows

the LO input equivalent circuit and the associated match-

ing network.

Single-ended to differential conversion at the LO inputs

can also be implemented using a discrete L-C circuit to

produce a balanced waveform without a transformer.

An alternative solution is a simple single-ended termina-

tion. However, the LO feedthrough to RF may be degraded.

Ω source

with a matching circuit, while the other input is connected

to ground through a 100pF bypass capacitor.

Table 2 shows the differential input impedance of the LO

input port.

Table 2. LO Input Differential Impedance

FREQUENCY

DIFFERENTIAL INPUT

DIFFERENTIAL S11

(MHz)

IMPEDANCE (

Ω)

MAG

ANGLE (˚)

800

134.7-j65.1

0.552

–22.5

900

128.5-j66.7

0.517

–25.4

1000

121.8-j67.5

0.512

–28.5

1100

115.7-j67.2

0.505

–31.8

1200

109.3-j66.1

0.498

–35.0

1300

103.0-j64.4

0.490

–38.3

1400

96.7-j62.1

0.480

–42.0

1500

91.0-j59.4

0.469

–45.8