LTC6948 datasheet(18/32 Pages) AD

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LTC5594

Rev A

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APPLICATIONS INFORMATION

The LTC5594 is an IQ demodulator designed for high

dynamic range receiver applications. It consists of an RF

balun, I/Q mixers, quadrature LO amplifiers, IF amplifiers,

and correction circuitry for DC offset, image rejection,

and nonlinearity.

Operation

As shown in the Block Diagram for the LTC5594, the RF

input signal is converted to a differential signal by the

on-chip balun transformer before going to the I and Q

channel mixers.

The LO inputs are impedance matched using a program-

mablenetwork,andthenaccuratelyshiftedinphaseby90°

by an internal precision phase shifter. This phase shifter

maintains the accurate quadrature relation over the full LO

input range from 300MHz to 9GHz. In addition, the phase

shifter allows fine tuning of the phase difference between

the I- and Q-channel LO with a resolution of around 0.05

degrees to compensate for any phase mismatch between

themixersandphasemismatchintroducedintotheIFpath

by any filter component mismatch.

The differential mixer IF output signals are filtered off-chip

to remove the fRF + fLO signal and other high frequency

mixing products before being applied to the on-chip IF

amplifiers. The IF amplifiers have adjustable gain and

common mode output voltage to allow for direct interfac-

ing with A/D converters. The gain balance between both

IF output channels of the LTC5594 can be fine tuned with

a resolution of about 0.016dB in order to compensate

for gain mismatches in the IF signal path, either caused

internally by the device or by external amplifiers and fil-

ters. The DC offset in both IF channels can be adjusted in

order to minimize the accumulated DC offset at the A/D

converter input.

The IF gain, gain error and phase error adjust, DC offset

adjust, and nonlinearity adjust registers are digitally con-

trolled through a 4-wire serial interface. The register map

is detailed in the Appendix.

RF Input Port

Figure 4 shows a simplified schematic of the demodula-

tor’s RF input connected to an on-chip balun transformer.

External DC voltage should not be applied to the RF input

pin. DC current flowing into the pin may cause damage

to the chip. Series DC blocking capacitors should be

used to couple the RF input pin to the RF signal source.

As shown in Figure 5, the RF input port is well matched

with return loss greater than 10dB over the frequency

range of 500MHz to 9GHz with a 0.2pF capacitor on C2.

A 0.1pF capacitor on C3 placed 3mm away from C1 on

the 50Ω input transmission line may improve return loss

at higher frequencies. The RF pin can also be externally

matched over the 300MHz to 500MHz frequency range

by changing C2 to 6.8pF. Table 1 shows the impedance

and input reflection coefficient for the RF input with C2 =

0.2pF. The input transmission line length is de-embedded

from the measurement.

Figure 4. Simplified Schematic of the RF Input with External Matching Components

1000pF

LTC5594

Z0 = 50

INPUT

(MATCHED)

GND

VCC

L = 3mm

TL1

5594 F04

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