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LTM9003

9003p

DESCRIPTION

The LTM9003 is an integrated system in a package (SiP)

that includes a high-speed 12-bit A/D converter, a wideband

ﬁlter and an active mixer. The LTM9003 is designed for IF

sampling, digital pre-distortion (DPD) applications, also

known as transmit observation path receivers, with RF input

frequenciesupto3.8GHz.Typicalapplicationsincludemulti-

carrier base stations and telecom test instrumentation.

Digital pre-distortion is a technique often used in third-

generation (3G) wireless base stations to improve the

linearity of power ampliﬁers (PA). Improved PA linearity

allows for a lower power PA to be used and therefore save

a signiﬁcant amount of power in the base station. The DPD

receiver captures the PA output, digitizes it and feeds it back

where the distortion can be analyzed. A complementary

distortion is then introduced to the transmit DAC thereby

pre-distorting the signal.

A signiﬁcant factor in PA linearity is the distortion caused

by the odd order intermodulation (IM) products. The band-

width to be digitized is equivalent to the signal bandwidth

multiplied by the order of the IM product to be canceled.

For example, four carrier WCDMA consumes 20MHz of

signal bandwidth; therefore, to capture the ﬁfth order IM

product requires 100MHz. The Nyquist theory requires that

the ADC sample rate be at least twice that frequency.

However, simply doubling the captured bandwidth to set

the sample rate may not be the best choice. Selecting the

exact ADC sample rate and intermediate frequency (IF)

depends on other factors within the system. To simplify

ﬁltering, the sample rate is often set at a multiple of the

chip rate. The chip rate for WCDMA is 3.84MHz; select-

ing an ADC sample rate of 64 times the chip rate gives

245.76Msps. Placing the IF at 3/4ths the sample rate (fS)

gives 184.32MHz and allows the entire bandwidth to fall

within the second Nyquist zone. Many other frequency

plans may be acceptable.

The following sections describe in further detail the opera-

tion of each functional element of the LTM9003. The SiP

technology allows the LTM9003 to be customized and

this is described in the Semi-Custom Options section.

The outline of the remaining sections follows the basic

functional elements as shown in Figure 2.

Figure 2. Basic Functional Elements

ADC

MIXER

FILTER

AMPLIFIER

9003 F02

The mixer dominates the noise ﬁgure calculation as would

be expected. The overall gain is optimized for the dynamic

range of the ADC relative to the RF input level allowed by

the mixer. The equivalent cascaded noise ﬁgure is 9.1dB

(LTM9003-AA) and 9.9dB (LTM9003-AB). The bandpass

ﬁlter is a second order L-C ﬁlter following the mixer and

a lowpass ﬁlter following the ampliﬁer provides anti-alias

and noise limiting.

SEMI-CUSTOM OPTIONS

The μModule construction affords a new level of ﬂexibility

in application-speciﬁc standard products. Standard ADC

and ampliﬁer components can be integrated regardless

of their process technology and matched with passive

components to a particular application. The LTM9003-AA,

as the ﬁrst example, is conﬁgured with a 12-bit ADC

sampling at rates up to 250Msps. The total system gain

is approximately 10.8dB. The IF is ﬁxed by the bandpass

ﬁlter at 184MHz with 125MHz bandwidth. The RF range

is matched for 1.1GHz to 1.8GHz with low side LO.

However, other options are possible through Linear

Technology’s semi-custom development program. Linear

Technology has in place a program to deliver other speed,

resolution, IF range, gain and ﬁlter conﬁgurations for nearly

any speciﬁed application. These semi-custom designs are

based on existing ADCs and ampliﬁers with an appropriately

modiﬁed matching network. The ﬁnal subsystem is then

tested to the exact parameters deﬁned for the application.

The ﬁnal result is a fully integrated, accurately tested and

optimized solution in the same package. For more details

on the semi-custom receiver subsystem program, contact

Linear Technology.

Down-Converting Mixer

The mixer stage consists of a high linearity double-bal-

anced mixer, RF buffer ampliﬁer, high speed limiting LO

buffer ampliﬁer and bias/enable circuits. The RF and LO

OPERATION

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