DC1437B-AA Linear Technology, DC1437B-AA Datasheet - Page 13

• Manufacturer Part Number: DC1437B-AA
• Description: BOARD EVAL LTM9003-AA
• Manufacturer: Linear Technology
• Type: Pre-Distortion Receiver Subsystemr

Specifications of DC1437B-AA

• Design Resources: DC1437 Schematic
• Frequency: 184MHz
• Features: LTM9003 12bit Predistortion Receiver Subsystem
• Tool / Board Applications: Wireless Connectivity-ZigBee, RF, Infrared, USB
• Mcu Supported Families: LTM9003
• Development Tool Type: Hardware - Eval/Demo Board
• For Use With/related Products: LTM9003
• Lead Free Status / RoHS Status: Not applicable / Not applicable

operaTion
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
frequencies up to 3.8GHz. Typical applications include multi-
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 (f
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.
S
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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
MIXER
Figure 2. Basic Functional Elements
FILTER
AMPLIFIER
IF
FILTER
LTM9003
ADC
9003 F02

9003f