LTM9004_15 Datasheet, PDF (14/28 Pages) Linear Technology – 14-Bit Direct Conversion Receiver Subsystem

LTM9004
Operation
DESCRIPTION
The LTM9004 is a direct conversion receiver targeting
high linearity receiver applications, such as wireless
infrastructure with RF input frequencies up to 2.7GHz.
It is an integrated μModule receiver utilizing system in a
package (SiP) technology to combine a dual, high speed
14-bit A/D converter, lowpass filters, two low noise dif-
ferential amplifiers per channel with fixed gain, and an I/Q
demodulator with DC offset adjustment.
The direct conversion receiver architecture offers several
advantages over the traditional superheterodyne. It eases
the requirements for RF front-end bandpass filtering, as it
is not susceptible to signals at the image frequency. The
RF bandpass filters need only attenuate strong out-of-band
signals to prevent them from overloading the front end.
Also, direct conversion eliminates the need for IF ampli-
fiers and bandpass filters. Instead, the RF input signal is
directly converted to baseband.
Direct conversion does, however, come with its own set
of implementation issues. Since the receive LO signal is at
the same frequency as the RF signal, it can easily radiate
from the receive antenna and violate regulatory standards.
Unwanted baseband signals can also be generated by 2nd
order nonlinearity of the receiver. A tone at any frequency
entering the receiver will give rise to a DC offset in the
baseband circuits. The 2nd order nonlinearity of the receiver
also allows a modulated signal, even the desired signal,
to generate a pseudo-random block of energy centered
about DC.
For this reason, the LTM9004 provides for DC offset cor-
rection immediately following the I/Q demodulator stage.
Once generated, straightforward elimination of DC offset
becomes very problematic. Necessary gain in the baseband
amplifiers increases the offset because their frequency
response extends to DC.
The following sections describe in further detail the opera-
tion of each section. The μModule technology allows the
LTM9004 to be customized and this is described in the
first section. The outline of the remaining sections follows
the basic functional elements as shown in Figure 2.
V_CC1
V_CC2
V_CC3
V_DD
OV_DD
RF
1^ST
2ND
AMP
AMP
LPF
ADC
MIXER
OGND
LO OFFSET ADJ
GND
9004 F02
ADC
CLK
Figure 2. Basic Functional Elements (Only Half Shown)
SEMI-CUSTOM OPTIONS
The μModule construction affords a new level of flexibility
in application-specific standard products. Standard ADC,
amplifier and RF components can be integrated regardless
of their process technology and matched with passive
components to a particular application. The LTM9004-AA,
as the first example, is configured with a dual 14-bit ADC
sampling at rates up to 125Msps. The amplifiers provide a
total voltage gain of 14dB (including the gain of the mixer).
The lowpass filter limits the bandwidth to 1.92MHz. The
RF and LO inputs of the I/Q demodulator have integrated
transformers and present 50Ω single-ended inputs. An
external DAC can be used for DC offset cancellation.
However, other options are possible through Linear
Technology’s semi-custom development program. Linear
Technology has in place a program to deliver other sample
rate, resolution, gain and filter configurations for nearly
any specified application. These semi-custom designs
are based on existing components with an appropriately
modified passive 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.
MIXER OPERATION
The RF signal is applied to the inputs of the RF trans-
conductance amplifiers and is then demodulated into I/Q
baseband signals using quadrature LO signals which are
internally generated from an external LO source by preci-
sion 90° phase shifters.
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For more information www.linear.com/LTM9004