Datasheet AD8340 (Analog Devices) - 10

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AD8340 Data Sheet THEORY OF OPERATION
The AD8340 is a linear RF vector modulator with Cartesian Pure amplitude modulation is represented by radial movement baseband controls. In the simplified block diagram shown in of the gain vector tip at a fixed angle, while pure phase modula- Figure 26, the RF signal propagates from the left to the right tion is represented by rotation of the tip around the circle at a while baseband controls are placed above and below. The RF fixed radius. Unlike traditional I-Q modulators, the AD8340 is input is first split into in-phase (I) and quadrature (Q) compo- designed to have a linear RF signal path from input to output. nents. The variable attenuators independently scale the I and Q Traditional I-Q modulators provide a limited LO carrier path components of the RF input. The attenuator outputs are then through which any amplitude information is removed. summed and buffered to the output.
VBBI
By controlling the relative amounts of I and Q components that
I CHANNEL INPUT
are summed, the AD8340 allows continuous magnitude and
LINEAR ATTENUATOR
phase control of the gain. Consider the vector gain representa-
V-I
tion of the AD8340 expressed in polar form in Figure 27. The
SINGLE-ENDED OR SINGLE-ENDED OR
attenuation factors for the I and Q signal components are
DIFFERENTIAL 0°/90° I-V DIFFERENTIAL 50Ω INPUT Z 50Ω OUTPUT
represented on the x- and y-axis, respectively, by the baseband
V-I
inputs, V
OUTPUT
BBI and VBBQ. The resultant vector sum represents the
LINEAR DISABLE
vector gain, which can also be expressed as a magnitude and
ATTENUATOR
026 phase. By applying different combinations of baseband inputs,
Q CHANNEL INPUT V
any vector gain within the unit circle can be programmed. 04699-
BBQ
Figure 26. Simplified Architecture of the AD8340 A change in sign of VBBI or VBBQ can be viewed as a change
V
in sign of the gain or as a 180° phase change. The outermost
q
circle represents the maximum gain magnitude of unity. The
+0.5 MAX GAIN = 0dB
circle origin implies, in theory, a gain of 0. In practice, circuit mismatches and unavoidable signal feedthrough limit the
A
minimum gain to approximately −40 dB. The phase angle
|A|
between the resultant gain vector and the positive x-axis is
θ Vi
defined as the phase shift. Note that there is a nominal,
–0.5 +0.5
systematic insertion phase through the AD8340 to which the phase shift is added. In the following discussions, the systematic insertion phase is normalized to 0°.
MIN GAIN < –30dB
027
–0.5
The correspondence between the desired gain and phase 04699- setpoints, GainSP and PhaseSP, and the Cartesian inputs, VBBI Figure 27. Vector Gain Representation and VBBQ, is given by simple trigonometric identities.
RF QUADRATURE GENERATOR
Gain = ( + SP VBBI /V )2 O (VBBQ /VO )2 The RF input is directly coupled differentially or single-endedly to the quadrature generator, which consists of a multistage RC Phase = SP arctan( BBQ V / BBI V ) polyphase network tuned over the operating frequency range of where: 700 MHz to 1000 MHz. The recycling nature of the polyphase VO is the baseband scaling constant (500 mV). network generates two replicas of the input signal, which are VBBI and VBBQ are the differential I and Q baseband voltages, in precise quadrature, that is, 90°, to each other. Because the respectively. passive network is perfectly linear, the amplitude and phase Note that when evaluating the arctangent function, the proper information contained in the RF input is transmitted faithfully phase quadrant must be selected. For example, if the principal to both channels. The quadrature outputs are then separately value of the arctangent (known as the arctangent(x)) is used, buffered to drive the respective attenuators. The characteristic Quadrant 2 and Quadrant 3 would be interpreted mistakenly impedance of the polyphase network is used to set the input as Quadrant 4 and Quadrant 1, respectively. In general, both impedance to the AD8340. VBBI and VBBQ are needed in concert to modulate the gain and the phase. Rev. C | Page 10 of 20 Document Outline Features Applications Functional Block Diagram General Description Table of Contents Revision History Specifications Absolute Maximum Ratings ESD Caution Pin Configuration and Function Descriptions Typical Performance Characteristics Theory of Operation RF Quadrature Generator I-Q Attenuators and Baseband Amplifiers Output Amplifier Noise and Distortion Gain and Phase Accuracy RF Frequency Range Applications Information Using the AD8340 RF Input and Matching RF Output and Matching Driving the I-Q Baseband Controls Interfacing to High Speed DACs CDMA2000 Application Evaluation Board Schematic and Artwork Outline Dimensions Ordering Guide