Datasheet AD8341 (Analog Devices) - 10

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AD8341 Data Sheet THEORY OF OPERATION
The AD8341 is a linear RF vector modulator with Cartesian Pure amplitude modulation is represented by radial movement of baseband controls. In the simplified block diagram given in the gain vector tip at a fixed angle, while pure phase modulation Figure 26, the RF signal propagates from the left to the right is represented by rotation of the tip around the circle at a fixed while baseband controls are placed above and below. The RF radius. Unlike traditional I-Q modulators, the AD8341 is designed input is first split into in-phase (I) and quadrature (Q) components. to have a linear RF signal path from input to output. Traditional The variable attenuators independently scale the I and Q compo- I-Q modulators provide a limited LO carrier path through which nents of the RF input. The attenuator outputs are then summed any amplitude information is removed. and buffered to the output.
VBBI
By control ing the relative amounts of I and Q components that
I CHANNEL INPUT
are summed, continuous magnitude and phase control of the
LINEAR ATTENUATOR
gain is possible. Consider the vector gain representation of the
V-I
AD8341 expressed in polar form in Figure 27. The attenuation
SINGLE-ENDED OR SINGLE-ENDED OR
factors for the I and Q signal components are represented on
DIFFERENTIAL 0
°
/90
°
I-V DIFFERENTIAL 50
Ω
INPUT Z 50
Ω
OUTPUT
the x- and y-axis, respectively, by the baseband inputs, VBBI and
V-I
V
OUTPUT
BBQ. The resultant of their vector sum represents the vector
LINEAR DISABLE
gain, which can also be expressed as a magnitude and phase. By
ATTENUATOR
applying different combinations of baseband inputs, any vector
Q CHANNEL INPUT VBBQ
gain within the unit circle can be programmed. 04700-026 Figure 26. Simplified Architecture of the AD8341 A change in sign of VBBI or VBBQ can be viewed as a change in
V
sign of the gain or as a 180° phase change. The outermost circle
q
represents the maximum gain magnitude of unity. The circle
+0.5 MAX GAIN
origin implies, in theory, a gain of 0. In practice, circuit mismatches and unavoidable signal feedthrough limit the minimum gain to
A
approximately −34.5 dB. The phase angle between the resultant
|A|
gain vector and the positive x-axis is defined as the phase shift. θ
Vi
Note that there is a nominal, systematic insertion phase through
–0.5 +0.5
the AD8341 to which the phase shift is added. In the fol owing discussions, the systematic insertion phase is normalized to 0°. The correspondence between the desired gain and phase setpoints,
MIN GAIN –0.5
GainSP and PhaseSP, and the Cartesian inputs, VBBI and VBBQ, is 04700-027 given by simple trigonometric identities Figure 27. Vector Gain Representation Gain = ( +
RF QUADRATURE GENERATOR
SP [ BBI V /V )2 O ( BBQ V / O V )2] The RF input is directly coupled differentially or single-ended PhaseSP = arctan( BBQ V / BBI V ) to the quadrature generator, which consists of a multistage RC where: polyphase network tuned over the operating frequency range of VO is the baseband scaling constant (500 mV). 1.5 GHz to 2.4 GHz. The recycling nature of the polyphase net- VBBI and VBBQ are the differential I and Q baseband voltages, work generates two replicas of the input signal, which are in respectively. precise quadrature, i.e., 90°, to each other. Because the passive Note that when evaluating the arctangent function, the proper network is perfectly linear, the amplitude and phase information phase quadrant must be selected. For example, if the principal contained in the RF input is transmitted faithful y to both chan- value of the arctangent (known as the Arctangent(x)) is used, nels. The quadrature outputs are then separately buffered to drive quadrants 2 and 3 could be interpreted mistakenly as quadrants the respective attenuators. The characteristic impedance of the 4 and 1, respectively. In general, both V polyphase network is used to set the input impedance of the BBI and VBBQ are needed in concert to modulate the gain and the phase. AD8341. Rev. B | 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 AD8341 RF INPUT AND MATCHING RF OUTPUT AND MATCHING DRIVING THE I-Q BASEBAND CONTROLS INTERFACING TO HIGH SPEED DACs CDMA2000 APPLICATION WCDMA APPLICATION EVALUATION BOARD OUTLINE DIMENSIONS ORDERING GUIDE