AD8341

Rev. 0 | Page 10 of 20

THEORY OF OPERATION

The AD8341 is a linear RF vector modulator with Cartesian

baseband controls. In the simplified block diagram given in

Figure 26, the RF signal propagates from the left to the right

while baseband controls are placed above and below. The RF

input is first split into in-phase (I) and quadrature (Q) compo-

nents. The variable attenuators independently scale the I and Q

components of the RF input. The attenuator outputs are then

summed and buffered to the output.

By controlling the relative amounts of I and Q components that

are summed, continuous magnitude and phase control of the

gain is possible. Consider the vector gain representation of the

AD8341 expressed in polar form in Figure 27. The attenuation

factors for the I and Q signal components are represented on

gain, which can also be expressed as a magnitude and phase. By

applying different combinations of baseband inputs, any vector

gain within the unit circle can be programmed.

sign of the gain or as a 180° phase change. The outermost

circle represents the maximum gain magnitude of unity. The

circle origin implies, in theory, a gain of 0. In practice, circuit

mismatches and unavoidable signal feedthrough limit the

minimum gain to approximately −34.5 dB. The phase angle

between the resultant gain vector and the positive x-axis is de-

fined as the phase shift. Note that there is a nominal, systematic

insertion phase through the AD8341 to which the phase shift is

added. In the following discussions, the systematic insertion

phase is normalized to 0°.

The correspondence between the desired gain and phase set-

()

()

2

/

/

O

BBQ

O

BBI

SP

V

V

V

V

Gain

+

=

(

)

BBI

BBQ

SP

V

V

Phase

/

arctan

=

where:

respectively.

Note that when evaluating the arctangent function, the proper

phase quadrant must be selected. For example, if the principal

value of the arctangent (known as the Arctangent(x)) is used,

quadrants 2 and 3 could be interpreted mistakenly as quadrants

in concert to modulate the gain and the phase.

Pure amplitude modulation is represented by radial movement

of the gain vector tip at a fixed angle, while pure phase modula-

tion is represented by rotation of the tip around the circle at a

fixed radius. Unlike traditional I-Q modulators, the AD8341 is

designed to have a linear RF signal path from input to output.

Traditional I-Q modulators provide a limited LO carrier path

through which any amplitude information is removed.

LINEAR

ATTENUATOR

LINEAR

ATTENUATOR

V-I

V-I

0

°/90°

I-V

VBBQ

Q CHANNEL INPUT

SINGLE-ENDED OR

DIFFERENTIAL

50

Ω INPUT Z

VBBI

I CHANNEL INPUT

OUTPUT

DISABLE

SINGLE-ENDED OR

DIFFERENTIAL

50

Ω OUTPUT

Figure 26. Simplified Architecture of the AD8341

|A|

θ

A

+0.5

–0.5

+0.5

–0.5

MIN GAIN

MAX GAIN

Figure 27. Vector Gain Representation

RF QUADRATURE GENERATOR

The RF input is directly coupled differentially or single-ended

to the quadrature generator, which consists of a multistage RC

polyphase network tuned over the operating frequency range of

1.5 GHz to 2.4 GHz. The recycling nature of the polyphase net-

work generates two replicas of the input signal, which are in

precise quadrature, i.e., 90°, to each other. Since the passive

network is perfectly linear, the amplitude and phase information

contained in the RF input is transmitted faithfully to both chan-

nels. The quadrature outputs are then separately buffered to

drive the respective attenuators. The characteristic impedance

of the polyphase network is used to set the input impedance of

the AD8341.