AD8340 Data Sheet
THEORY OF OPERATION
The AD8340 is a linear RF vector modulator with Cartesian
baseband controls. In the simplified block diagram shown 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, the AD8340 allows continuous magnitude and
phase control of the gain. Consider the vector gain representa-
tion of the AD8340 expressed in polar form in Figure 27. The
attenuation factors for the I and Q signal components are
represented on the x- and y-axis, respectively, by the baseband
inputs, V
BBI
and V
BBQ
. The resultant vector sum represents the
vector 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.
A change in sign of V
BBI
or V
BBQ
can be viewed as a change
in 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 −40 dB. The phase angle
between the resultant gain vector and the positive x-axis is
defined as the phase shift. Note that there is a nominal,
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°.
The correspondence between the desired gain and phase
setpoints, Gain
SP
and Phase
SP
, and the Cartesian inputs, V
BBI
and V
BBQ
, is given by simple trigonometric identities.
( )
( )
2
2
/
/
O
BBQO
BBI
SP
VVV
VGain +
=
( )
BBI
BBQSP
VVPhase /arctan=
where:
V
O
is the baseband scaling constant (500 mV).
V
BBI
and V
BBQ
are the differential I and Q baseband voltages,
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,
Quadrant 2 and Quadrant 3 would be interpreted mistakenly
as Quadrant 4 and Quadrant 1, respectively. In general, both
V
BBI
and V
BBQ
are needed 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 AD8340 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.
04699-026
LINEAR
ATTENUATOR
LINEAR
ATTENUATOR
V-I
V-I
0°/90°
I-V
V
BBQ
Q CHANNEL INPUT
SINGLE-ENDED OR
DIFFERENTIAL
50Ω INPUT Z
V
BBI
I CHANNEL INPUT
OUTPUT
DISABLE
SINGLE-ENDED OR
DIFFERENTIAL
50Ω OUTPUT
Figure 26. Simplified Architecture of the AD8340
04699-027
|A|
θ
A
+0.5–0.5
+0.5
–0.5
V
i
V
q
MIN GAIN < –30dB
MAX GAIN = 0dB
Figure 27. Vector Gain Representation
RF QUADRATURE GENERATOR
The RF input is directly coupled differentially or single-endedly
to the quadrature generator, which consists of a multistage RC
polyphase network tuned over the operating frequency range of
700 MHz to 1000 MHz. The recycling nature of the polyphase
network generates two replicas of the input signal, which are
in precise quadrature, that is, 90°, to each other. Because the
passive network is perfectly linear, the amplitude and phase
information contained in the RF input is transmitted faithfully
to both channels. 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 to the AD8340.
Rev. C | Page 10 of 20
