Application Note

AN-1039

Rev. 0 | Page 3 of 8

After low-pass filtering, the two word streams are applied to a

pair of digital-to-analog converters (DAC). The DAC outputs

drive two low-pass filters whose primary role is to remove

Nyquist images. The outputs of these filters then drive the

baseband inputs of the IQ modulator. The local oscillator (LO)

input of the modulator is driven by a relatively pure CW signal

generated by a phase-locked loop (PLL) such as the ADF4106

from Analog Devices, Inc. Now, take a closer look at the

operation of the IQ modulator.

The LO signal is split into two signals, equal in amplitude but

with a phase difference of exactly 90°. These two quadrature

signals drive the inputs of the two mixers that, for the purposes

of this application note, are viewed as analog multipliers. The

outputs of these two multipliers are added together (in the

Σ block of the IQ modulator) to provide the IQ modulator’s

output.

While it is apparent that the baseband data streams have

been filtered, instead briefly consider them as the original bit

streams. Instead of a stream of 1s and 0s, think of them as two

streams switching between a value of +1 and –1. So, the output

of the I multiplier consists of a vector which is flipping in-phase

between 0° and 180°as the bit stream alternates. Likewise, the

output of the Q multiplier is a vector that flips between +90°

and –90° as the bit stream modulates the original 90° vector.

Thus, if at a particular instant, both the I and Q bit streams are

equal to +1, the result at the output of the IQ modulator is the

sum of the 90° and 0° vectors, that is, a +45° vector. Likewise,

I and Q bit combinations of −1/+1, −1/−1, and +1/−1 produce

vectors (commonly called symbols) all of equal amplitude at

+135°, −135°, and −45°, respectively. If these vectors were

plotted, observe the constellation of the modulated carrier

(see Figure 2A).

(A)

(B)

(C)

(D)

(E)

(F)

Figure 2. Error Vector Magnitude Constellations that Result from Various

Modulator Imperfections

MODULATOR IMPERFECTIONS

Contrary to the previous hypothetical situation, in a real IQ

modulator, things do not look so perfect. A series of effects in

the IQ modulator conspire to create QPSK (or QAM) vectors

that are neither equal in amplitude nor separated by exactly 45°.

Consider first what happens if for some reason the gain of the I

path is greater than that of the Q channel; this could be caused

by a DAC gain mismatch, low-pass filter insertion loss, mismatch,

or gain imbalance inside the IQ modulator. Regardless of where

this gain imbalance comes from, its effect is the same. Because

the 0°/180° vectors at the output of the I multiplier are larger

than the +90°/−90° vectors from the Q multiplier, the shape

of the constellation becomes rectangular (see Figure 2B). This

degrades signal integrity at the receiver because the receiver is

expecting a perfectly square constellation. In the QPSK example

shown in Figure 2B, a slight gain imbalance is unlikely to result

in an incorrect bit decision in the receiver unless the received

signal is very small. However, in higher order modulation

schemes such as 16 QAM or 64 QAM (see Figure 2E and

Figure 2F), the increased density of the constellation points

could easily combine with an IQ gain imbalance to produce

an incorrect symbol decision in the receiver.

In most IQ modulators, the 90° phase split of the LO is achieved

using either a polyphase filter or a divide-by-two flip-flop circuit

(which requires an external LO that is twice the desired output

frequency). In either circuit, the 90° phase split or quadrature is

never perfect. For example, if there is a 1° quadrature error, the

shape of the resulting constellation is slightly trapezoidal (see

Figure 2C). Just like IQ gain imbalance, this can result in

incorrect bit decisions in the receiver.

Now consider what happens if either the I or Q paths have

unwanted dc offset errors. This results in the +1/−1 multipli-

cation being skewed. For example, an offset that is equal to 1%

of the baseband signal amplitude causes the +1/−1 multipliers

to be modified to +1.01/−0.99. This has the effect of shifting

the center of the constellation off the origin, on either the I or

Q axis, most likely in both (see Figure 2

D). In the frequency

domain, this manifests itself as a small portion of the unmodu-

lated carrier appearing at the output of the modulator. In the

frequency domain, this LO leakage (also referred to as LO

feedthrough) appears at the center of the modulated spectrum.

Because of parasitic capacitances within the silicon die and

bond-wire to bond-wire coupling, the signal that is applied

to the LO port of the IQ modulator may also couple directly

to the RF output. This leakage is independent of the offset

multiplication effect that was described previously. However,

its manifestation, that is, the presence of the unmodulated

carrier in the output spectrum, is exactly the same. Thus, the

net LO leakage seen at the output of the IQ modulator is the

vector sum of these two components. Fortunately, as discussed

in the Correcting Modulator Imperfections section, the com-

posite LO leakage at the output can be mitigated by a single

compensation technique.