RF9678PCBA datasheet(5/16 Pages) RFMD

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Preliminary

5-95

RF9678

Rev A4 010622

Application Notes

Quadrature modulator performance can be correlated to a set of specifications known as Carrier and Sideband Suppres-

sion. In addition, Sideband Suppression can be correlated with the amplitude and phase balance of the In-Phase (I) and

Quadrature (Q) signals and Carrier Suppression can be correlated to the DC offset between the I and Q signals (see Fig-

ure 1). For a more thorough discussion of the theory and mathematics behind these specifications refer to RF Micro

Devices application note AN0001.

Effects of Carrier Suppression and Sideband Suppression on W-CDMA (QPSK) Modulation

W-CDMA signals may be displayed on a vector signal analyzer as a collection of points called a constellation. Each point

in the constellation is called a symbol and is representative of a bit sequence. In QPSK modulation, there are four sym-

bols and each symbol is representative of two data bits (see Figure 2). The I and Q signals are added together to create

a vector of precise phase and amplitude. The vector is then sampled at a rate called the symbol rate and it's position at

these intervals corresponds to the target symbol locations. Errors in the phase and amplitude of the I and Q signals will

translate to errors in the vector's phase and amplitude. This phase and amplitude error will result in a displacement of the

vector from it's target symbol point. A measurement of this error is called the Error Vector Magnitude (EVM) and it repre-

sents the magnitude of the displacement of the actual vector from it's target location.

QPSK constellation points exist on a circle of constant radius around the origin. Amplitude errors result in symbol points

being displaced either inside or outside of their target locations on this circle. Phase errors result in symbol points being

displaced on an arc either to the left or right of their target location. Finally, DC offset errors cause the origin to shift,

resulting in a constant I and Q offset of all target points (see Figure 3).

In-Phase Signal

Quadrature Signal

LO Signal

RF Output Signal

0°

90°

Figure 1. Quadrature Modulator Block Diagram

Ref Lvl

0dBm

Ref Lvl

0dBm

-1.875

1.875

REAL

380 MHz

Meas Signal

Constellation

IMAG

EXT

3.84 MHz

Demod

QPSK

-1.5

1.5

Date:

6.FEB.2001

01:20:38

Figure 2. W-CDMA (QPSK) Constellation

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