AD9775 datasheet(37/56 Pages) AD

37 / 56 page

AD9775

Rev. E | Page 37 of 56

COMPLEX MODULATION AND IMAGE REJECTION

OF BASEBAND SIGNALS

In traditional transmit applications, a two-step upconversion is

done in which a baseband signal is modulated by one carrier to

an intermediate frequency (IF) and then modulated a second

time to the transmit frequency. Although this approach has

several benefits, a major drawback is that two images are

created near the transmit frequency. Only one image is needed,

the other being an exact duplicate. Unless the unwanted image

is filtered, typically with analog components, transmit power is

wasted and the usable bandwidth available in the system is reduced.

A more efficient method of suppressing the unwanted image

can be achieved by using a complex modulator followed by a

quadrature modulator. Figure 78 is a block diagram of a

quadrature modulator. Note that it is in fact the real output half

of a complex modulator. The complete upconversion can

actually be referred to as two complex upconversion stages, the

real output of which becomes the transmitted signal.

The entire upconversion, from baseband to transmit frequency,

is represented graphically in Figure 79. The resulting spectrum

shown in Figure 79 represents the complex data consisting of

the baseband real and imaginary channels, now modulated onto

orthogonal (cosine and negative sine) carriers at the transmit

frequency. It is important to remember that in this application

(two baseband data channels) the image rejection is not

dependent on the data at either of the AD9775 input channels.

In fact, image rejection still occurs with either one or both of

the AD9775 input channels active. Note that by changing the

sign of the sinusoidal multiplying term in the complex

modulator, the upper sideband image could have been

suppressed while passing the lower one. This is easily done in

the AD9775 by selecting the e+jωt bit (Register 0x01, Bit 1). In

purely complex terms, Figure 79 represents the two-stage

upconversion from complex baseband to carrier.

INPUT

(REAL)

OUTPUT

INPUT

(IMAGINARY)

COS

ωt

SIN

ωt

Figure 78. Quadrature Modulator

REAL CHANNEL (OUT)

IMAGINARY CHANNEL (OUT)

A/2

–B/2J

B/2J

–A/2J

A/2J

–FC1

–FC

A/2

B/2

COMPLEX

MODULATOR

TO QUADRATURE

MODULATOR

REAL CHANNEL (IN)

IMAGINARY CHANNEL (IN)

A/4 + B/4J

A/4 – B/4J

A/4 + B/4J

A/4 – B/4J

–A/4 – B/4J

A/2 + B/2J

A/2 – B/2J

A/4 – B/4J

A/4 + B/4J –A/4 + B/4J

–FQ – FC

–FQ + FC

–FQ2

–FQ

FQ – FC

FQ + FC

QUADRATURE

MODULATOR

OUT

REAL

IMAGINARY

REJECTED IMAGES

1FC = COMPLEX MODULATION FREQUENCY

2FQ = QUADRATURE MODULATION FREQUENCY

Figure 79. Two-Stage Upconversion and Resulting Image Rejection

AD9775 Folha de dados(PDF) 37 Page - Analog Devices

AD9775 Folha de dados(HTML) 37 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

Folha de dados Download

Ligação URL