ADRF6510ACPZ-R7 datasheet(20/28 Pages) AD

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ADRF6510

Rev. 0 | Page 20 of 28

An overall RF-to-baseband EVM performance was obtained

with the ADL5387 IQ demodulator preceding the ADRF6510.

An 840 MHz RF signal with a modulation setting of 64 QAM

and a 7 MHz symbol rate was used. The local oscillator (LO) is

set at 1680 MHz to obtain 840 MHz at the quadrature core after

the divide-by-2 stage. The analog gain of the ADRF6510 was

adjusted to maintain 1.5 V p-p into a 1 kΩ load impedance.

Figure 49 shows EVM vs. input power and the corresponding

analog gain voltage.

–65

–5

RF INPUT POWER (dBm)

0.5

1.0

1.5

2.0

2.5

–55

–45

–35

–25

–15

Figure 49. EVM vs. RF Input Power Level

LOW IF IMAGE REJECTION

The image rejection ratio is the ratio of the intermediate frequency

(IF) signal level produced by the desired input frequency to that

produced by the image frequency. The image rejection ratio is

expressed in decibels (dB). Appropriate image rejection is critical

because the image power can be much higher than that of the

desired signal, thereby plaguing the downconversion process.

Figure 51 illustrates the image problem. If the upper sideband is

the desired band, a 90° shift to the Q channel cancels the image

at the lower sideband. In the same way, if the lower sideband is

the desired band, a 90° shift to the I channel cancels the image

at the upper sideband. Phase and gain balance between the I

and Q channels are critical for high levels of image rejection.

Figure 50 shows the image rejection of the ADL5387 and the

ADRF6510 for various baseband frequencies. The modulation

is 64 QAM with a 7 MHz symbol rate. Note the following:

•

To the right of the 5 MHz center frequency, the filter is

programmed to be 5 MHz greater than the centered base-

band frequency. This ensures that the signal edge is well

within the pass band of the filter. In such cases, where the

filter bandwidth is set to be greater than the signal bandwidth,

the image rejection of the ADL5387 tends to be the limiting

factor, and the ADRF6510 has minimal effects.

•

To the left of 5 MHz—specifically at a center frequency

of 3.5 MHz—the filter corner is lowered to the baseband

signal edge, degrading the image rejection. When the

centered baseband frequency is 3.5 MHz and the filter is

set at 7 MHz (instead of a safer 8 MHz), the filter corner

conflicts with the edge of the modulated signal. Channel

mismatch in group delay characteristics and variation in

absolute group delay (from the normal flat response) tend

to degrade image rejection.

BASEBAND FREQUENCY (MHz)

579

AT CENTER FREQUENCY = 3.5MHz,

FILTER CORNER IS SET TO 7MHz

AT CENTER FREQUENCIES GREATER

THAN 5MHz, FILTER CORNER IS

SET TO 5MHz HIGHER THAN SIGNAL EDGE

Figure 50. Image Rejection of the ADL5387 and ADRF6510

0°

–90°

+90°

ωIF

ωLSB

USB

SIN

ωLOt

COS

ωLOt

–

ωIF

–

ωIF

Figure 51. Illustration of the Image Problem

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