ADL5387ACPZ-R71 datasheet(18/28 Pages) AD

18 / 28 page

ADL5387

Rev. 0 | Page 18 of 28

0°

SIN

ωLOt

COS

ωLOt

ωIF

ωLSB

ωUSB

–

ωIF

–

ωIF

ωLO

–90°

+90°

Figure 54. Illustration of the Image Problem

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. Appropriate image

rejection is critical because the image power can be much

higher than that of the desired signal, thereby plaguing the

down conversion process. Figure 54 illustrates the image

problem. If the upper sideband (lower sideband) is the desired

band, a 90° shift to the Q channel (I channel) cancels the image

at the lower sideband (upper sideband).

Figure 55 shows the excellent image rejection capabilities of the

ADL5387 for low IF applications, such as CDMA2000. The

ADL5387 exhibits image rejection greater than 45 dB over the

broad frequency range for an IF = 1.23 MHz.

–70

–60

–50

–40

–30

–20

–10

250

450

650

850

1050 1250 1450 1650 1850

RF INPUT FREQUENCY (MHz)

Figure 55. Image Rejection vs.

RF Input Frequency for a CDMA2000 Signal, IF = 1.23 MHz

EXAMPLE BASEBAND INTERFACE

In most direct conversion receiver designs, it is desirable to

select a wanted carrier within a specified band. The desired

channel can be demodulated by tuning the LO to the appropriate

carrier frequency. If the desired RF band contains multiple

carriers of interest, the adjacent carriers would also be down

converted to a lower IF frequency. These adjacent carriers can

be problematic if they are large relative to the wanted carrier as

they can overdrive the baseband signal detection circuitry. As a

result, it is often necessary to insert a filter to provide sufficient

rejection of the adjacent carriers.

It is necessary to consider the overall source and load impedance

presented by the ADL5387 and ADC input to design the filter

network. The differential baseband output impedance of the

ADL5387 is 50 Ω. The ADL5387 is designed to drive a high

impedance ADC input. It may be desirable to terminate the

ADC input down to lower impedance by using a terminating

resistor, such as 500 Ω. The terminating resistor helps to better

define the input impedance at the ADC input. The order and

type of filter network depends on the desired high frequency

rejection required, pass-band ripple, and group delay. Filter

design tables provide outlines for various filter types and orders,

illustrating the normalized inductor and capacitor values for a

1 Hz cutoff frequency and 1 Ω load. After scaling the normalized

prototype element values by the actual desired cut-off frequency

and load impedance, the series reactance elements are halved to

realize the final balanced filter network component values.

ADL5387ACPZ-R71 数据表(PDF) 18 Page - Analog Devices

ADL5387ACPZ-R71 数据表(HTML) 18 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

数据表下载