ADRF6620ACPZ-R7 datasheet(36/52 Pages) AD

36 / 52 page

ADRF6620

Data Sheet

Rev. 0 | Page 36 of 52

Most important, the low-pass interstage filter must attenuate the

sum term (fRF + fLO) and LO feedthrough to prevent unnecessary

overdrive of the DGA. The level of attenuation that is required to

achieve optimal OIP3 performance is shown in Figure 89, where

OIP3 vs. (fRF + fLO) amplitude is plotted. To maintain performance,

attenuate the amplitude of the sum term to at least −16 dBm (see

Figure 89). Beyond this point, the OIP3 degrades decibel per

decibel for increased amplitudes.

–20

–18

–16

–14

–12

–10

–8

–6

–4

AMPLITUDE (dBm)

Figure 89. OIP3 vs. (fRF + fLO) Amplitude

The ADRF6620 is optimized for use in digital predistortion

(DPD) receivers. An example filter design for DPD is shown in

Figure 91. Table 17 lists the interstage filter design targets.

In most DPD systems for cellular transmission, the pass band

is between 50 MHz and 500 MHz. For this reason, the pull-up

inductors have a low frequency cutoff of 50 MHz, and the pass-

band edge of the interstage low-pass filter is 500 MHz. This results

in a band-pass filter profile with a maximally flat pass band from

50 MHz to 500 MHz. The stop-band attenuation at 1400 MHz is

20 dB, which typically provides the necessary attenuation of the

mixer sum term with some margin.

Table 17. Example Filter Design

Parameter

Value

255 Ω

150 Ω

Pass-Band Edge

500 MHz

Attenuation at Pass-Band Edge

0.5 dB

Stop-Band Edge

1400 MHz

Attenuation at Stop-Band Edge

20 dB

Filter Type

Third-order Chebyshev

Using filter equations from a textbook or filter design software,

a third-order Chebyshev filter can be designed to satisfy all the

specifications in Table 17, as shown in Figure 91. The mixer output

capacitance of 1.1 pF can be absorbed into the filter, resulting in a

reduction in C1 from 2 pF to 0.8 pF. In addition, depending on the

PCB board stack-up, C2 can be further reduced, or eliminated,

because the capacitance of the PCB board can be used as the

third pole of the filter. The components used in the simulation

were the Coilcraft 0805CS inductors and Murata GRM15 series

capacitors. Figure 90 shows the filter profile that satisfies all the

filter specifications in Table 17.

–40

–35

–30

–25

–20

–15

–10

–5

200

400

600

800

1000 1200 1400 1600 1800 2000

FREQUENCY (MHz)

Figure 90. Third-Order Chebyshev Filter Profile

1.1pF

2.5pF

82.5Ω

90Ω

MXOUT+

MXOUT–

24nH

0.1µF

+5V

IFIN+

IFIN–

MIXER OUTPUT IMPEDANCE

EQUIVALENT MODEL

THIRD-ORDER CHEBYSHEV

FILTER

DC BLOCKING

CAPS

150Ω

IDEAL IF AMP

INPUT IMPEDANCE

470nH

0.8pF

1pF

Figure 91. Low-Pass Interstage Filter Design

ADRF6620ACPZ-R7 Datasheet(PDF) 36 Page - Analog Devices

ADRF6620ACPZ-R7 Datasheet(HTML) 36 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

Datasheet Descarga

Enlace URL