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ADRF6821

Data Sheet

Rev. A | Page 28 of 61

LOW-PASS FILTER BANDWIDTH SELECTION

The ADRF6821 incorporates an on-chip, third-order, low-pass

filter (LPF) between the demodulator and the output buffer. The

filter has four different bandwidth settings. The EN_LPF_LB_I

(Register 0x0060, Bit 0) and the EN_LPF_LB_Q (Register 0x0060,

Bit 1) bits enable various LPF bandwidth modes, as detailed in

Table 12.

The ADRF6821 incorporates a wideband buffer at the I and Q

outputs that poses a challenge for the linearity of the overall

RFIC. For RF and LO frequencies lower than 1000 MHz, the

mixing product, RF + LO, is amplified by the wideband buffer

and, in turn, deteriorates the overall linearity. The on-chip LPF

can improve the leakage rejection of the high frequency mixing

product. Depending on the I/Q bandwidth requirement of the

system, the LPF can be set to lower bandwidths to provide

rejection at RF and LO frequencies. Table 16 can determine the

LPF bandwidth according to RF and LO frequency of operation.

Table 16. LPF Bandwidth Selection for Various RF and LO

Frequencies

LO Frequency

(MHz)

LPF Bandwidth

Setting (MHz)

EN_LPF_LB_I,

EN_LPF_LB_Q

450 to 1000

250

1, 1

1000 to 1200

500

1, 0

1200 to 2000

750

0, 1

2000 to 2800

1000

0, 0

Moreover, the on-chip LPF can improve the RF to IF and LO to

IF leakage performance for RF and LO frequencies higher than

1 GHz. However, note the gain flatness degradation with the use

of various LPF settings (see Figure 48).

100

200

300

400

500

IF FREQUENCY (MHz)

250MHz BANDWIDTH

500MHz BANDWIDTH

750MHz BANDWIDTH

1000MHz BANDWIDTH

Figure 48. Gain vs. IF Frequency for Various Low-Pass Filter Settings,

LO = 2000 MHz

Figure 49 and Figure 50 illustrate the effect of the LPF on the RF

to IF leakage and LO to IF leakage.

–70

–60

–50

–40

–30

–20

–10

450

920

1390

1860

2330

2800

RF FREQUENCY (MHz)

250MHz BANDWIDTH

500MHz BANDWIDTH

750MHz BANDWIDTH

1000MHz BANDWIDTH

Figure 49. RF to IF Rejection for RF Frequency at 100 MHz and a Low-Pass

Filter Setting of 250 MHz

–80

–70

–60

–50

–40

–30

–20

–10

450

920

1390

1860

2330

2800

LO FREQUENCY (MHz)

250MHz BANDWIDTH

500MHz BANDWIDTH

750MHz BANDWIDTH

1000MHz BANDWIDTH

Figure 50. LO to IF Rejection for LO Frequency at 100 MHz and a Low-Pass

Filter Setting of 250 MHz

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