ADF7021 datasheet(30/64 Pages) AD

30 / 64 page

ADF7021

Data Sheet

Rev. B | Page 30 of 64

RECEIVER SECTION

RF FRONT END

The ADF7021 is based on a fully integrated, low IF receiver

architecture. The low IF architecture facilitates a very low

external component count and does not suffer from powerline-

induced interference problems.

Figure 44 shows the structure of the receiver front end. The

many programming options allow users to trade off sensitivity,

linearity, and current consumption to best suit their application.

To achieve a high level of resilience against spurious reception,

the low noise amplifier (LNA) features a differential input.

Switch SW2 shorts the LNA input when transmit mode is

selected (R0_DB27 = 0). This feature facilitates the design of a

combined LNA/PA matching network, avoiding the need for an

external Rx/Tx switch. See the LNA/PA Matching section for

details on the design of the matching network.

SW2

LNA

RFIN

RFINB

Tx/Rx SELECT

(R0_DB27)

LNA MODE

(R9_DB25)

LNA CURRENT

(R9_DB[26:27])

MIXER LINEARITY

(R9_DB28)

I (TO FILTER)

Q (TO FILTER)

LNA GAIN

(R9_DB[20:21])

LNA/MIXER ENABLE

(R8_DB6)

Figure 44. RF Front End

The LNA is followed by a quadrature downconversion mixer,

which converts the RF signal to the IF frequency of 100 kHz.

An important consideration is that the output frequency of the

synthesizer must be programmed to a value 100 kHz below the

center frequency of the received channel. The LNA has two

basic operating modes: high gain/low noise mode and low

gain/low power mode. To switch between these two modes, use

the LNA_MODE bit (R9_DB25). The mixer is also configurable

between a low current and an enhanced linearity mode using

the MIXER_LINEARITY bit (R9_DB28).

Based on the specific sensitivity and linearity requirements of

the application, it is recommended to adjust the LNA_MODE

bit and MIXER_LINEARITY bit as outlined in Table 14.

The gain of the LNA is configured by the LNA_GAIN bits

(R9_DB[20:21]) and can be set by either the user or the

automatic gain control (AGC) logic.

IF FILTER

IF Filter Settings

Out-of-band interference is rejected by means of a fifth-order

Butterworth polyphase IF filter centered on a frequency of

100 kHz. The bandwidth of the IF filter can be programmed to

12.5 kHz, 18.75 kHz, or 25 kHz by R4_DB[30:31] and should be

chosen as a compromise between interference rejection and

attenuation of the desired signal.

If the AGC loop is disabled, the gain of the IF filter can be set to one

of three levels by using the FILTER_GAIN bits (R9_DB[22:23]).

The filter gain is adjusted automatically if the AGC loop is

enabled.

IF Filter Bandwidth and Center Frequency Calibration

To compensate for manufacturing tolerances, the IF filter should be

calibrated after power-up to ensure that the bandwidth and

center frequency are correct. Coarse and fine calibration

schemes are provided to offer a choice between fast calibration

(coarse calibration) and high filter centering accuracy (fine

calibration). Coarse calibration is enabled by setting R5_DB4

high. Fine calibration is enabled by setting R6_DB4 high.

For details on when it is necessary to perform a filter

calibration, and in what applications to use either a coarse

calibration or fine calibration, refer to the IF Filter Bandwidth

Calibration section.

It is necessary to do a coarse calibration before doing a fine

calibration. If the IF_FINE_CAL bit (R6_DB4) has already been

configured high, it is possible to do a fine calibration by writing

only to Register 5. Once initiated by writing to the part, the cali-

bration is performed automatically without any user intervention.

Calibration time is 200 μs for coarse calibration and a few

milliseconds for fine calibration, during which time the ADF7021

should not be accessed. The IF filter calibration logic requires

that the IF_FILTER_DIVIDER bits (R5_DB[5:13]) be set such that

kHz

[Hz]



DIVIDER

FILTER

XTAL

IF Filter Fine Calibration Overview

The fine calibration uses two internally generated tones at

certain offsets around the IF filter. The two tones are attenuated

by the IF filter, and the level of this attenuation is measured

using the RSSI. The filter center frequency is adjusted to allow

equal attenuation of both tones. The attenuation of the two test

tones is then remeasured. This continues for a maximum of

10 RSSI measurements, at which stage the calibration algorithm

sets the IF filter center frequency to within 0.5 kHz of 100 kHz.

The frequency of these tones is set by the following bits:



IF_CAL_LOWER_TONE_DIVIDER (R6_DB[5:12])



IF_CAL_UPPER_TONE_DIVIDER (R6_DB[13:20])

It is recommended to place the lower and upper tones as close

as possible to 65.8 kHz and 131.5 kHz, respectively, as outlined

in the following equations:

kHz





DIVIDE

TONE

LOWER

CAL

XTAL

kHz

131





DIVIDE

TONE

UPPER

CAL

XTAL

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