ADF7021 datasheet(18/44 Pages) AD

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ADF7021

Preliminary Technical Data

Rev. PrI | Page 18 of 44

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 power-

line-induced interference problems.

Figure 19 shows the structure of the receiver front end. The

many programming options allow users to trade off sensitivity,

linearity, and current consumption against each other in the

way best suited for their applications. 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 19. ADF7021 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 control bits LNA

mode (R9_DB25) and mixer linearity (R9_DB28) as outlined in

Table 8.

The gain of the LNA is configured by the LNA gain field,

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

automatic gain control (AGC) logic.

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

between 12.5 kHz and 25 kHz by Control Bits R4_DB[30:31], and

should be chosen as a compromise between interference

rejection, attenuation of the desired signal, and the AFC pull-in

range.

Coarse/Fine Filter Calibration

To compensate for manufacturing tolerances, the IF filter

should be calibrated once after power-up. Coarse and fine

calibration schemes are provided in order to offer a choice

between fast calibration with the coarse scheme and high filter

centering accuracy with fine calibration. Coarse calibration is

enabled by setting Bit R5_DB4 high. Fine calibration is enabled

by setting Bit R6_DB4 high. It is necessary to do a coarse cali-

bration 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 calibration is performed automatically

without any user intervention. Calibration time is 200 μs for coarse

calibration and a few milliseconds for fine calibration, during

which the ADF7021 should not be accessed. The IF filter

calibration logic requires that the IF filter divider in Bits

R5_DB[5:13] be set depending on the crystal frequency. It is

important not to initiate the calibration cycle before the crystal

oscillator has fully settled. If the AGC loop is disabled, the gain

of IF filter can be set to three levels using the Filter Gain Field,

R9_DB[22:23]. The filter gain is adjusted automatically if the

AGC loop is enabled.

Table 8. LNA/Mixer Modes

Receiver Mode

LNA Mode

(R9_DB25)

LNA

Gain Value

(R9_DB[20:21])

Mixer

Linearity

(R9_DB28)

Sensitivity

(DR = 9.6 kbps,

FDEV = 4.8 kHz)

Rx Current

Consumption

(mA)

Input IP3

(dBm)

High Sensitivity Mode (default)

−117

TBD

Rx Mode2

TBD

Low Current Mode

TBD

Enhanced Linearity Mode

TBD

Rx Mode5

TBD

Rx Mode6

TBD

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