ADF7021 datasheet(40/64 Pages) AD

40 / 64 page

ADF7021

Data Sheet

Rev. B | Page 40 of 64

INTERNAL

SIGNAL

SOURCE

MUX

RFIN

RFINB

LNA

ADF7021

POLYPHASE

IF FILTER

PHASE ADJUST

FROM LO

GAIN ADJUST

PHASE ADJUST

SERIAL

INTERFACE

MICROCONTROLLER

RSSI/

LOG AMP

7-BIT ADC

RSSI READBACK

I/Q GAIN/PHASE ADJUST AND

RSSI MEASUREMENT

ALGORITHM

Figure 51. Image Rejection Calibration Using the Internal Calibration Source and a Microcontroller

Using the internal RF source, the RF frequencies that can be

utilized for image calibration are programmable and are odd

multiples of the reference frequency.

Calibration Using External RF Source

IR calibration can also be implemented using an external RF

source. The IR calibration procedure is the same as that used for

the internal RF source, except that an RF tone is applied to the

LNA input.

Calibration Procedure and Setup

The IR calibration algorithm available from Analog Devices, Inc. is

based on a low complexity, 2D optimization algorithm that can

be implemented in an external microprocessor or microcontroller.

To enable the internal RF source, the IR_CAL_SOURCE_

DRIVE_LEVEL bits (R6_DB[28:29]) should be set to the

maximum level. The LNA should be set to its minimum gain

setting, and the AGC should be disabled if the internal source is

being used. Alternatively, an external RF source can be used.

The magnitude of the phase adjust is set by using the IR_PHASE_

ADJUST_MAG bits (R5_DB[20:23]). This correction can be

applied to either the I channel or Q channel, depending on the

value of the IR_PHASE_ADJUST_DIRECTION bit (R5_DB24).

The magnitude of the I/Q gain is adjusted by the IR_GAIN_

ADJUST_MAG bits (R5_DB[25:29]). This correction can be

applied to either the I or Q channel, depending on the value of

IR_GAIN_ADJUST_I/Q bit (R5_DB30), whereas the

IR_GAIN_ADJUST_UP/DN bit (R5_DB31) sets whether

the gain adjustment defines a gain or an attenuation adjust.

The calibration results are valid over changes in the ADF7021

supply voltage. However, there is some variation with temperature.

A typical plot of variation in image rejection over temperature

after initial calibrations at −40°C, +25°C, and +85°C is shown in

Figure 52. The internal temperature sensor on the ADF7021 can

be used to determine if a new IR calibration is required.

–60

–40

–20

100

VDD = 3.0V

IF BW = 25kHz

WANTED SIGNAL:

RF FREQ = 430MHz

MODULATION = 2FSK

DATA RATE = 9.6kbps,

PRBS9

fDEV = 4kHz

LEVEL= –100dBm

INTERFERER SIGNAL:

RF FREQ = 429.8MHz

MODULATION = 2FSK

DATA RATE = 9.6kbps,

PRBS11

fDEV = 4kHz

TEMPERATURE (°C)

CAL AT +25°C

CAL AT +85°C

CAL AT –40°C

Figure 52. Image Rejection Variation with Temperature after Initial

Calibrations at −40°C, +25°C, and +85°C

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