EVAL-ADF7021DBZ5 Datasheet, PDF (40/64 Pages) Analog Devices – High Performance Narrow-Band Transceiver IC

ADF7021
Data Sheet
RFIN
RFINB
LNA
INTERNAL
SIGNAL
SOURCE
ADF7021
MUX
POLYPHASE
IF FILTER
RSSI/
LOG AMP
7-BIT ADC
PHASE ADJUST
I
Q
FROM LO
SERIAL
INTERFACE
4
PHASE ADJUST
REGISTER 5
GAIN ADJUST
REGISTER 5
4
RSSI READBACK
MICROCONTROLLER
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.
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
CAL AT +25°C
50
To enable the internal RF source, the IR_CAL_SOURCE_
DRIVE_LEVEL bits (R6_DB[28:29]) should be set to the
40
CAL AT +85°C
CAL AT –40°C
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).
30
V_DD = 3.0V
IF BW = 25kHz
20
WANTED SIGNAL:
INTERFERER SIGNAL:
RF FREQ = 430MHz
RF FREQ = 429.8MHz
MODULATION = 2FSK MODULATION = 2FSK
10 DATA RATE = 9.6kbps, DATA RATE = 9.6kbps,
PRBS9
PRBS11
f_DEV = 4kHz
f_DEV = 4kHz
LEVEL= –100dBm
0
–60 –40 –20
0
20
40
60
80 100
The magnitude of the I/Q gain is adjusted by the IR_GAIN_
TEMPERATURE (°C)
ADJUST_MAG bits (R5_DB[25:29]). This correction can be
Figure 52. Image Rejection Variation with Temperature after Initial
Calibrations at −40°C, +25°C, and +85°C
Rev. B | Page 40 of 64