Image Rejection Calibration - Analog Devices ADF7021-V Manual

ADF7021-V
External Tx/Rx Switch
Figure 50 shows a configuration using an external Tx/Rx switch.
This configuration allows independent optimization of the
matching and filter network in the transmit and receive paths.
Therefore, it is more flexible and less difficult to design than the
configuration using the internal Tx/Rx switch. The PA is biased
through Inductor L1, whereas C1 blocks dc current. Together,
L1 and C1 form the matching network that transforms the source
impedance into the optimum PA load impedance, Z
ADG919
OPTIONAL
LPF
ANTENNA
OPTIONAL
BPF
(SAW)
Rx/Tx – SELECT
Figure 50. ADF7021-V with External Tx/Rx Switch
Z _PA depends on various factors, such as the required out-
OPT
put power, the frequency range, the supply voltage range, and
the temperature range. Selecting an appropriate Z
to minimize the Tx current consumption in the application.
The AN-764 Application Note and the AN-859 Application
Note contain a number of Z
OPT
conditions. Under certain conditions, however, it is recom-
mended that a suitable Z _PA value be obtained by means
OPT
of a load-pull measurement.
Due to the differential LNA input, the LNA matching network
must be designed to provide both a single-ended-to-differential
conversion and a complex, conjugate impedance match. The net-
work with the lowest component count that can satisfy these
requirements is the configuration shown in Figure 50, consist-
ing of two capacitors and one inductor.
Depending on the antenna configuration, the user may need a
harmonic filter at the PA output to satisfy the spurious emission
requirement of the applicable government regulations. The har-
monic filter can be implemented in various ways, for example, a
discrete LC pi or T-stage filter. The immunity of the ADF7021-V
to strong out-of-band interference can be improved by adding a
band-pass filter in the Rx path. Alternatively, the ADF7021-V
blocking performance can be improved by selecting one of the
enhanced linearity modes, as described in Table 14.

IMAGE REJECTION CALIBRATION

The image channel in the ADF7021-V is 200 kHz below the
desired signal. The polyphase filter rejects this image with an
asymmetric frequency response. The image rejection (IR)
_PA.
OPT
V
BAT
ADF7021-V
L1
C1
RFOUT
PA
Z _PA
OPT
Z _RFIN
IN
C
A
RFIN
L LNA
A
RFIN
Z _RFIN
IN
C
B
_PA helps
OPT
_PA values for representative
performance of the receiver is dependent on how well matched
the I and Q signals are in amplitude and how well matched the
quadrature is between them (that is, how close to 90° apart they
are). The uncalibrated image rejection performance is approxi-
mately 29 dB (at 460 MHz). However, it is possible to improve
this performance by as much as 20 dB by finding the optimum
I/Q gain and phase adjust settings.
Calibration Using Internal RF Source
With the LNA powered off, an on-chip generated, low level RF
tone is applied to the mixer inputs. The LO is adjusted to make
the tone fall at the image frequency where it is attenuated by the
image rejection of the IF filter. The power level of this tone is
then measured using the RSSI readback. The I/Q gain and phase
adjust DACs (Register 5, Bits[DB31:DB20]) are adjusted and the
RSSI is remeasured. This process is repeated until the optimum
values for the gain and phase adjust are found that provide the
lowest RSSI readback level, thereby maximizing the image
rejection performance of the receiver.
Using the internal RF source, the RF frequencies that can be
used 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 (Register 6, Bits[DB29:DB28]) 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 RF source
is being used. Alternatively, an external RF source can be used.
The magnitude of the phase adjust is set using the IR_PHASE_
ADJUST_MAG bits (Register 5, Bits[DB23:DB20]). This correc-
tion can be applied to either the I or Q channel, depending on
the value of the IR_PHASE_ADJUST_DIRECTION bit
(Register 5, Bit DB24).
The magnitude of the I/Q gain is adjusted using the IR_GAIN_
ADJUST_MAG bits (Register 5, Bits[DB29:DB25]). This correction
can be applied to either the I or Q channel, depending on the
value of the IR_GAIN_ADJUST_I/Q bit (Register 5, Bit DB30),
whereas the IR_GAIN_ADJUST_UP/DN bit (Register 5,
Bit DB31) sets whether the gain adjustment defines a gain or
an attenuation adjust.
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