EVAL-ADF7021DBZ5 Datasheet, PDF (39/64 Pages) Analog Devices – High Performance Narrow-Band Transceiver IC

Data Sheet
ADF7021
ANTENNA _OPTIONAL
BPF OR LPF
V_BAT
L1
C1
PA_OUT
Z_OPT_PA
Z_IN_RFIN
C_A
RFIN
L_A
RFINB
PA
LNA
Z_IN_RFIN
C_B
ADF7021
Figure 49. ADF7021 with Internal Rx/Tx Switch
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
network with the lowest component count that can satisfy these
requirements is the configuration shown in Figure 50, consisting
of two capacitors and one inductor. A first-order implementation
of the matching network can be obtained by understanding the
arrangement as two L-type matching networks in a back-to-
back configuration. Due to the asymmetry of the network with
respect to ground, a compromise between the input reflection
coefficient and the maximum differential signal swing at the
LNA input must be established. The use of appropriate CAD
software is strongly recommended for this optimization.
The procedure typically requires several iterations until an
acceptable compromise has been reached. The successful imple-
mentation of a combined LNA/PA matching network for the
ADF7021 is critically dependent on the availability of an accurate
electrical model for the PCB. In this context, the use of a suitable
CAD package is strongly recommended. To avoid this effort, a
small form-factor reference design for the ADF7021 is provided,
including matching and harmonic filter components. The
design is on a 2-layer PCB to minimize cost. Gerber files are
available at www.analog.com.
External Rx/Tx Switch
Figure 50 shows a configuration using an external Rx/Tx switch.
This configuration allows an independent optimization of the
matching and filter network in the transmit and receive path.
Therefore, it is more flexible and less difficult to design than the
configuration using the internal Rx/Tx switch. The PA is biased
through Inductor L1, while C1 blocks dc current. Together, L1
and C1 form the matching network that transforms the source
impedance into the optimum PA load impedance, ZOPT_PA.
V_BAT
ANTENNA
L1
C1
PA_OUT
OPTIONAL
LPF
PA
Z_OPT_PA
Z_IN_RFIN
OPTIONAL ^CA
RFIN
BPF
(SAW)
L_A
RFINB
LNA
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
harmonic filter can be implemented in various ways, such as
a discrete LC pi or T-stage filter. Dielectric low-pass filter
components, such as the LFL18924MTC1A052 (for operation
in the 915 MHz and 868 MHz band) by Murata Manufacturing
Co. Ltd., represent an attractive alternative to discrete designs.
The immunity of the ADF7021 to strong out-of-band interference
can be improved by adding a band-pass filter in the Rx path.
Apart from discrete designs, SAW or dielectric filter components
such as the SAFCH869MAM0T00, SAFCH915MAL0N00,
DCFB2869MLEJAA-TT1, or DCFB3915MLDJAA-TT1, all by
Murata Manufacturing Co. Ltd., are well-suited for this purpose.
Alternatively, the ADF7021 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 is 200 kHz below the desired
signal. The polyphase filter rejects this image with an asymme-
tric frequency response. The image rejection 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 approximately
29 dB (at 450 MHz). However, it is possible to improve on this
performance by as much as 20 dB by finding the optimum I/Q
gain and phase adjust settings.
ADG919
Rx/Tx – SELECT
Z_IN_RFIN
C_B
ADF7021
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
Figure 50. ADF7021 with External Rx/Tx Switch
the tone fall at the image frequency where it is attenuated by the
ZOPT_PA depends on various factors, such as the required
output power, the frequency range, the supply voltage range,
and the temperature range. Selecting an appropriate ZOPT_PA
helps to minimize the Tx current consumption in the application.
Application Note AN-764 contains a number of ZOPT_PA values
for representative conditions. Under certain conditions, however,
it is recommended to obtain a suitable ZOPT_PA value by means
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 (R5_DB[20:31]) 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.
of a load-pull measurement.
Rev. B | Page 39 of 64