ADF7021 datasheet(39/64 Pages) AD

39 / 64 page

Data Sheet

ADF7021

Rev. B | Page 39 of 64

LNA

PA_OUT

RFIN

RFINB

VBAT

ADF7021

OPTIONAL

BPF OR LPF

ZIN_RFIN

ZOPT_PA

ZIN_RFIN

ANTENNA

Figure 49. ADF7021 with Internal Rx/Tx Switch

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.

LNA

PA_OUT

RFIN

RFINB

VBAT

ADF7021

ADG919

OPTIONAL

BPF

(SAW)

OPTIONAL

LPF

ZIN_RFIN

ZOPT_PA

ZIN_RFIN

ANTENNA

Rx/Tx – SELECT

Figure 50. ADF7021 with External Rx/Tx Switch

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

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

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.

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.

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 (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.

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