ADF7021 datasheet(23/44 Pages) AD

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Preliminary Technical Data

ADF7021

Rev. PrI | Page 23 of 44

APPLICATIONS

LNA/PA MATCHING

The ADF7021 exhibits optimum performance in terms of

sensitivity, transmit power, and current consumption, only if its

RF input and output ports are properly matched to the antenna

impedance. For cost-sensitive applications, the ADF7021 is

equipped with an internal Rx/Tx switch that facilitates the use

of a simple, combined passive PA/LNA matching network.

Alternatively, an external Rx/Tx switch such as the ADG919 can

be used, which yields a slightly improved receiver sensitivity

and lower transmitter power consumption.

External Rx/Tx Switch

Figure 23 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. Both

elements, L1 and C1, also 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 23. 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 23,

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 might 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 SAFCH869MAM0T00B0S,

SAFCH915MAL0N00B0S, 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

the high linearity mode, as described in Table 8.

Internal Rx/Tx Switch

Figure 24 shows the ADF7021 in a configuration where the

internal Rx/Tx switch is used with a combined LNA/PA

matching network. For most applications, the slight performance

degradation of 1 dB to 2 dB caused by the internal Rx/Tx switch

is acceptable, allowing the user to take advantage of the cost-

saving potential of this solution. The design of the combined

matching network must compensate for the reactance presented

by the networks in the Tx and the Rx paths, taking the state of

the Rx/Tx switch into consideration.

LNA

PA_OUT

RFIN

RFINB

VBAT

ADF7021

OPTIONAL

BPF OR LPF

ZIN_RFIN

ZOPT_PA

ZIN_RFIN

ANTENNA

Figure 24. ADF7021 with Internal Rx/Tx Switch

The procedure typically requires several iterations until an

acceptable compromise has been reached. The successful

implementation of a combined LNA/PA matching network for

the ADF7021 is critically dependent upon the availability of an

accurate electrical model for the PC board. In this context, the

use of a suitable CAD package is strongly recommended. To

avoid this effort, the reference design provided for the ADF7021

RF module can be used. Gerber files are available on request.

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