ADF7020 datasheet(21/40 Pages) AD

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Preliminary Technical Data

ADF7020

Rev. PrH | Page 21 of 40

APPLICATIONS SECTION

LNA/PA MATCHING

The ADF7020 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 ADF7020 is

equipped with an internal Rx/Tx switch, which facilitates the

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

Alternatively, an external Rx/Tx switch such as the Analog

Devices ADG919 can be used, which yields a slightly improved

receiver sensitivity and lower transmitter power consumption.

External Rx/Tx Switch

Figure 18 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,

and is, therefore, 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, which

transforms the source impedance into the optimum PA load

impedance, ZOPT_PA.

LNA

PA_OUT

RFIN

RFINB

VBAT

ADF7020

ADG919

OPTIONAL

BPF

(SAW)

OPTIONAL

LPF

ZIN_RFIN

ZOPT_PA

ZIN_RFIN

ANTENNA

Rx/Tx – SELECT

Figure 18. ADF7020 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. This

datasheet contains a number of ZOPT_PA values for representa-

tive 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 18, which

consists 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 compro-

mise 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-filter. Dielectric low-pass filter components such as

the LFL18924MTC1A052 (for operation in the 915 MHz band),

or LFL18869MTC2A160 (for operation in the 868 MHz band),

both by Murata Mfg. Co., Ltd., represent an attractive alternative

to discrete designs. The immunity of the ADF7020 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 Mfg. Co., Ltd., are well suited for this purpose.

Internal Rx/Tx Switch

Figure 19 shows the ADF7020 in a configuration where the

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

matching network. Depending on the application, the slight

performance degradation caused by the internal Rx/Tx switch

might be 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

ADF7020

OPTIONAL

BPF OR LPF

ZIN_RFIN

ZOPT_PA

ZIN_RFIN

ANTENNA

Figure 19. ADF7020 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 ADF7020 is critically dependent on the availability of an

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