AD8364ACPZ-R2 datasheet(29/44 Pages) AD

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Data Sheet

AD8364

Rev. C | Page 29 of 44

GAIN-STABLE TRANSMITTER/RECEIVER

There are many applications for a transmitter or receiver with a

highly accurate temperature-stable gain. For example, a multi-

carrier base station high power amplifier (HPA) using digital

predistortion has a power detector and an auxiliary receiver. The

power detector and all parts associated with it can be removed if

the auxiliary receiver has a highly accurate temperature-stable

gain. With a set gain receiver, the ADC on the auxiliary receiver

can not only determine the overall power being transmitted but can

also determine the power in each carrier for a multicarrier HPA.

In controller mode, the AD8364 can be used to hold the

receiver gain constant over a broad input power/temperature

range. In this application, the difference outputs are used to

hold the receiver gain constant.

The RF input is connected to INPA, using a 19.1 dB coupler,

and the down converted output from our signal chain is connected

to INPB, using a 10.78 dB coupler. A 0.1 µF capacitor is connected

between FBKA and OUTP, forming an integrator. OUTA is

connected to VLVL, forcing OUTP to adjust the VGA so that

OUTB is equal to OUTA. The circuit gain is set by the difference in

the coupling values of the input and output couplers. As noted,

OUTP is used to drive the gain control of the ADL5330 by

adjusting the gain up or down as needed to force the power at

the AD8364 inputs to be equal in amplitude. Since operating at

different frequencies, the appropriate voltages on the ADJ[A, B]

pins must be supplied. Because INPA is operating at 1880 MHz,

ADJA is set to 0.75 V. Likewise, because INPB is operating at

80 MHz, ADJB is set to 0 V.

Because the difference in the coupler values is 8.32 dB, a fixed

gain of −8.32 dB is expected. In practice, there is a gain of −13 dB.

This is caused by the intercept shift of the AD8364 due to the

frequency response, the insertion loss of the output coupler, and

the insertion loss differences of the baluns used on the input of

the AD8364. In this configuration, approximately 33 dB of

control range with 0.5 dB drift over temperature is obtained.

Figure 66 shows a gain-stable receiver amplifier circuit using

the AD8364 to control an ADL5330 VGA and the AD8343

mixer. The input power was swept from +3 dBm to −35 dBm,

the output power was measured, and the gain was calculated at

multiple temperatures between −40°C and +85°C. Note that the

gain changed less than ±0.45 dB over this range (Figure 65).

Most of the gain change was caused by performance differences

at different frequencies.

PIN (dBm)

–40

–25

–15

–5

–20

–35

–30

–10

–11.0

–11.5

–12.0

–12.5

–13.0

–13.5

–14.0

–40°C

+25°C

+85°C

Figure 65. Performance of Gain-Stable Receiver

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