ADL5519ACPZ-R2 datasheet(32/39 Pages) AD

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ADL5519

Data Sheet

Rev. B | Page 32 of 39

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 can have 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 determine not only the overall power being transmitted but

also the power in each carrier for a multicarrier HPA. Without the

use of a detector, the auxiliary receiver is very difficult to calibrate

accurately over temperature due to the part-to-part variation of the

components in the auxiliary receiver.

In controller mode, the ADL5519 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. Figure 69 shows an example of how this can be done.

The RF input is connected to INHB, using a 19 dB coupler, and

the down-converted output from the signal chain is connected

to INHA, using a 19 dB coupler. A 100 pF 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 and the

differences in path losses to the detector. Because they are operating

at different frequencies, the appropriate voltages on the ADJA,

ADJB pins must be supplied. ADJA is set to 0.6 V and ADJB is

set to 0.65 V to set the −40oC/+85oC crossover point toward the

center of the input power range. Using the suggested ADJA value

for 80 MHz would put the crossover point at a higher power level.

Figure 68 shows the results of the circuit in Figure 69. The input

power is swept from −47 dBm to +8 dBm. The output power is

measured, and the gain is calculated at +25°C, −40°C and +85°C.

With equal valued couplers used on the input and output, the

expected gain is about 0 dB. Due to path loss differences and

differences due to using two separate frequencies, the average

gain is about 2.5 dB. In this configuration, approximately 50 dB of

control range with 0.2 dB drift over temperature is obtained. For

an auxiliary receiver, less than 5 dB of variation is expected over

temperature. If the power levels are chosen to coincide with the

temperature crossover point, approximately 0.1 dB of temperature

variation can be expected. Most of the gain change over input

power level is caused by performance differences at different

frequencies.

4.0

1.5

2.0

2.5

3.0

3.5

1.0

0.5

–50

–40

–30

–20

–10

PIN (dBm)

GAIN +25°C

GAIN –40°C

GAIN +85°C

Figure 68. Performance of Gain-Stable Receiver

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