ADL5519ACPZ-R7 datasheet(34/39 Pages) AD

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ADL5519

Data Sheet

Rev. B | Page 34 of 39

MEASURING VSWR

Measurement of reflected power in wireless transmitters is a critical

auxiliary function that is often overlooked. The power reflected

back from an antenna is specified using either the voltage standing

wave ratio (VSWR) or the reflection coefficient (also referred to

as the return loss). Poor VSWR can cause shadowing in a TV

broadcast system because the signal reflected off the antenna

reflects again off the power amplifier and is then rebroadcast. In

wireless communications systems, shadowing produces multipath-

like phenomena. Poor VSWR can degrade transmission quality;

the catastrophic VSWR that results from damage to a co-axial

cable or to an antenna can, at its worst, destroy the transmitter.

The ADL5519 delivers an output voltage proportional to the log

of the input signal over a large dynamic range. A log-responding

device offers a key advantage in VSWR measurement applications.

To compute gain or reflection loss, the ratio of the two signal

powers (either OUTPUT/INPUT or REVERSE/FORWARD)

must be calculated. An analog divider must be used to perform

this calculation with a linear-responding diode detector, but

only simple subtraction is required when using a log-responding

detector (because log(A/B) = log(A) − log(B)).

A dual RF detector has an additional advantage compared to

a discrete implementation. There is a natural tendency for two

devices (RF detectors, in this case) to behave similarly when they

are fabricated on a single piece of silicon, with both devices having

similar temperature drift characteristics, for example. At the

summing node, this drift cancels to yield a result that is more

temperature stable.

In Figure 71, two directional couplers are used, one to measure

forward power and one to measure reverse power. Additional

attenuation is required before applying these signals to the

detectors. The ADL5519 dual detector has a measurement range

of 50 dB in each detector. Care must be taken in setting the attenua-

tion levels so the reflection coefficient can be measured over the

desired output power range.

The level planning used in this example is graphically depicted

in Figure 70. In this example, the expected output power range

from the HPA is 30 dB, from 20 dBm to 50 dBm. Over this

power range, the ADL5519 can accurately measure reflection

coefficients from 0 dB (short, open, or load) to −20 dB.

Each ADL5519 detector has a nominal input range from −5 dBm

to −55 dBm. In this example, the maximum forward power of

+50 dBm is attenuated to −10 dBm at the detector input (this

attenuation is achieved through the combined coupling factor

of the directional coupler and the subsequent attenuation). This

puts the maximum power at the detector comfortably within its

linear operating range. Also, when the HPA is transmitting at its

lowest power level of +20 dBm, the detector input power is

−40 dBm, which is still within its input operating range.

50dBm

40dBm

30dBm

20dBm

10dBm

0dBm

–10dBm

–20dBm

–30dBm

–40dBm

–50dBm

–60dBm

FORWARD

POWER

RANGE

POWER

AT INPUT A

REVERSE

POWER

RANGE

POWR

AT INPUT B

55dB

ATTENUATION

DECTOR A/B

INPUT RANGE

60dB

ATTENUATION

Figure 70. ADL5519 VSWR Level Planning

Careful level planning should be used to match the input power

levels in a dual detector and to place these power levels within

the linear operating range of the detectors. The power from the

reverse path is attenuated by 55 dB, which means that the detector

is capable of measuring reflected power up to 0 dB. In most appli-

cations, the system is designed to shut down when the reflection

coefficient degrades below a certain minimum (for example,

10 dB). Full reflection is allowed when using the ADL5519 because

of its large dynamic range. In the case of very little reflection

(a return loss of 20 dB) and the HPA is transmitting +20 dBm, the

reverse path detector has an input power of −55 dBm.

The application circuit in Figure 71 provides a direct reading of

return loss, forward power, and reverse power. If the forward and

reverse phase difference (phase angle) is needed to optimize the

power delivered to the antenna, the AD8302 should be used.

It provides one output that represents the return loss and one

output that represents the phase difference between the two

signals. However, the AD8302 does not provide the absolute

forward or reverse power.

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