AD8302 datasheet(20/24 Pages) AD

20 / 24 page

REV. 0

AD8302

–20–

The measurement accuracy can be compromised if board

level details are not addressed. Minimize the physical distance

between the series connected couplers since the extra path

length adds phase error to . Keep the paths from the couplers

to the AD8302 as well matched as possible since any differences

introduce measurement errors. The finite directivity, D, of the

couplers sets the minimum detectable reflection coefficient, i.e.,

|GMIN(dB)|<|D(dB)|.

SOURCE

1dB

20dB

INCIDENT

WAVE

REFLECTED

WAVE

ZLOAD

COMM

MFLT 14

INPA

VMAG

213

OFSA

MSET

312

VPOS

VREF

411

OFSB

PSET

510

INPB

VPHS

COMM

PFLT

AD8302

Figure 13. Using the AD8302 to Measure the Vector

Reflection Coefficient Off an Arbitrary Load

Reflectometer

The AD8302 can be configured to measure the magnitude ratio

and phase difference of signals that are incident on and reflected

from a load. The vector reflection coefficient,

is defined as,

= Reflected Voltage/Incident Voltage = (ZL – ZO)/(ZL + ZO), (16)

where ZL is the complex load impedance and ZO is the charac-

teristic system impedance.

The measured reflection coefficient can be used to calculate the

level of impedance mismatch or standing wave ratio (SWR) of a

particular load condition. This proves particularly useful in diag-

nosing varying load impedances such as antennas that can degrade

performance and even cause physical damage. The vector

reflectometer arrangement given in Figure 13 consists of a pair

of directional couplers that sample the incident and reflected sig-

nals. The attenuators reposition the two signal levels within the

dynamic range of the AD8302. In analogy to Equations 14 and

15, the attenuation factors and coupling coefficients are given by,

CB + LB = PIN – POPT

(17)

CA + LA = PIN + NOM – POPT

(18)

where NOM is the nominal reflection coefficient in dB and is

negative for passive loads. Consider the case where the incident

signal is 10 dBm and the nominal reflection coefficient is –19 dB.

As shown in Figure 13, using 20 dB couplers on both sides and

–30 dBm for POPT, the attenuators for Channel A and B paths are

1 dB and 20 dB, respectively. The magnitude and phase of the

reflection coefficient are available at the VMAG and VPHS pins

scaled to 30 mV/dB and 10 mV/degree. When

is –19 dB, the

VMAG output is 900 mV.

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