LF-to-2.5GHz Dual Logarithmic Detector/

Controller for Power, Gain, and VSWR Measurements

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11

The differential RF inputs allow for the measurement of

broadband signals ranging from low frequency to

2.5GHz. For single-ended signals, RFINA- and RFINB-

are AC-coupled to ground. The RF inputs are internally

biased and need to be AC-coupled. Using 680pF

capacitors, as shown in the Typical Application Circuit,

results in a 10MHz highpass corner frequency. An

internal 50

Ω resistor between RFINA+ and RFINA- (as

well as RFINB+ and RFINB-) produces a good low-fre-

quency to 3.0GHz match.

SETA, SETB, and SETD Inputs

The SET_ inputs are used for loop control when the

device is in controller mode. Likewise, these same

SET_ inputs are used to set the slope of the output sig-

nal (mV/dB) when the MAX2016 is in detector mode.

The center node of the internal resistor-divider is fed to

the negative input of the power detector’s internal out-

put op amp.

Reference

The MAX2016 has an on-chip 2V voltage reference.

The internal reference output is connected to REF. The

output can be used as a reference voltage source for

the comparators or other components and can source

up to 2mA.

OUTA and OUTB

Each OUT_ is a DC voltage proportional to the RF input

power level. The change of OUT_ with respect to the

Ω).

The input power level can be determined by the following

equation:

the output voltage intersects the horizontal axis.

OUTD

OUTD is a DC voltage proportional to the difference of

the input RF power levels. The change of the OUTD

with respect to the power difference is -25mV/dB (R3 =

0

Ω). The difference of the input power levels (gain) can

be determined by the following equation:

Applications Information

Monitoring VSWR and Return Loss

The MAX2016 can be used to measure the VSWR of an

RF signal, which is useful for detecting the presence or

absence of a properly loaded termination, such as an

antenna (see Figure 1). The transmitted wave from the

power amplifier is coupled to RFINA and to the anten-

na. The reflected wave from the antenna is connected

to RFINB through a circulator. When the antenna is

missing or damaged, a mismatch in the nominal load

PP

VV

SLOPE

RFINA

RFINB

OUTD

CENTER

−

=

−

()

P

V

SLOPE

P

RFIN

OUT

INT

_

_

=+

MAX2016

CSETL

COUTL

LOGARITHMIC

DETECTOR

LOGARITHMIC

DETECTOR

COUTL

OUTD

SETD

20k

Ω

RFINB

RFINA

OUTD

TRANSMITTER

COUPLER

CIRCULATOR

ATTENUATOR

GND

Figure 1. VSWR Monitoring Configuation