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 follow-

ing 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

impedance results, leading to an increase in reflected

power and subsequent change in the transmission

line’s VSWR. This increase in reflected power is mani-

fested by a reduction in the voltage at OUTD. An alarm

condition can be set by using the low comparator out-

put (COUTL) as shown in Figure 1. The comparator

automatically senses the change in VSWR, yielding a

logic 0 as it compares OUTD to a low DC voltage at

CSETL. CSETL, in turn, is set by using the internal refer-

ence voltage and an external resistor-divider network.

Figure 1 illustrates a simple level detector. For window-

detector implementation, see the Comparator/Window

Detector section.

PP

VV

SLOPE

RFINA

RFINB

OUTD

CENTER

−

=

−

()

P

V

SLOPE

P

RFIN

OUT

INT

_

_

=+

DESIGNATION

VALUE

DESCRIPTION

C1, C2, C8, C9

680pF

Microwave capacitors (0402)

C3, C6, C10, C13

33pF

Microwave capacitors (0402)

C4, C7, C11, C14

0.1µF

Microwave capacitors (0603)

C5, C12, C15

Not used

Capacitors are optional for frequency compensation

C18

10µF

Tantalum capacitor (C case)

R1, R2, R3

0

Ω

Resistors (0402)

0

Ω

R6

37.4

Ω

Table 1. Component Values Used in the Typical Application Circuit