LF-to-2.5GHz Dual Logarithmic Detector/

Controller for Power, Gain, and VSWR Measurements

14

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Measuring Power (RSSI Detector Mode)

In detector mode, the MAX2016 acts like a receive-sig-

nal-strength indicator (RSSI), which provides an output

voltage proportional to the input power. This is accom-

plished by providing a feedback path from OUTA

(OUTB) to SETA (SETB) (R1/R2 = 0

Ω; see Figure 5).

By connecting SET_ directly to OUT_, the op-amp gain

is set to 2V/V due to two internal 20k

Ω feedback resis-

tors. This provides a detector slope of approximately

18mV/dB with a 0.5V to 1.8V output range.

Gain-Controller Mode

The MAX2016 can be used as a gain controller within

an automatic gain-control (AGC) loop. As shown in

Figure 6, RFINA and RFINB monitor the VGA’s input

and output power levels, respectively. The MAX2016

produces a DC voltage at OUTD that is proportional to

the difference in these two RF input power levels. An

internal op amp compares the DC voltage with a refer-

ence voltage at SETD. The op amp increases or

decreases the voltage at OUTD until OUTD equals

SETD. Thus, the MAX2016 adjusts the gain of the VGA

to a level determined by the voltage applied to SETD.

Place the nominal signal levels of RFINA and RFINB

near the middle of their respective dynamic ranges to

accommodate the largest range of gain compensation.

This is nominally -25dBm to -30dBm. If so selected, the

nominal voltage applied to SETD will be approximately

1.0V. Operate the SETD voltage within the range of

0.5V to 1.5V for the greatest accuracy of gain control.

MAX2016

RFIN+A

RFIN-A

OUTA

IN_

GND

R1/R2

20k

Ω

20k

Ω

DETECTORS

SETA

OUTA

RFIN+B

RFIN-B

OUTB

IN_

R1/R2

20k

Ω

20k

Ω

DETECTORS

SETB

OUTB

Figure 5. In Detector Mode (RSSI), OUTA/OUTB is a DC

Voltage Proportional to the Input Power

MAX2016

LOGARITHMIC

DETECTOR

LOGARITHMIC

DETECTOR

20k

Ω

VGA

GAIN CONTROL INPUT

RFINA

RFINB

OUTD

SETD

VGA INPUT

VGA OUTPUT

SET-POINT

DAC

COUPLER

COUPLER

Figure 6. In Gain-Controller Mode, the OUTD Maintains the

Gain of the VGA