LF-to-2.5GHz Dual Logarithmic Detector/

Controller for Power, Gain, and VSWR Measurements

______________________________________________________________________________________

13

VSWR can similarly be calculated through the following

relationship:

Measuring Gain

The MAX2016 can be used to measure the gain of an

RF block (or combination of blocks) through the imple-

mentation outlined in Figure 3. As shown, a coupled

signal from the input of the block is fed into RFINA,

while the coupled output is connected to RFINB. The

DC output voltage at OUTD is proportional to the power

difference (i.e., gain).

The gain of a complete receiver or transmitter lineup

can likewise be measured since the MAX2016 accepts

RF signals that range from low frequency to 2.5GHz;

see Figure 4. The MAX2016 accurately measures the

gain, regardless of the different frequencies present

within superheterodyne architectures.

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.

Power-Controller Mode

The MAX2016 can also be used as a power

detector/controller within an AGC loop. Figure 7 depicts

a scenario where the MAX2016 is employed as the

AGC circuit. As shown in the figure, the MAX2016 mon-

itors the output of the PA through a directional coupler.

An internal integrator (Figure 5) compares the detected

The integrator, acting like a comparator, increases or

decreases the voltage at OUT_, according to how

erence. The MAX2016 maintains the power of the PA to

a level determined by the voltage applied to SET_.

VSWR

RL

RL

= +

−

−

−

⎛

⎝

⎞

⎠

⎛

⎝

⎞

⎠

110

110

20

20

MAX2016

RFINA

RFINB

IN

LOGARITHMIC

DETECTOR

LOGARITHMIC

DETECTOR

GND

OUTD

SETD

20k

Ω

COUPLER

COUPLER

RF BLOCK

OUT

OUTD

Figure 3. Gain Measurement Configuration