LF-to-2.5GHz Dual Logarithmic Detector/

Controller for Power, Gain, and VSWR Measurements

12

______________________________________________________________________________________

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 an increase 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.

For accurate measurement of signals carrying signifi-

cant amplitude modulation, limit the bandwidth of the

difference amplifier to be less than the lowest modula-

tion frequency. This will minimize the ripple in the

OUTD waveform. This is particularly appropriate if the

system-level time delay between the two sense points

is significant with respect to the period of modulation.

Figure 1 illustrates a simple level detector. For window-

detector implementation, see the Comparator/Window

Detector section.

Measuring VSWR and Return Loss

In Figure 2, the two logarithmic amplifiers measure the

incident and the reflected power levels to produce two

proportional output voltages at OUTA and OUTB. Since

OUTD is a DC voltage proportional to the difference of

OUTA and OUTB, return loss (RL) and VSWR can be

easily calculated within a microprocessor using the

following relationships:

where return loss (RL) is expressed in decibels,

-25mV/dB (for R3 = 0

Ω).

VSWR can similarly be calculated through the following

relationship:

VSWR

RL

RL

= +

−

−

−

⎛

⎝

⎞

⎠

⎛

⎝

⎞

⎠

110

110

20

20

RL

P

P

VV

SLOPE

RFINA

RFINB

OUTD

CENTER

=−

=

−

()

MAX2016

RFINA

RFINB

GND

ADC

μP

IN

LOAD

4-PORT DIRECTIONAL

COUPLER

LOGARITHMIC

DETECTOR

LOGARITHMIC

DETECTOR

OUTD

SETD

20k

Ω

Figure 2. Measuring Return Loss and VSWR of a Given Load