Datasheet
MAX2016
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,
V
CENTER
is the output voltage (typically 1V) when
P
RFINA
= P
RFINB
, and SLOPE is typically equal to
-25mV/dB (for R3 = 0Ω).
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
μ
P
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