MAX9930–MAX9933
2MHz to 1.6GHz 45dB RF-Detecting
Controllers and RF Detector
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Filter Capacitor and Transient Response
In general, for the MAX9930/MAX9931/MAX9932, the
choice of filter capacitor only partially determines the
time-domain response of a PA control loop. However,
some simple conventions can be applied to affect tran-
sient response. A large filter capacitor, C
CLPF
, domi-
nates time-domain response, but the loop bandwidth
remains a factor of the PA gain-control range. The
bandwidth is maximized at power outputs near the cen-
ter of the PA’s range, and minimized at the low and
high power levels, where the slope of the gain-control
curve is lowest.
A smaller valued C
CLPF
results in an increased loop
bandwidth inversely proportional to the capacitor value.
Inherent phase lag in the PA’s control path, usually
caused by parasitics at OUT, ultimately results in the
addition of complex poles in the AC loop equation. To
avoid this secondary effect, experimentally determine
the lowest usable C
CLPF
for the power amplifier of inter-
est. This requires full consideration to the intricacies of
the PA control function. The worst-case condition,
where the PA output is smallest (gain function is steep-
est) should be used because the PA control function is
typically nonlinear. An additional zero can be added to
improve loop dynamics by placing a resistor in series
with C
CLPF
. See Figure 4 for the gain and phase
response for different C
CLPF
values.
Additional Input Coupling
There are three common methods for input coupling:
broadband resistive, narrowband reactive, and series
attenuation. A broadband resistive match is implement-
ed by connecting a resistor to ground at the external
AC-coupling capacitor at RFIN as shown in Figure 5. A
50Ω resistor (use other values for different input imped-
ances) in this configuration, in parallel with the input
impedance of the MAX9930–MAX9933, presents an
input impedance of approximately 50Ω. These devices
require an additional external coupling capacitor in
series with the RF input. As the operating frequency
increases over 2GHz, input impedance is reduced,
resulting in the need for a larger-valued shunt resistor.
Use a Smith Chart for calculating the ideal shunt resis-
tor value. Refer to the MAX4000/MAX4001/MAX4002
data sheet for narrowband reactive and series attenua-
tion input coupling.
GAIN AND PHASE vs. FREQUENCY
SMALL-SIGNAL BANDWIDTH vs. C
