RF3140 データシートの表示（PDF） - RF Micro Devices
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RF3140 Datasheet PDF : 16 Pages
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Power control loop stability often presents many chal-
lenges to transmitter design. Designing a proper power
control loop involves trade-offs affecting stability, tran-
sient spectrum and burst timing.
In conventional architectures the PA gain (dB/ V) varies
across different power levels, and as a result the loop
bandwidth also varies. With some power amplifiers it is
possible for the PA gain (control slope) to change from
100dB/V to as high as 1000dB/V. The challenge in this
scenario is keeping the loop bandwidth wide enough to
meet the burst mask at low slope regions which often
causes instability at high slope regions.
The RF3140 loop bandwidth is determined by internal
bandwidth and the RF output load and does not
change with respect to power levels. This makes it eas-
ier to maintain loop stability with a high bandwidth loop
since the bias voltage and collector voltage do not vary.
An often overlooked problem in PA control loops is that
a delay not only decreases loop stability it also affects
the burst timing when, for instance the input power
from the VCO decreases (or increases) with respect to
temperature or supply voltage. The burst timing then
appears to shift to the right especially at low power lev-
els. The RF3140 is insensitive to a change in input
power and the burst timing is constant and requires no
software compensation.
Switching transients occur when the up and down
ramp of the burst is not smooth enough or suddenly
changes shape. If the control slope of a PA has an
inflection point within the output power range or if the
slope is simply too steep it is difficult to prevent switch-
ing transients. Controlling the output power by chang-
ing the collector voltage is as earlier described based
on the physical relationship between voltage swing and
output power. Furthermore all stages are kept con-
stantly biased so inflection points are nonexistent.
RF3140
Harmonics are natural products of high efficiency
power amplifier design. An ideal class “E” saturated
power amplifier will produce a perfect square wave.
Looking at the Fourier transform of a square wave
reveals high harmonic content. Although this is com-
mon to all power amplifiers, there are other factors that
contribute to conducted harmonic content as well. With
most power control methods a peak power diode
detector is used to rectify and sense forward power.
Through the rectification process there is additional
squaring of the waveform resulting in higher harmon-
ics. The RF3140 address this by eliminating the need
for the detector diode. Therefore the harmonics coming
out of the PA should represent the maximum power of
the harmonics throughout the transmit chain. This is
based upon proper harmonic termination of the trans-
mit port. The receive port termination on the T/R switch
as well as the harmonic impedance from the switch
itself will have an impact on harmonics. Should a prob-
lem arise, these terminations should be explored.
The RF3140 incorporates many circuits that had previ-
ously been required external to the power amplifier.
The shaded area of the diagram below illustrates those
components and the following table itemizes a compar-
ison between the RF3140 Bill of Materials and a con-
ventional solution:
Component
Power Control ASIC
Directional Coupler
Buffer
Attenuator
Various Passives
Mounting Yield
(other than PA)
Total
Conventional
Solution
$0.80
$0.20
$0.05
$0.05
$0.05
$0.12
$1.27
RF3140
N/A
N/A
N/A
N/A
N/A
N/A
$0.00
Rev A6 040113
2-501
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