RF3166
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 levels. The RF3166 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 pre-
vent switching transients. Controlling the output power by changing the collector voltage is as earlier described based on the
physical relationship between voltage swing and output power. Furthermore all stages are kept constantly biased so inflection
points are nonexistent.
Harmonics are natural products of high efficiency power amplifier design. An ideal class “E” saturated power amplifier will pro-
duce a perfect square wave. Looking at the Fourier transform of a square wave reveals high harmonic content. Although this is
common 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 pro-
cess there is additional squaring of the waveform resulting in higher harmonics. The RF3166 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 har-
monics throughout the transmit chain. This is based upon proper harmonic termination of the transmit 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 problem arise, these terminations should be explored.
Rev A3 DS061031
7628 Thorndike Road, Greensboro, NC 27409-9421 · For sales or technical
support, contact RFMD at (+1) 336-678-5570 or sales-support@rfmd.com.
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