RF5146 Datasheet, PDF (15/18 Pages) RF Micro Devices – QUAD-BAND GSM850/GSM900/DCS/PCS POWER AMP MODULE

RF5146
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 RF5146 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 diffi-
cult to prevent 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 produce 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 process there is additional squaring of the waveform resulting in higher harmonics. The RF5146 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
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 A1 050928
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