RF7115 Datasheet, PDF (14/22 Pages) RF Micro Devices – QUAD-BAND GSM850/GSM900/DCS/PCS TRANSMIT MODULE

RF7115
The RF7115 loop bandwidth is determined by internal bandwidth and does not change with respect to power levels. This
makes it easier 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 levels. The RF7115 is insensi-
tive to a change in input power and the burst timing is constant and requires no software compensation. Switching tran-
sients 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
inflexion 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 RF7115 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.
Performance under VSWR
Often overlooked when designing transmitters is the fact that they normally operate under mismatch conditions while
they are designed to operate only under perfect 50 ohm loads. This means that in the real application, performance is
degraded. This performance degradation may include reduction in output power, increased harmonic levels, increased
transient spectrum and catastrophic failures, breakdown. Traditionally designers have verified that the PA does not break
during mismatch and this is all verification that has been carried out during mismatch. Modern antennas in handsets
often present a load that significantly deviates from nominal impedance. A VSWR of 5:1 in not uncommon. In order not to
disturb other phones in the same and close by cells, it is important that the ETSI specifications for transient spectrum,
bust timing and spurious emission are fulfilled even during mismatch conditions. The RF7115 is designed to maintain its
performance even under high antenna mismatch conditions.
If power variation into a mismatch condition presents a problem, a current limiting option maybe utilized. The current lim-
iter can be set by adding a resistor to determine the set point. When open, the current limiter feature is fully engaged and
shorter the current limiter is disabled. Please refer to application note for further details.
Unlike a current controlled power control loop, the voltage controlled loop is almost impossible to force out of lock. For
the current controlled loop this easily happens as the current to the power amplifier that the controller tries to keep con-
stant can not be maintained during some phase angles. If the output stage of the power amplifier faces a high imped-
ance due to mismatch at the antenna, then the last stage simply cannot sink the current it does in a 50Ω load condition.
As the loop detects the lower current, the control voltage to the power amplifier increases in an attempt to keep the cur-
rent constant. As it is impossible to reach the desired current, the control voltage for the power amplifier rails and the
error is accumulated in the integrator in the control loop. When the reference value is lowered when the down ramp
starts, the integrator still contains the accumulated error and the control voltage to the power amplifier does not track the
reference signal. This means that the burst will be too long and that when the error finally reaches zero in the integrator,
the control voltage to the power amplifier suddenly decreases and this will contribute to increased levels of transient
spectrum at the down ramp.
The Power Star methodology is superior to the traditional current control method; it allows the transient spectrum in nor-
mal operation to be in the order of -35dBm to -40dBm but also both transient spectrum and the power versus time per-
formance is unaffected even with severe mismatch. In addition to this, the harmonics of the RF7115 is designed to be
within ETSI limits for usage with realistic antennas.
11-378
Rev A0 060808