RF7115_1 Datasheet, PDF (14/22 Pages) RF Micro Devices – QUAD-BAND GSM850/GSM900/DCS/PCS TRANSMIT MODULE

RF7115
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 sig-
nificantly 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 ful-
filled even during mismatch conditions. The RF7115 is designed to maintain its performance even under high antenna mis-
match conditions.
If power variation into a mismatch condition presents a problem, a current limiting option maybe utilized. The current limiter
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 cur-
rent controlled loop this easily happens as the current to the power amplifier that the controller tries to keep constant can not
be maintained during some phase angles. If the output stage of the power amplifier faces a high impedance 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 current constant. As it is impossi-
ble 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 accumu-
lated 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 normal
operation to be in the order of -35dBm to -40dBm but also both transient spectrum and the power versus time performance 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.
TX/RX Switch
The pHEMT switch integrated in the RF7115 allows for a low loss connection between the antenna port and the four RX ports.
The insertion loss in the TX and RX paths is lower than the loss for a traditional pin-diode switch solution, which means lower
current consumption in TX mode and better receiver sensitivity.
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Rev A0 DS060808