RF7115 Datasheet, PDF (13/22 Pages) RF Micro Devices – QUAD-BAND GSM850/GSM900/DCS/PCS TRANSMIT MODULE

RF7115
Talk time and power management are key concerns in transmitter design since the power amplifier is the leading current
consumer in a mobile terminal. Considering only the power amplifier's efficiency does not provide a true picture for the
total system efficiency. It is important to consider effective efficiency which is represented by η_EFF. (η_EFF considers the
loss between the PA and antenna and is a more accurate measurement to determine how much current will be drawn in
the application). η_EFF is defined by the following relationship (Equation 2):
P_{----P---A----+-----}P_{----L---O---S---S}
-^P---^I--^N-
η_EFF
=
1----0------------¹_---⁰_{-------------–-----1---0----}¹_--⁰_--
V_BAT ⋅ I_BAT ⋅ 10
(Eq. 2)
Where P_PA is the output power from the PA, P_LOSS the insertion loss and P_IN the input power to the PA. The RF7115
improves the effective efficiency by minimizing the P_LOSS term in the equation. An ASM may have a typical loss of 1.2dB
in LB and 1.4dB in high band. To be added to this is trace losses and mismatch losses. A post PA loss of 1.5dB in LB
and 1.8dB in HB is common. With the integration of a low loss pHEMT switch and matching network in the same module,
higher system efficiency can be achieved.
Output power does not vary due to supply voltage under normal operating conditions if V_RAMP is sufficiently lower than
V_BATT. By regulating the collector voltage to the PA the voltage sensitivity is essentially eliminated. This covers most
cases where the PA will be operated. However, as the battery discharges and approaches its lower power range the
maximum output power from the PA will also drop slightly. In this case, it is important to also decrease V_RAMP to prevent
the power control from inducing switching transients. These transients occur as a result of the control loop slowing down
and not regulating power in accordance with V_RAMP. The relationship for V_RAMPMAX based on V_BATT is expressed in
equation 3.
V_RAMPMAX
=
3--
8
×
V_BATT
+
0.15
≤
1.5V
(Eq. 3)
The components following the power amplifier often have insertion loss variation with respect to frequency. Usually, there
is some length of microstrip that follows the power amplifier. There is also a frequency response found in directional cou-
plers due to variation in the coupling factor over frequency, as well as the sensitivity of the detector diode. Since the
RF7115 does not use a directional coupler with a diode detector, these variations do not occur. Also the TX/RX switch
with low pass filters that usually follows the PA may contribute to frequency variation. The TX/RX switch incorporated in
the RF7115 is very broadband and does not contribute to frequency roll off. Traditionally working with PA modules, some
matching network is necessary between the PA output and the input of the TX/RX switch in order to get best possible
performance. This work no longer has to be carried out, as this matching network is included in the RF7115.
Noise power in PA's where output power is controlled by changing the bias voltage is often a problem when backing off of
output power. The reason is that the gain is changed in all stages and according to the noise formula (Equation 4),
F_TOT
=
F1
+
F_{-----2----–-----1-}
G1
+
G^{--F---1-3----}⋅^-–--G^--1--2--
(Eq. 4)
the noise figure depends on noise factor and gain in all stages. Because the bias point of the RF7115 is kept constant
the gain in the first stage is always high and the overall noise power is not increased when decreasing output power.
Power control loop stability often presents many challenges to transmitter design. Designing a proper power control loop
involves trade-offs affecting stability, transient spectrum and burst timing.
Rev A0 060808
11-377