RF60 Datasheet, PDF (43/157 Pages) –

11. Power Amplifier
RF60 CRYSTAL-LESS SOC TRANSMITTER v1.0
TXP
INPUT
PA
TXM
Itail
FEEDBACK (HW, SW)
FREQUENCY TUNE, CONST PWR
Figure 11.1. Simplified PA Block Diagram
The CMOS power amplifier (PA) is a differential open drain amplifier capable of delivering +10 dBm of out-
put power. Maximum power can be transferred to an inductive antenna load when the antenna and output
driver of the PA are at resonance and the real component of the combined load is equal to the optimum
load resistance of V_pk/(4/Pi * I_tail/2) where V_pk is the peak differential voltage of the PA and I_tail is the tail
current of the PA. This optimum load resistance is the parallel combination of the PA output resistance and
the differential antenna resistance. At higher resistances the PA is voltage limited and at lower resistances
the PA is current limited. The PA tail current is programmable from 740uA up to 7mA (SFR register
PA_LVL) in 0.25 dB steps and there is a boost current bit (XREG PA_TRIM.PA_MAX_DRV) that multiplies
the tail current by 1.5 times allowing it to go up to 10.5mA. The maximum differential peak-to-peak voltage
is 4V when the supply is 2.2 to 3.6 V and drops linearly down to 3.4V when the supply is at 1.8V
The calculator spreadsheet tool computes the required antenna impedance and API settings to achieve
the user desired output power. Proper layout and matching techniques are all necessary to ensure optimal
performance. Figure 9.1 shows a typical application schematic of the RF60 with the associated matching
circuitry for a differential loop antenna. With proper filtering and layout techniques, the RF60 can conform
to US FCC part 15.231 and European EN 300 220 regulations. Edge rate control is also included for OOK
mode to reduce harmonics that may otherwise violate government regulations. Edge shaping is
accomplished by gradually turning on and off the driver transistors of the PA. The edge shaping
parameters are controlled by the ODS block and is auto- matically determined by the calculator spread
sheet based on the desired data rate and encoding method. Users must comply with local radio frequency
transmission regulations.
Variations in the transmit center frequency due to off-chip capacitor tolerances, loop antenna manufactur-
ing tolerances, and environmental variations can lead to large antenna inefficiencies and wasted power
especially in high-Q power amplifiers. The RF60 includes an automatic antenna tuning circuit to improve
the antenna efficiency of the transmitter by adjusting the on-chip variable capacitor to resonate with the
inductance of the antenna. The PA output has 2.7 to 13pF of variable capacitance that is adjusted to tune
the antenna to the correct frequency using a firmware assisted algorithm and on-chip hardware.The vari-
able capacitance is adjusted at the start of each packet transmission during the preamble. The switching
network in the capacitor array is compensated over process, voltage, and temperature (PVT) to keep its
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