KESTX01
v CC
v CC
TXEN
v EE
power dn
power up
ACTIVE CIRCUITS
v EE
Fig. 3
Figure 3 TXEN power-up operation
APPLICATIONS INFORMATION
Power control
The bias current for the power amplifier directly controls the
output current (and hence the output power). The bias current
is set by the external resistor connected between PWRC and
ground. The bias voltage on pin PWRC is nominally 1.20V and
hence the modulation current Imod is given by 1.20/R.
To a first order neither the linearity (harmonic spurii relative
to fundamental) nor the amplifier efficiency are affected by
Imod. The graph below shows typical simulation results for the
amplifier current output with Imod variation.
9
8
OUTPUT CURRENT (mA)
OUTPUT CURRENT VS Imod
7
6
5
4
3
2
1
37
100
200
300
400
500
600
MODULATION CURRENT Imod (uA)
Figure 4 PWRC power output control
Frequency accuracy
The stability of the output frequency is equal to that of the
crystal referenced oscillator and shift in the VCO frequency
during data modulation. To operate with a final output
accuracy of
±
66KHz at 433.92MHz (as required for use with
the receiver KESRX01) would require a crystal with a
tolerance specification of
±
150ppm. This tolerance should
encompass e.g. initial accuracy, temperature stability and
ageing.
Operation at a final output frequency of 433.92MHz
requires a crystal specified for operation at 6.78MHz.
Antenna interface
The IC is capable of directly interfacing to a PCB loop
antenna as shown in the applications diagram.
Figure 4 is an equivalent circuit for a PCB loop
antenna. The inductance of the loop is Lant and this is in series
with two resistors. These represent Rr the radiation resistance
and Rs the series resistance of the antenna.
The Q of the antenna is defined as (ωo*Lant/(Rs+Rr)
where
ωo
is the resonant frequency (rad/s) of the antenna. At
resonance the antenna can be transformed to the equivalent
circuit on the right hand side. Here the equivalent parallel
resistance Rp is given by
5
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