6. General Description
The fully-integrated PLL transmitter that allows particularly simple, low-cost RF miniature trans-
mitters to be assembled. The VCO is locked to 64 × fXTAL, thus, a 13.5672 MHz crystal is
needed for a 868.3 MHz transmitter and a 14.2969 MHz crystal for a 915 MHz transmitter. All
other PLL and VCO peripheral elements are integrated.
The XTO is a series resonance oscillator so that only one capacitor together with a crystal con-
nected in series to GND are needed as external elements.
The crystal oscillator together with the PLL needs typically < 1 ms until the PLL is locked and the
CLK output is stable. A wait time of ≥ 4 ms must be used until the CLK is used for the microcon-
troller and the PA is switched on.
The power amplifier is an open-collector output delivering a current pulse which is nearly inde-
pendent from the load impedance. The delivered output power is controlled via the connected
load impedance.
This output configuration enables a simple matching to any kind of antenna or to 50 Ω. A high
power efficiency of η= Pout/(IS,PA × VS ) of 24% for the power amplifier at 868.3 MHz results
when an optimized load impedance of ZLoad = (166 + j226) Ω is used at 3 V supply voltage.
7. Functional Description
If ENABLE = L and PA_ENABLE = L, the circuit is in standby mode consuming only a very small
amount of current, so that a lithium cell used as power supply can work for several years.
With ENABLE = H, the XTO, PLL and the CLK driver are switched on. If PA_ENABLE remains
L, only the PLL and the XTO are running and the CLK signal is delivered to the microcontroller.
The VCO locks to 64 times the XTO frequency.
With ENABLE = H and PA_ENABLE = H, the PLL, XTO, CLK driver and the power amplifier are
on. With PA_ENABLE, the power amplifier can be switched on and off, which is used to perform
the ASK modulation.
7.1
7.2
ASK Transmission
The PLL transmitter block is activated by ENABLE = H. PA_ENABLE must remain L for t ≥ 4 ms,
then the CLK signal can be taken to clock the microcontroller and the output power can be mod-
ulated by means of pin PA_ENABLE. After transmission, PA_ENABLE is switched to L and the
microcontroller switches back to internal clocking. The ATAM862-8 is switched back to standby
mode with ENABLE = L.
FSK Transmission
The PLL transmitter block is activated by ENABLE = H. PA_ENABLE must remain L for t ≥ 4 ms,
then the CLK signal can be taken to clock the microcontroller and the power amplifier is switched
on with PA_ENABLE = H. The chip is then ready for FSK modulation. The microcontroller starts
to switch on and off the capacitor between the XTAL load capacitor and GND with an open-drain
output port, thus changing the reference frequency of the PLL. If the switch is closed, the output
frequency is lower than if the switch is open. After transmission PA_ENABLE is switched to L
and the microcontroller switches back to internal clocking. The PLL transmitter block is switched
back to standby mode with ENABLE = L.
The accuracy of the frequency deviation with XTAL pulling method is about ±25% when the fol-
lowing tolerances are considered.
6
ATAM862-8
4590G–4BMCU–06/07
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