5
ATA8403 [DATASHEET]
4983F–INDCO–08/15
3. General Description
This fully integrated PLL transmitter allows particularly simple, low-cost RF miniature transmitters to be assembled. The
VCO is locked to 64 fXTAL, and therefore a 13.5672MHz crystal is needed for a 868.3MHz transmitter and a 14.2969MHz
crystal for a 915MHz 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 connected in series to GND are
needed as external elements.
The crystal oscillator together with the PLL typically needs < 1ms until the PLL is locked and the CLK output is stable. There
is a wait time of ≥4ms must be used until the CLK is used for the microcontroller and the PA is switched on.
The power amplifier is an open-collector output delivering a current pulse, which is nearly independent from the load
impedance. The delivered output power is therefore controllable via the connected load impedance.
This output configuration enables a simple matching to any kind of antenna or to 50. A high power efficiency of
=P
out/(IS,PA VS) of 24% for the power amplifier at 868.3MHz results when an optimized load impedance of
ZLoad = (166 + j226) is used at 3V supply voltage.
4. Functional Description
If ENABLE = L and the 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. The power amplifier can
be switched on and off with PA_ENABLE. This is used to perform the ASK modulation.
4.1 ASK Transmission
The ATA8403 is activated by ENABLE = H. PA_ENABLE must remain L for t ≥4ms, then the CLK signal can be taken to
clock the microcontroller, and the output power can be modulated by means of the PA_ENABLE pin. After transmission,
PA_ENABLE is switched to L, and the microcontroller switches back to internal clocking. The ATA8403 is switched back to
standby mode with ENABLE = L.
4.2 FSK Transmission
The ATA8403 is activated by ENABLE = H. PA_ENABLE must remain L for t ≥4ms, 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 ATA8403 is switched back to standby mode with ENABLE = L.
The accuracy of the frequency deviation with XTAL pulling method is about ±25% when the following tolerances are
considered.
Figure 4-1. Tolerances of Frequency Modulation
Using C4=9.2pF±2%, C
5= 6.8pF ±5%, a switch port with CSwitch = 3pF ±10%, stray capacitances on each side of the crystal
of CStray1 =C
Stray2 = 1pF ±10%, a parallel capacitance of the crystal of C0= 3.2pF ±10% and a crystal with CM= 13fF ±10%,
typically results in an FSK deviation of ±21.5kHz with worst case tolerances of ±16.8kHz to ±28.0kHz.