ATA5771_09 Datasheet, PDF (18/287 Pages) ATMEL Corporation – Microcontroller with UHF ASK/FSK Transmitter

3. General Description
This fully integrated PLL transmitter allows particularly simple, low-cost RF miniature transmit-
ters to be assembled. The VCO is locked to 64 × f_XTAL hence 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. There is a wait time of ≥ 4 ms 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 inde-
pendent from the load impedance. The delivered output power is hence controllable via the
connected load impedance.
This output configuration enables a simple matching to any kind of antenna or to 50Ω. A high
power efficiency of η= P_out/(I_S,PA × V_S) of 24% for the power amplifier at 868.3 MHz results when
an optimized load impedance of Z_Load = (166 + j226)Ω 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 is 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.
4.1 ASK Transmission
The T5750 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 modulated by
means of pin PA_ENABLE. After transmission PA_ENABLE is switched to L and the microcon-
troller switches back to internal clocking. The T5750 is switched back to standby mode with
ENABLE = L.
4.2 FSK Transmission
The T5750 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 T5750 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.
4 T5750
4546F–RKE–12/08