T5754C_14 Datasheet, PDF (5/13 Pages) ATMEL Corporation – UHF ASK/FSK Transmitter

3. General Description
This fully integrated PLL transmitter allows particularly simple, low-cost RF miniature transmitters to be assembled. The
VCO is locked to 32 f_XTAL hence a 13.56MHz crystal is needed for a 433.92MHz 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 needs typically < 1 ms until the PLL is locked and the CLK output is stable. There
is a wait time of ≥ 1ms 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 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 36% for the power amplifier results when an optimized load impedance of Z_Load = (166 + j223)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 32 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 Atmel^® T5754C is activated by ENABLE = H. PA_ENABLE must remain L for t ≥1ms, 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 microcontroller switches back to internal clocking. The Atmel T5754C is switched back
to standby mode with ENABLE = L.
4.2 FSK Transmission
The Atmel T5754C is activated by ENABLE = H. PA_ENABLE must remain L for t ≥1ms, 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 Atmel T5754C 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
V_S
XTAL
C_Stray1
L_M
C_M
R_S
C_Stray2
C₄
C₀
Crystal equivalent circuit
C₅
C_Switch
Using C₄ = 9.2pF ±2%, C₅ = 6.8pF ±5%, a switch port with C_Switch = 3pF ±10%, stray capacitances on each side of the crystal
of C_Stray1 = C_Stray2 = 1pF ±10%, a parallel capacitance of the crystal of C₀ = 3.2pF ±10% and a crystal with C_M = 13fF ±10%,
an FSK deviation of ±21kHz typical with worst case tolerances of ±16.3kHz to ±28.8kHz results.
T5754C [DATASHEET]
5
4511K–RKE–03/14