4

4546F–RKE–12/08

T5750

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

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 50

Ω. A high

power efficiency of

η=P

an optimized load impedance of Z

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.