Introduction
The Atmel ATA8404/ATA8405 is a PLL transmitter IC, which has been developed for the demands of RF low-cost transmission systems at data rates up to 20kBaud Manchester coding and 40kBaud NRZ coding. The transmitting frequency range is 313MHz to 317MHz (ATA8404) and 432MHz to 448MHz (ATA8405), respectively. It can be used in both FSK and ASK systems.
This fully integrated PLL transmitter allows the design of simple, low-cost RF miniature transmitters for remote control and other industrial applications. The VCO is locked to 24×f_XTAL/32×f_XTAL for ATA8404/ATA8405. Thus, a 13.125MHz/13.56MHz crystal is needed for a 315MHz/433.92MHz transmitter. All other PLL and VCO peripheral elements are integrated.
The XTO is a series resonance (current mode) oscillator. Only one capacitor and a crystal connected in series to GND are needed as external elements in an ASK system. The internal FSK switch, together with a second capacitor, can be used for FSK modulation. The crystal oscillator needs typically 0.6ms until the CLK output is activated if a crystal as defined in the electrical characteristics is used (e.g., TPM crystal). For most crystals used in RKE systems, a shorter time will result.
The CLK output is switched on if the amplitude of the current flowing through the crystal has reached 35% to 80% of its final value. This is synchronized with the 1.64/1.69MHz CLK output. As a result, the first period of the CLK output is always a full period. The PLL is then locked <250μs after CLK output activation. This means an additional wait time of ≥250μs is necessary before the PA can be switched on and the data transmission can start. This results in a significantly lower time of about 0.85ms between enabling the ATA8404/ATA8405 and the beginning of the data transmission which saves battery power.
The power amplifier is an open-collector output delivering a current pulse which is nearly independent from the load impedance and can therefore be controlled via the connected load impedance.
This output configuration enables a simple matching to any kind of antenna or to 50Ω. A high power efficiency for the power amplifier results if an optimized load impedance of z_Load,opt = 380Ω + j340Ω (ATA8404) at 315MHz and z_Load,opt = 280Ω + j310Ω (ATA8405) at 433.92MHz is used at the 3-V supply voltage.
If ASK = Low, FSK = Low, and ENABLE = open or Low, the circuit is in power-down mode consuming only a very small amount of current so that a lithium cell used as power supply can work for many years.
If the ENABLE pin is left open, ENABLE is the logical OR operation of the ASK and FSK input pins. This means, the IC can be switched on by either the FSK of the ASK input.
If the ENABLE pin is Low and ASK or FSK are High, the IC is in idle mode where the PLL, XTO, and power amplifier are off and the microcontroller ports controlling the ASK and FSK inputs can be used to control other devices. This can help to save ports on the microcontroller in systems where other devices with 3-wire interface are used.
With FSK = High, ASK = Low, and ENABLE = open or High, the PLL and the XTO are switched on and the power amplifier is off. When the amplitude of the current through the crystal has reached 35% to 80% of its final amplitude, the CLK driver is automatically activated. The CLK output stays Low until the CLK driver has been activated. The driver is activated synchronously with the CLK output frequency, hence, the first pulse on the CLK output is a complete period. The PLL is then locked within <250μs after the CLK driver has been activated, and the transmitter is then ready for data transmission.
With ASK = High, the power amplifier is switched on. This is used to perform the ASK modulation. During ASK modulation, the IC is enabled with the FSK or the ENABLE pin.
With FSK = Low the switch at pin XTO2 is closed, with FSK = High the switch is open. To achieve a faster start-up of the crystal oscillator, the FSK pin should be High during start-up of the XTO because the series resistance of the resonator seen from pin XTO1 is lower if the switch is off.
