AX5243 datasheet(28/65 Pages) ONSEMI

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Circuit Description

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AX5243

5.6. Power-on-reset (POR)

AX5243 has an integrated power-on-reset block. No external POR circuit is required.

After POR the AX5243 can be reset by first setting the SPI SEL pin to high for at least

100 ns, then setting followed by resetting the bit RST in the

PWRMODE register.

After POR or reset all registers are set to their default values.

5.7. RF Frequency Generation Subsystem

The RF frequency generation subsystem consists of a fully integrated synthesizer, which

multiplies the reference frequency from the crystal oscillator to get the desired RF

frequency. The advanced architecture of the synthesizer enables frequency resolutions of

1 Hz, as well as fast settling times of 5 – 50 s depending on the settings (see section

4.3: AC Characteristics). Fast settling times mean fast start-up and fast RX/TX switching,

which enables low-power system design.

For receive operation the RF frequency is fed to the mixer, for transmit operation to the

power-amplifier.

The frequency must be programmed to the desired carrier frequency.

The synthesizer loop bandwidth can be programmed, this serves three purposes:

1. Start-up time optimization, start-up is faster for higher synthesizer loop

bandwidths

2. TX spectrum optimization, phase-noise at 300 kHz to 1 MHz distance from the

carrier improves with lower synthesizer loop bandwidths

3. Adaptation of the bandwidth to the data-rate. For transmission of FSK and MSK it

is required that the synthesizer bandwidth must be in the order of the data-rate.

VCO

An on-chip VCO converts the control voltage generated by the charge pump and loop

filter into an output frequency. This frequency is used for transmit as well as for receive

operation. The frequency can be programmed in 1 Hz steps in the

FREQ registers. For

operation in the 433 MHz band, the RFDIV bit in the

PLLVCODIV register must be

programmed.

The fully integrated VCO allows to operate the device in the frequency ranges 800 –

1050 MHz and 400 – 525 MHz.

The carrier frequency range can be extended to 54 – 525 MHz and 27 – 262 MHz by

using an appropriate external inductor between device pins L1 and L2. The bit VCO2INT

in the

PLLVCODIV register must be set high to enter this mode.

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