AX5243 datasheet(26/65 Pages) ONSEMI

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Circuit Description

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AX5243

5.2. Crystal Oscillator and TCXO Interface

The AX5243 is normally operated with an external TCXO, which is required by most

narrow-band regulation with a tolerance of 0.5 ppm to 1.5 ppm depending on the

regulation. The on-chip crystal oscillator allows the use of an inexpensive quartz crystal

as the RF generation subsystem’s timing reference when possible from a regulatory point

of view.

A wide range of reference frequencies can be handled by the crystal oscillator circuit. As

the reference frequency impacts both the spectral performance of the transmitter as well

as the current consumption of the receiver, the choice of reference frequency should be

made according to the regulatory regime targeted by the application. For guide-lines see

the separate Application Notes for usage of AX5243 in compliance with various

regulatory regimes.

The crystal or TCXO reference frequency should be chosen so that the RF carrier

frequency is not an integer multiple of the crystal or TCXO frequency.

The oscillator circuit is enabled by programming the

PWRMODE register. At power-up it

is enabled.

To adjust the circuit’s characteristics to the quartz crystal being used, without using

additional external components, the tuning capacitance of the crystal oscillator can be

programmed. The transconductance of the oscillator is automatically regulated, to allow

for fastest start-up times together with lowest power operation during steady-state

oscillation.

The integrated programmable tuning capacitor bank makes it possible to connect the

oscillator directly to pins CLK16N and CLK16P without the need for external capacitors. It

is programmed using bits XTALCAP[5:0] in register

XTALCAP.

To synchronize the receiver frequency to a carrier signal, the oscillator frequency could

be tuned using the capacitor bank however, the recommended method to implement

frequency synchronization is to make use of the high resolution RF frequency generation

sub-system together with the Automatic Frequency Control, both are described further

down.

Alternatively a single ended reference (TXCO, CXO) may be used. For detailed TCXO

network recommendations depending on TCXO output swing refer to the AX5243

Application Note: Use with a TXCO Reference Clock.

5.3. Low Power Oscillator and Wake on Radio (WOR) Mode

The AX5243 features an internal lowest power fully integrated oscillator. In default

mode the frequency of oscillation is 640 Hz +/- 1.5%, in fast mode it is 10.2 kHz +/-

1.5%.

These accuracies are reached after the internal hardware has been used to

calibrate the low power oscillator versus the RF reference clock. This procedure can be

run in the background during transmit or receive operations.

The low power oscillator makes a WOR mode with a power consumption of 500nA

possible.

AX5243 Datasheet(PDF) 26 Page - ON Semiconductor

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