AX5042 datasheet(21/42 Pages) ONSEMI

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Circuit Description

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AX5042

5.6.

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 μs 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 RF frequency shift

by the IF frequency that is required for RX operation, is automatically set when the receiver is

activated and does not need to be programmed by the user. The default IF frequency is 1

MHz. It can be programmed to other values. Changing the IF frequency and thus the centre

frequency of the digital channel filter can be used to adapt the blocking performance of the

device to specific system requirements.

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

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

2. TX spectrum optimisation, 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, GFSK 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 BANDSEL bit in the PLLLOOP register must be programmed.

VCO Auto-Ranging

The AX5042 has an integrated auto-ranging function, which allows to set the correct VCO

range for specific frequency generation subsystem settings automatically. Typically it has to

be executed after power-up. The function is initiated by setting the RNG_START bit in the

PLLRANGING register. The bit is readable and a 0 indicates the end of the ranging process. If

the bit RNGERR is 0, then the auto-ranging has been executed successfully.

AX5042 Datasheet(PDF) 21 Page - ON Semiconductor