ax5042-1 AXSEM, ax5042-1 Datasheet - Page 22

• Manufacturer Part Number: ax5042-1
• Description: Rf Transceiver
• Manufacturer: AXSEM

22
Circuit Description
5.6.
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:
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
433 MHz band, the BANDSEL bit in the
VCO Auto-Ranging
The
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
The RNGERR bit indicates the correct execution of the auto-ranging.
Version 2.2
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
3. Adaptation of the bandwidth to the data-rate. For transmission of FSK, GFSK and MSK
AX5042
RF Frequency Generation Subsystem
improves with lower synthesizer loop bandwidths
it is required that the synthesizer bandwidth must be in the order of the data-rate.
has an integrated auto-ranging function, which allows to set the correct VCO
register. The bit is readable and a 0 indicates the end of the ranging process.
PLLLOOP
register must be programmed.
FREQ
registers. For operation in the
Datasheet AX5042