AX8052F131

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21

TRANSMITTER

The transmitter block is controllable through its registers,

which are mapped into the X data space of the

micro−controller. The transmitter block features its own

32 word

× 10 bit FIFO. The microcontroller can either be

interrupted at a programmable FIFO fill level, or one of the

DMA channels can be instructed to transfer between XRAM

and the transmitter FIFO.

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

ms depending on the settings (see section AC

Characteristics). Fast settling times mean fast start−up,

which enables low−power system design.

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.

The frequency can be programmed in 1 Hz steps in the

AX5031_FREQ registers. For operation in the 433 MHz

band, the BANDSEL bit in the AX5031_PLLLOOP

register must be programmed.

VCO Auto−Ranging

The AX8052F131 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 AX5031_PLLRANGING register.

The bit is readable and a 0 indicates the end of the ranging

process. The RNGERR bit indicates the correct execution of

the auto−ranging.

Loop Filter and Charge Pump

The AX8052F131 internal loop filter configuration

together with the charge pump current sets the synthesizer

loop band width. The loop−filter has three configurations

that can be programmed via the register bits FLT[1:0] in

register AX5031_PLLLOOP, the charge pump current can

be programmed using register bits PLLCPI[1:0] also in

register AX5031_PLLLOOP. Synthesizer bandwidths are

typically

50

–

500

kHz

depending

on

the

AX5031_PLLLOOP settings, for details see the section:

AC Characteristics.

Registers

Table 16. REGISTERS

Register

Bits

Purpose

AX5031_PLLLOOP

FLT[1:0]

Synthesizer loop filter bandwidth, recommended usage is to increase the bandwidth for faster

settling time, bandwidth increases of factor 2 and 5 are possible.

PLLCPI[2:0]

Synthesizer charge pump current, recommended usage is to decrease the bandwidth (and

improve the phase−noise) for low data−rate transmissions.

BANDSEL

Switches between 868 MHz / 915 MHz and 433 MHz bands

AX5031_FREQ

Programming of the carrier frequency

AX5031_FREQB

FREQSEL = 1

AX5031_PLLRANGING

Initiate VCO auto−ranging and check results

RF Input and Output Stage (ANTP/ANTN)

The AX8052F131 uses fully differential antenna pins.

The PA drives the signal generated by the frequency

generation subsystem out to the differential antenna

terminals. The output power of the PA is programmed via

bits TXRNG[3:0] in the register AX5031_TXPWR. Output

power as well as harmonic content will depend on the

external impedance seen by the PA, recommendations are

given in section: Antenna Interface Circuitry.

Encoder

The encoder is located between the Framing Unit and the

Modulator. It can optionally transform the bit−stream in the

following ways:

zero is transmitted as no change in the level, and a one

is transmitted as a change in the level. Differential