ATA5425-PLSW datasheet(18/100 Pages) ATMEL

18 / 100 page

4841D–WIRE–10/07

ATA5423/ATA5425/ATA5428/ATA5429

3.10

Frequency Synthesizer

The synthesizer is a fully integrated fractional-N design with internal loop filters for receive and

transmit mode. The XTO frequency f

XTO is the reference frequency FREF for the synthesizer.

The bits FR0 to FR12 in control registers 2 and 3 (see Table 7-7 on page 39 and Table 7-10 on

page 40) are used to adjust the deviation of f

XTO. In transmit mode, at 433.92 MHz, the carrier

has a phase noise of –111 dBC/Hz at 1 MHz and spurious emissions at FREF of –66 dBC with a

high PLL loop bandwidth allowing the direct modulation of the carrier with 20 Kbit/s Manchester

data. Due to the closed loop modulation any spurious emissions caused by this modulation are

effectively filtered out as can be seen in Figure 3-9 on page 20. In RX mode the synthesizer has

a phase noise of –120 dBC/Hz at 1 MHz and spurious emissions of –75 dBC.

The initial tolerances of the crystal oscillator due to crystal tolerances, internal capacitor toler-

ances and the parasitics of the board have to be compensated at manufacturing setup with

control registers 2 and 3 as can be seen in Table 4-1 on page 28. The other control words for the

synthesizer needed for ASK, FSK and receive/transmit switching are calculated internally. The

RF (Radio Frequency) resolution is equal to the XTO frequency divided by 16384 which is

777.1 Hz at 315.0 MHz, 851.1 Hz at 345.0 MHz, 808.9 Hz at 433.92 MHz, 818.6 Hz at

868.3 MHz and 862.6 Hz at 915.0 MHz.

For the multi-channel system the frequency control word FREQ in control registers 2 and 3 can

be programmed in the range of 1000 to 6900, this is equivalent to a programmable tuning range

of ±2.5 MHz hence every frequency within the 315 MHz, 345 MHz, 433 MHz, 868 MHz and

915 MHz ISM bands can be programmed as receive and as transmit frequency, and the position

of channels within these ISM bands can be chosen arbitrarily (see Table 4-1).

Care must be taken as to the harmonics of the CLK output signal as well as to the harmonics

produced by a microprocessor clocked with it, since these harmonics can disturb the reception

of signals. In a single-channel system, using FREQ = 3803 to 4053 ensures that harmonics of

this signal do not disturb the receive mode.

3.11

FSK/ASK Transmission

Due to the fast modulation capability of the synthesizer and the high resolution, the carrier can

be internally FSK modulated, which simplifies the application of the transceiver. The deviation of

the transmitted signal is ±20 digital frequency steps of the synthesizer which is equal to

±15.54 kHz for 315 MHz, ±17.02 kHz for 345 MHz, ±16.17 kHz for 433.92 MHz, ±16.37 kHz for

868.3 MHz and ±17.25 kHz for 915 MHz.

Due to closed loop modulation with PLL filtering the modulated spectrum is very clean, meeting

ETSI and CEPT regulations when using a simple LC filter for the power amplifier harmonics as it

is shown in Figure 2.2 on page 8. In ASK mode the frequency is internally connected to the cen-

ter of the FSK transmission and the power amplifier is switched on and off to perform the

modulation. Figure 3-7 on page 19 to Figure 3-9 on page 20 show the spectrum of the FSK mod-

ulation with pseudo-random data with 20 Kbit/s/±16.17 kHz/Manchester and 5 dBm output

power.

ATA5425-PLSW Datasheet(PDF) 18 Page - ATMEL Corporation

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