ATA5428-PLSW datasheet(14/100 Pages) ATMEL

14 / 100 page

4841D–WIRE–10/07

ATA5423/ATA5425/ATA5428/ATA5429

3.4

Frequency Accuracy of the Crystals

The XTO is an amplitude regulated Pierce oscillator with integrated load capacitors. The initial

tolerances (due to the frequency tolerance of the XTAL, the integrated capacitors on XTAL1,

XTAL2 and the XTO’s initial transconductance gm) can be compensated to a value within

±0.5 ppm by measuring the CLK output frequency and programming the control registers 2 and

3 (see Table 7-7 on page 39 and Table 7-10 on page 40). The XTO then has a remaining influ-

ence of less than ±2 ppm over temperature and supply voltage due to the band gap controlled

gm of the XTO.

The needed frequency stability of the used crystals over temperature and aging is hence

±58 kHz/315 MHz – 2

× ±2.5 ppm = ±179.2 ppm for 315 MHz,

±58 kHz/345 MHz – 2

× ±2.5 ppm = ±163.2 ppm for 345 MHz,

±58kHz/433.92MHz –2

× ±2.5 ppm = ±128.6 ppm for 433.92 MHz,

±58 kHz/868.3 MHz – 2

× ±2.5 ppm = ±61.8 ppm for 868.3 MHz and

±58 kHz/915 MHz – 2

× ±2.5 ppm = ±58.4 ppm for 915 MHz.

Thus, the used crystals in receiver and transmitter each need to be better than ±89.6 ppm for

315 MHz, ±81.6 ppm for 345 MHz, ±64.3 ppm for 433.92 MHz, ±30.9 ppm for 868.3 MHz and

±29.2 ppm for 915 MHz. In access control systems it may be advantageous to have a more tight

tolerance at the Base-station in order to relax the requirement for the remote control unit.

3.5

RX Supply Current versus Temperature and Supply Voltage

Table 3-5 shows the typical supply current at 433.92 MHz of the transceiver in RX mode versus

supply voltage and temperature with VS = VS1 = VS2. As can be seen, the supply current at

2.4 V and –40°C is less than the typical supply current; this is useful because this is also the

operation point where a lithium cell has the worst performance. The typical supply current at

315 MHz, 345 MHz, 868.3 MHz or 915 MHz in RX mode is about the same as for 433.92 MHz.

3.6

Blocking, Selectivity

As can be seen in Figure 3-3 and Figure 3-4 on page 15, the receiver can receive signals 3 dB

higher than the sensitivity level in the presence of very large blockers of –47 dBm/–34 dBm with

small frequency offsets of ±1

/ ±10 MHz.

Figure 3-3 shows narrow band blocking and Figure 3-4 wide band blocking characteristics. The

measurements were done with a signal of 433.92 MHz/FSK/20 Kbit/s/±16 kHz/ Manchester, and

with a level of –106 dBm + 3 dB = –103 dBm which is 3 dB above the sensitivity level. The fig-

ures show how much larger than –103dBm a continuous wave signal can be before the BER is

higher than 10

–3. The measurements were done at the 50

Ω input according to Figure 3-1 on

page 11

. At 1 MHz, for example, the blocker can be 56 dB higher than –103 dBm which is

-103 dBm + 56 dB = –47 dBm. These values, together with the good intermodulation perfor-

mance, avoid the need for a SAW filter in the remote control unit application.

Table 3-5.

Measured 433.92 MHz Receive Supply Current in FSK Mode

VS = VS1 = VS2

2.4V

3.0V

3.6V

amb = –40°C

8.4 mA

8.8 mA

9.2 mA

amb = 25°C

9.9 mA

10.3 mA

10.8 mA

amb = 85°C

10.9 mA

11.3 mA

11.8 mA

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