ATA5811 datasheet(22/90 Pages) ATMEL

22 / 90 page

ATA5811/ATA5812 [Preliminary]

4689B–RKE–04/04

Note that if matching to 50

Ω, like in Figure 5 on page 7, a high Q wire wound inductor

with a Q > 70 should be used for L2 to minimize its contribution to RX losses which will

otherwise be dominant. The RX and TX losses will be in the range of 1.0 dB there.

XTO

The XTO is an amplitude regulated Pierce oscillator type with integrated load capaci-

tances (2

× 18 pF with a tolerance of ±17%) hence C

Lmin = 7.4 pF and CLmax = 10.6 pF.

The XTO oscillation frequency fXTO is the reference frequency FREF for the fractional-N

synthesizer. When designing the system in terms of receiving and transmitting fre-

quency offset the accuracy of the crystal and XTO have to be considered.

The synthesizer can adjust the local oscillator frequency for more than ±150 ppm at

433.92 MHz/315 MHz and up to ±118 ppm at 868.3 MHz of initial frequency error in

fXTO. This is done at nominal supply voltage and temperature with the control registers 2

and 3 (see Table 20 on page 35 and Table 23 on page 36). The remaining local oscilla-

tor tolerance at nominal supply voltage and temperature is then < ±0.5 ppm. A XTO

frequency error of ±150 ppm/±118 ppm can hence be tolerated due to the crystal toler-

ance at 25°C and the tolerances of CL1 and CL2. The XTO’s gm has very low influence of

less than ±2 ppm on the frequency at nominal supply voltage and temperature.

Over temperature and supply voltage, the XTO's additional pulling is only ±2 ppm if

Cm ≤7 fF. The XTAL versus temperature and its aging is then the main source of fre-

quency error in the local oscillator.

The XTO frequency depends on XTAL properties and the load capacitances C

L1, 2 at pin

XTAL1 and XTAL2. The pulling of f

XTO from the nominal fXTAL is calculated using the fol-

lowing formula:

ppm.

m is the crystal's motional, C0 the shunt and CLN the nominal load capacitance of the

XTAL found in its data sheet. C

L is the total actual load capacitance of the crystal in the

circuit and consists of C

L1 and CL2 in series connection.

Figure 18. XTAL with Load Capacitance

Table 11. Impedance RF_OUT Pin in RX Mode

Frequency

Z(RF_OUT)RX

RP//CP

315

MHz

Ω −j 502 Ω

7 k

Ω / / 1.0 pF

433.92

MHz

Ω −j 366 Ω

7 k

Ω / / 1.0 pF

868.3

MHz

2.8

Ω −j 141Ω

7 k

Ω / / 1.3 pF

--------

–

()

-------------------------------------------------------------

XTAL

L = CL1

L2/(CL1 + (CL2)

Crystal equivalent circuit

ATA5811 Datasheet(PDF) 22 Page - ATMEL Corporation

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