ATA5425-PLSW datasheet(25/100 Pages) ATMEL

25 / 100 page

4841D–WIRE–10/07

ATA5423/ATA5425/ATA5428/ATA5429

XTO

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

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

Lmin = 7.4 pF and CLmax = 10.6 pF. The XTO oscil-

lation frequency f

XTO is the reference frequency FREF for the fractional-N synthesizer. When

designing the system in terms of receiving and transmitting frequency offset, the accuracy of the

crystal and XTO have to be considered.

The synthesizer can adjust the local oscillator frequency for the initial frequency error in f

XTO.

This is done at nominal supply voltage and temperature with the control registers 2 and 3 (see

Table 7-7 and Table 7-10). The remaining local oscillator tolerance at nominal supply voltage

and temperature is then < ±0.5 ppm. The XTO’s gm has very low influence of less than ±2 ppm

on the frequency at nominal supply voltage and temperature.

In a single channel system less than ±150 ppm should be corrected to avoid that harmonics of

the CLK output disturb the receive mode. If the CLK is not used or if it is carefully laid out on the

application PCB (as needed for multi channel systems), more than ±150 ppm can be

compensated.

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

versus temperature and its aging is then the main source of frequency 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 following 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 con-

sists of C

L1 and CL2 in series connection.

Figure 4-1.

XTAL with Load Capacitance

With C

m ≤14 fF, C0 ≥ 1.5 pF, CLN = 9 pF and CL = 7.4 pF to 10.6 pF, the pulling amounts to

≤±100 ppm and with C

m ≤7fF, C0 ≥ 1.5 pF, CLN = 9 pF and CL = 7.4 pF to 10.6 pF, the pulling

is P

≤ ±50 ppm.

Since typical crystals have less than ±50 ppm tolerance at 25°C, the compensation is not criti-

cal, and can in both cases be done with the ±150 ppm.

--------

–

()

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

CL2

CL1

CL = CL1 × CL2/ (CL1 + CL2)

XTAL

Crystal equivalent circuit

ATA5425-PLSW Datasheet(PDF) 25 Page - ATMEL Corporation

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