ATA5745 datasheet(14/43 Pages) ATMEL

14 / 43 page

4596A–RKE–05/06

ATA5745/ATA5746 [Preliminary]

As can be seen in Figure 2-9 on page 13, for single devices there is a variance over temperature

and supply voltage range of ±3 dB. The total variance over production, temperature, and supply

voltage range is ±9 dB.

2.9

Frequency Synthesizer

The LO generates the carrier frequency for the mixer via a PLL synthesizer. The XTO (crystal

oscillator) generates the reference frequency f

XTO. The VCO (voltage-controlled oscillator) gen-

erates the drive voltage frequency f

LO for the mixer. fLO is divided by the factor 24 (ATA5746) or

32 (ATA5745). The divided frequency is compared to f

XTO by the phase frequency detector. The

current output of the phase frequency detector is connected to the fully integrated loop filter, and

thereby generates the control voltage for the VCO. By means of that configuration, the VCO is

controlled in a way, such that f

LO /24 (fLO / 32) is equal to fXTO. If fLO is determined, fXTO can be

calculated using the following formula: f

XTO =fLO / 24 (fXTO =fLO / 32). The synthesizer has a

phase noise of –130 dBC/Hz at 3 MHz and spurs of –75 dBC.

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

produced by a microprocessor clocked using the signal, as these harmonics can disturb the

reception of signals.

XTO

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

× 16 pF). Due to additional internal and board parasitics (C

P) of approximately 2 pF on each

side, the load capacitance amounts to 2

× 18 pF (9 pF total).

The XTO oscillation frequency f

XTO is the reference frequency for the integer-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 XTO’s additional pulling (including the R

M tolerance) is only ±5 ppm. The XTAL versus tem-

perature, aging, and tolerances 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 (p) of f

XTO from the nominal fXTAL is calculated using the following for-

mula:

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

capacitance of the XTAL, are found in the datasheet. C

L is the total actual load capacitance of

the crystal in the circuit, and consists of C

L1 and CL2 connected in series.

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–

()

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-6

ppm

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