MRF49XA-ISLASHT datasheet(49/102 Pages) MICROCHIP

49 / 102 page

Preliminary

DS70590C-page 49

MRF49XA

3.6

Crystal Selection Guidelines

The crystal oscillator of MRF49XA requires a 10 MHz

Parallel mode crystal. The circuit contains an

integrated load capacitor in order to minimize the

external

component

count.

The

internal

load

capacitance value is programmable from 8.5 pF –

16 pF in 0.5 pF steps. With appropriate PCB layout, the

total load capacitance value can be 10 pF – 20 pF, so a

variety of crystal types can be used.

When the total load capacitance is not more than 20 pF,

and a worst case 7 pF Shunt Capacitance (CS) value is

expected for the crystal, the oscillator is able to start-up

with any crystal having less than 100

Ω Equivalent

Series Loss Resistance (ESR). However, the low CS

and ESR values ensure the faster oscillator start-up.

The Crystal Frequency (fref) is used as the reference of

the PLL, which generates the Local Oscillator

Frequency (fLO). Therefore, fLO is directly proportional to

fref. The accuracy requirements for production tolerance,

temperature drift and aging can thus be determined from

the maximum allowable local oscillator frequency error.

Whenever a low-frequency error is essential for the

application, it is possible to “pull” the crystal to the

accurate frequency by changing the load capacitor

value. The widest pulling range can be achieved if the

nominal required load capacitance of the crystal is in

the “midrange”; for example, 16 pF. The “pull-ability” of

the crystal is defined by its Motional Capacitance (Cm)

and shunt capacitance.

FIGURE 3-5:

MAXIMUM CRYSTAL TOLERANCES INCLUDING TEMPERATURE AND AGING (ppm)

105

120

433 MHz

100

868 MHz

30405060

915 MHz

30405050

Deviation [± kHz]

Bit Rate: 9.6 kbps

105

120

433 MHz

100

868 MHz

915 MHz

Deviation [± kHz]

Bit Rate: 2.4 kbps

105

120

433 MHz

Do Not Use

868 MHz

Do Not Use

915 MHz

Do Not Use

Deviation [± kHz]

Bit Rate: 38.4 kbps

105

120

135

150

165

180

195

433 MHz

Do Not Use

868 MHz

Do Not Use

915 MHz

Do Not Use

Deviation [± kHz]

Bit Rate: 115.2 kbps

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