MICRF405 datasheet(21/46 Pages) MICREL

21 / 46 page

Micrel

MICRF405

April 2006

M9999-041906

(408) 955-1690

Crystal Oscillator (XCO)

Adr

Data

A6..A0

0010101 ClkOut_1=0 ClkOut_0=0 XCO_Fast=1

XCOtune4=1

XCOtune3=0

XCOtune2=0

XCOtune1=0

XCOtune0=0

The crystal oscillator is a very critical block. As the

crystal oscillator is a reference for the RF output

frequency, very good phase and frequency stability

is required. When selecting crystal it should be paid

special attention to the total frequency tolerance and

load capacitance as these will directly influence on

the carrier frequency.

XTB, pin 7

XTA, pin 8

Figure 9. Crystal Oscillator Circuit.

The crystal should be connected between pins XTA

and XTB (pin 7 and 8). MICRF405 has an internal

crystal capacitor bank used for crystal tolerance

tuning during production. These internal capacitors

can be enabled using the XCOtune[4:0] bits. If

XCOtune[4:0]=0 then no internal capacitors are

connected to the crystal pins, while 18pF are

connected to each pin if XCOtune[4:0]=31. The unit

capacitance is about 0.6pF. The internal XCOtune

feature is optimized for a crystal with a load

capacitance of 9pF and will give the expected

oscillation frequency when no external capacitors

are connected and XCOtune[4:0]=16. If a crystal

requires

higher

load

capacitance,

additional

capacitors must be added off-chip (C1 and C2 in

Figure 9).

If XCOtune[4:0]=0, the loading capacitors can be

calculated by the following formula;

parasitic

The

parasitic

capacitance

the

input

capacitance and PCB stray capacitance. Typically

the total parasitic capacitance is around 6pF. For

instance, for a 9pF load crystal the recommended

values of the external load capacitors are 5.6pF.

The start-up time of a crystal oscillator is typically

around a millisecond. Therefore, to save current

consumption, the XCO is turned on before any other

circuit block. During start-up the XCO amplitude will

eventually reach a sufficient level to trigger the M-

counter. After counting 2 M-counter output pulses

the rest of the circuit will be turned on. The current

consumption during the prestart period is typically

205

µA. If the XCO_Fast bit is set, then XCO will

start up faster, typically in about 300

µs. This comes

at the expense of a higher current consumption of

typically 2mA during the period from start up until the

first output pulse of the M-divider.

If an external reference shall be used instead of a

crystal, the signal shall be applied to pin 7, XTB.

Due

internal

biasing,

coupling

recommended for use between the external

reference and the XTB-pin.

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