MICRF505BML datasheet(14/27 Pages) MICREL

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MICRF505

Micrel

M9999-051304

May 13, 2004

parasitic

The parasitic capacitance is the pin input capacitance and

PCB stray capacitance. Typically the total parasitic capaci-

tance is around 6pF. For instance, for a 9pF load crystal the

recommended values of the external load capacitors are

5.6pF.

It is also possible to tune the crystal oscillator internally by

switching in internal capacitance using 5 tune bits XCOtune4

- XCOtune0. When XCOtune4 - XCOtune0 = 0 no internal

capacitors are connected to the crystal pins. When XCOtune4

- XCOtune0 = 1 all of the internal capacitors are connected to

the crystal pins. Figure 3 shows the tuning range for two

different capacitor values, these are 1.5pF and no capacitors.

The crystal used is a TN4-26011 from Toyocom. Specifica-

tion: Package TSX-10A, Nominal frequency 16.000000 MHz,

frequency tolerance

±10ppm, frequency stability ±9ppm,

load capacitance 9pF, pulling sensitivity 15ppm/pF.

-60,0

-40,0

-20,0

0,0

20,0

40,0

60,0

80,0

100,0

XCO bitvalue

2x1.5pF

2x0pF

Figure 6. XCO Tuning

The start up time is given in Table 7. As can be seen, more

capacitance will slow down the start up time.

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 approximately

280

mA.

XCO Bitvalue

Start-up Time (ms)

590

700

810

1140

2050

Table 7. Typical values with

CEXT= 1.5pF

If an external reference shall be used instead of a crystal, the

signal shall be applied to pin 24, XtalOut. Due to internal DC

setting in the XCO, an AC coupling is recommended to be

used between the external reference and the XtalOut-pin.

VCO

A6..A0

0000011

‘1’

‘0’ VCO_IB2 VCO_IB1 VCO_IB0 VCO_freq1VCO_freq0

The VCO has no external components. It has three bit to set

the bias current and two bit to set the VCO frequency. These

five bit are set by the RF frequency, as follows:

RF freq.

VCO_IB2

VCO_IB1

VCO_IB0 VCO_freq1 VCO_freq0

850MHz

1110

868MHz

0110

915MHz

0011

950MHz

0001

Table 8. VCO Bit Setting

The bias bit will optimize the phase noise, and the frequency

bit will control a capacitor bank in the VCO. The tuning range,

the RF frequency versus varactor voltage, is dependent on

the VCO frequency setting, and can be shown in Figure 4.

When the tuning voltage is in the range from 0.9V to 1.4V, the

VCO gain is at its maximum, approximately 65 to 70MHz/V.

It is recommended that the varactor voltage stays in this

range.

The input capacitance at the varactor pin must be taken into

considerations when designing the PLL loop filter. This is

most critical when designing a loop filter with high bandwidth,

which gives relatively small component values. The input

capacitance is approximately 6pF.

Tuning range

800

820

840

860

880

900

920

940

960

980

1000

0,5

1,5

2,5

Varactor voltage (V)

'00'

'01'

'10'

'11'

Figure 7. RF Frequency vs. Varactor Voltage

and VCO Frequency bit (VDD = 2.25V)

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