MICRF506 datasheet(14/28 Pages) MICREL

14 / 28 page

Micrel

MICRF506

September 2004

M9999-092904

(408) 955-1690

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

approximately 280µA.

XCO Bitvalue

Start-up Time (ms)

590

700

810

1140

2050

Table 7. Typical values with C

EXT = 1.5pF

If an external reference is 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

D7D6D5

0000011

‘1’

‘0’

VCO_IB2

VCO_IB1

VCO_IB0

VCO_freq1

VCO_freq0

The VCO has no external components. If 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

410MHz

410-423MHz

423-436MHz

436-450MHz

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 7.

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

1.4V, the VCO gain is at its maximum, approximately

32 to 35MHz/V. It is recommended that the varactor

voltage stays in this range.

The input capacitance at the varactor pin must be

taken into consideration 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.

VCO frequency gain, Vdd=2.5V

380

390

400

410

420

430

440

450

460

470

480

0.4

0.8

1.2

1.6

2.4

V_varactor [V]

Figure 7. RF Frequency vs. Varactor Voltage

and VCO Frequency bit (V

DD = 2.25V)

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