MICRF506BML datasheet(16/42 Pages) MICREL

16 / 42 page

Micrel

MICRF506BML/YML

The lengths of the N, M, and A registers are 12, 12

and 6 respectively The values can be calculated

from the following formula:

()

VCO

XCO

PhD

M≠0

≤ A < N

where

fPhD: Phase detector comparison frequency

fXCO: Crystal oscillator frequency

fVCO: Voltage controlled oscillator frequency

fRF: RF carrier frequency

There are two sets of each of the divide factors (i.e.

A0 and A1). If modulation by using the dividers is

selected (that is Modulation1=1, Modulation0=0), the

two sets should be programmed to give two RF

frequencies, separated by two times the specified

frequency deviation. For all other modulation

methods, and also in receive mode, the 0-set will be

used.

Crystal Oscillator (XCO)

Adr

0001001

‘0’

‘1’

XCOtune4 XCOtune3 XCOtune2 XCOtune1

XCOtune0

July 2006

M9999-092904

+1 408-944-0800

The crystal oscillator is a very critical block. As the

crystal oscillator is a reference for the RF output

frequency and also for the LO frequency in the

receiver, very good phase and frequency stability is

required. The schematic of the crystal oscillator’s

external components for 16MHz are shown in Figure

C11

5.6pF

C10

5.6pF

TSX-10A

Pin 24

XTALOUT

Pin 23

XTALIN

Figure 5. Crystal Oscillator Circuit

The crystal should be connected between pins

XTALIN and XTALOUT (pin 23 and 24). In addition,

loading capacitors for the crystal are required. The

loading capacitor values depend on the total load

capacitance, CL, specified for the crystal. The load

capacitance seen between the crystal terminals

should be equal to CL for the crystal to oscillate at

the specified frequency.

CL =

C10

C11

+ C

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.

It is also possible to tune the crystal oscillator

internally by switching in internal capacitance using

5 tune bits XCOtune4 – XCOtun0. 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 6 shows the

tuning range for two different capacitor values, 1.5pF

and no capacitors.

The crystal used is a TN4-26011 from Toyocom.

Specification: Package TSX-10A, Nominal frequency

16.000000 MHz, frequency tolerance ±10ppm,

frequency stability ±9ppm, load capacitance 9pF,

pulling sensitivity 15ppm/pF. When the external

capacitors are set to 1.5pF and the XCOtune=16,

the total capacitance will normally be ~9pF.

-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.

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