MICRF507 datasheet(16/46 Pages) MICREL

16 / 46 page

Micrel, Inc.

MICRF507

March 2010

M9999-032210-B

Crystal Oscillator (XCO)

The crystal oscillator’s role as the synthesizer reference

demands very good phase and frequency stability. As

shown in Figure 7, the external components required for

the oscillator are a crystal, connected between pins 23

and 24, and loading capacitors.

Pin 24

XtalOut

Pin 23

XtalIn

1.5pF

TN4-26011

Figure 7. Crystal Oscillator Circuit

The load capacitance CL seen between the crystal

terminals is:

XCOtune

Where CXCOtune is the capacitance of the internal

adjustable capacitor bank, and Cpin is defined as the

internal chip capacitance when XCOtune bits are all

zeros, plus PCB stray capacitance across pins 23 and

24. The value of Cpin is about 6pF. The loading capacitor

values required depend on the total CL specified for the

crystal for oscillation at the desired frequency.

It is possible to tune the crystal oscillator internally by

giving the 5-bit register field XCOtune a non-zero value,

which causes internal capacitors to be switched across

the crystal. As this capacitance increases, frequency

decreases. When XCOtune is set to its maximum value

of 31, approximately 4.5pF additional capacitance is

connected across the crystal pins.

The XCO tuning can be used to cancel crystal resonant

frequency error, both initial and with temperature. It can

be used in combination with the Frequency Error

Estimator (FEE). See “FEE” section.

The crystal used is a TN4-26011 from Toyocom.

Specification:

•

Package TSX-10A,

•

Nominal frequency 16.000000MHz

•

Frequency tolerance ±10ppm

•

Frequency stability ±9ppm, load capacitance

9pF

•

Pulling sensitivity 15ppm/pF

To achieve 9pF load capacitance required to center

TN4-26011 at 16MHz, set the external capacitors to

1.5pF and XCOtune=16dec. Figure 8 shows the tuning

range for two different capacitor values, 1.5pF and zero

(external capacitors omitted). External capacitor values

will strongly affect the tuning range. Using 1.5pF with the

above crystal gives a tuning range that is approximately

symmetrical about the center frequency.

-60,0

-40,0

-20,0

0,0

20,0

40,0

60,0

80,0

100,0

XCO bitvalue

2x1.5pF

2x0pF

Figure 8. XCO Tuning with the XCOtune Field

The start-up time of the cystal oscillator, given in Table

4, is about a millisecond and increases with capacitance.

When the MICRF507’s main mode is switched from

Power down mode to Transmit mode via Standby mode,

or to Receive mode via Standby mode, only the XCO is

energized at first. Current consumption during this

prestart period is approximately 280μA (the same as for

Standby mode). After the XCO amplitude is sufficient to

trigger the M-counter and produce two pulses at its

output, the remaining circuits on the chip are powered

on.

XCOtune

Start-up Time (µs)

590

700

810

1140

2050

Table 4. Typical Crystal Oscillator Start-up Time

with C8 = C9 = 1.5pF

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