MICRF219A datasheet(18/23 Pages) MICREL

18 / 23 page

Micrel, Inc.

MICRF219A

July 2011

M9999-071811-A

RadioTech@micrel.com or (408) 944-0800

Crystal Selection

The crystal resonator provides a reference clock for all

the device internal circuits. Crystal tolerance needs to

be chosen such that the down-converted signal is

always inside the IF bandwidth of MICRF219A. From

this consideration, the tolerance should be

±50ppm on

both the transmitter and the MICRF219A side. The ESR

should be less than 300Ω, and the temperature range of

the crystal should match the range required by the

application. With the Abracon crystal listed in the Bill of

Materials, a typical MICRF219A crystal oscillator still

starts up at 105ºC with additional 400Ω series

resistance.

The oscillator of the MICRF219A is a Pierce-type

oscillator. Good care must be taken when laying out the

printed circuit board. Avoid long traces and place the

ground plane on the top layer close to the REFOSC pins

RO1 and RO2. When care is not taken in the layout, and

the crystals used are not verified, the oscillator may not

start or takes longer to start. Time-to-good-data will be

longer as well.

PCB Considerations and Layout

The MICRF219A evaluation board is a good starting

point for prototyping of most applications. The Gerber

files are downloadable from the Micrel website and

contain the remaining layers needed to fabricate this

board. When copying or making one’s own boards,

make the traces as short as possible. Long traces alter

the matching network and the values suggested are no

longer valid. Suggested matching values may vary due

to PCB variations. A PCB trace 100 mils (2.5mm) long

has about 1.1nH inductance. Optimization should always

be done with range tests. Make sure the individual

ground connection has a dedicated via rather then

sharing a few of ground points by a single via. Sharing

ground via will increase the ground path inductance.

Ground plane should be solid and with no sudden

interruptions. Avoid using ground plane on top layer next

to the matching elements. It normally adds additional

stray capacitance which changes the matching. Do not

use Phenolic materials as they are conductive above

200MHz. Typically, FR4 or better materials are

recommended. The RF path should be as straight as

possible to avoid loops and unnecessary turns.

Separate ground and VDD lines from other digital or

switching power circuits (such microcontroller…etc).

Known sources of noise should be laid out as far as

possible from the RF circuits. Avoid unnecessary wide

traces which would add more distribution capacitance

(between top trace to bottom GND plane) and alter the

RF parameters.

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