MICRF221 datasheet(9/26 Pages) MICREL

9 / 26 page

Micrel Inc.

MICRF221

October 2008

M9999-100108

(408) 955-1690

The reference oscillator in the MICRF221 uses a

basic Pierce crystal oscillator configuration with MOS

transconductor

provide

negative

resistance.

MICRF221 has built-in load capacitors for the crystal

oscillator, shown in Figure 4, even though external

load capacitors are still needed for tuning to the right

frequency. RO1 and RO2 are external pins of the

MICRF221 and are to connect to the reference

oscillator crystal.

MODE: Desense

No Desense - default

7dB Desense

19dB Desense

32dB Desense

42dB Desense

The reference oscillator crystal frequency can be

calculated as follows:

Reference Control

There are two components in the Reference and

Control sub-block: 1) Reference Oscillator and 2)

Control Logic, Serial Interface and Parallel Inputs.

FREF OSC = FRF/(64 + 1.1/12)

For 868.35 MHz,

Reference Oscillator

FREF OSC = 13.54856 MHz

V BIAS

RO2

RO1

For 915 MHz

FREF OSC = 14.27643 MHz

To operate the MICRF221 with minimum offset,

crystal frequencies should be specified with 10pF

loading capacitance.

Figure 4. MICRF221 Reference Oscillator Circuit

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