MICRF507 datasheet(27/46 Pages) MICREL

27 / 46 page

Micrel, Inc.

MICRF507

March 2010

M9999-032210-B

XCOtune Procedure Example

A procedure such as the algorithm given below can be

called during production (storing the calibrated XCO_tune

value), at regular intervals, or implemented in the

communication protocol when the frequency has changed.

The MICRF507 measures the frequency offset between

the demodulated signal and the LO, and a micro-controller

programs the XCO_tune bits to tune the XCO so the LO

frequency is equal to received carrier frequency.

Procedure description: A transmitter is assumed to be

sending a 1010… pattern at the correct frequency and bit

rate.

The FEE is enabled (FEEC[1:0] = 11) and the

number P of bit periods used in the measurement is 8 or

16. Only the sign of FEE is used.

Objective: The best XCO_tune value (giving the lowest

IFEEI). The desired frequency of the receiver’s PLL is

midway between the “0” and “1” frequencies.

Local variables:

XCO_Present: (5-bit) holds current value in XCO_tune

field

XCO_Step: (4-bit) holds amount by which XCO_tune will

be incremented or decremented

XCO_Sign: (1 bit) has a value of either POS or NEG,

determining respectively whether XCOtune is to be

incremented

decremented

(reducing

XCOtune

increases LO frequency)

XCO_tune_bits is a buffer, which is written to the XCOtune

field when ProgramRFChip is called.

XCO TUNE PROCEDURE

Initialization:

XCO_Present = 16;

XCO_Step = 16;

XCO_Sign = NEG;

Registers properly set for reception;

LOOP:

XCO_Step = XCO_Step/2;

If (XCO_Sign == POS) then

// If POS then increase LO frequency:

{XCO_present = XCO_present - XCO_Step}

Else

// If NEG then decrease LO frequency:

{XCO_present = XCO_present + XCO_Step}

;

XCO_tune_bits = XCO_Present;

ProgramRFChip;

Wait for > P bit periods;

Read FEE;

If (FEE > 0?) then

{XCO_Sign = POS}

else

{XCO_Sign = NEG} // negative or 0

;

If (XCO_Step > 1) then

{Branch to LOOP}

Else

{If(XCO_Sign == POS) then

{XCO_Present = XCO_Present – 1;

Return (XCO_Present)}; \\ done

}

;

MICRF507 Datasheet(PDF) 27 Page - Micrel Semiconductor

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