MICRF505BML datasheet(20/27 Pages) MICREL

20 / 27 page

MICRF505

Micrel

M9999-051304

May 13, 2004

FEE

A6..A0

0000001

—

FEEC_3

FEEC_2

FEEC_1

FEEC_0

0000001

FEE_7

FEE_6

FEE_5

FEE_4

FEE_3

FEE_2

FEE_1

FEE_0

The Frequency Error Estimator (FEE) uses information from

the demodulator to calculate the frequency offset between its

receive frequency and the transmitter frequency. The output

of the FEE can be used to tune the XCO frequency, both for

production calibration and for compensation for crystal tem-

perature drift and aging.

The input to the FEE circuit are the up and down pulses from

the demodulator. Every time a ‘1’ is updated, an UP-pulse is

coming out of the demodulator, and the same with the DN-

pulse every time the ‘0’ is updated. The expected no. of

pulses for every received symbol are 2 times the(modulation

index (

D).

The FEE can operate in three different modes; counting only

UP-pulses, only DN-pulses or counting UP+DN pulses. The

no. of received symbols to be counted are either 8, 16, 32 or

64. This is set by the FEEC_0..FEEC_3 control bit, as follows:

FEEC_1

FEEC_0

FEE Mode

Off

Counting UP pulses

Counting DN pulses

Counting UP and DN pulses. UP

increments the counter, DN decrements it.

FEEC_3

FEEC_2

No. of symbols used for the

measurement

Table 10. FEEC Control Bit

The result of the measurement is the FEE value, this can be

read from register with address 0010110b. Negative values

are stored as a binary no between 00000000 and 11111111.

To calculate the negative value, a two’s complement of this

value must be performed. Only FEE modes where DN-pulses

are counted (10 and 11) will give a negative value.

When the FEE value has been read, the frequency offset can

be calculated as follows:

Mode UP:

Foffset = R/(2P)

¥(FEE -DFp

Mode DN:

Foffset = R/(2P)

¥(FEE +DFp

Mode UP+DN:

Foffset = R/(4P)

¥(FEE

where FEE is the value stored in the FEE register, (Fp is the

single sided frequency deviation, P is the no. of symbols/data

bit counted and R is the symbol/data rate. A positive Foffset

means that the received signal has a higher frequency than

the receiver frequency. To compensate for this, the receivers

XCO frequency should be increased ( see ANNEX A ) on how

to tune the XCO frequency based on the FEE value).

It is recommended to use Mode UP+DN for two reasons, you

do not need to know the actual frequency deviation and this

mode gives the best accuracy.

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