MICRF506BML datasheet(22/42 Pages) MICREL

22 / 42 page

Micrel

MICRF506

July 2006

M9999-092904

+1 408-944-0800

FEE

A6..A0

0010101

FEEC_3

FEEC_2

FEEC_1

FEEC_0

0010110

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 it’s 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 temperature drift and aging.

The inputs 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 number of

pulses for every received symbol is 2 times the

modulation index (∆).

The FEE can operate in three different modes;

counting only UP-pulses, only DN-pulses or counting

UP+DN pulses. The number of received symbols to

be counted is 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

0000000 and 1111111. 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)x(FEE-∆Fp)

Mode DN:

Foffset = R/(2P)x(FEE+∆Fp)

Mode UP+DN: Foffset = R/(4P)x(FEE)

where FEE is the value stored in the FEE register,

(Fp is the single sided frequency deviation, P is the

number 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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