MICRF505BML datasheet(22/42 Pages) MICREL

22 / 42 page

Micrel

MICRF505

October 2006

M9999-103106

+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 DN

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 be set in an off state or set to count

the UP+DN pulses. The number of received symbols

to be counted is 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

Not in use

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.

The 8bit value can be read from register with

address 0010110b. After a load, the FEE will be

reset to 0, and will then be incremented or

decremented dependent of the frequency offset.

Positive FEE values are in the range 0 – 127, and

negative values in the range 128 – 255. For negative

values, the twos complement of the FEE value need

to be found before using the formula below. A FEE =

128 corresponds to ÷127 and FEE = 255

corresponds to ÷1.

When the FEE value has been read, the frequency

offset can be calculated as follows:

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

where FEE is the value stored in the FEE register,

(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

‘Using the XCO-tune Bits on how to tune the XCO

frequency based on the FEE value).

The most accurate reading will be achieved when

counting many symbols/data (P). However, attention

must be paid to the negative representation of the

FEE value. If the frequency offset is too large for the

chosen P, and the FEE is being incremented above

+127, the FEE value will be interpreted as a

negative number. If so, the P must be decremented.

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