Micrel MICRF506, Руководство пользователя

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Micrel MICRF506
July 2006 22 M9999-092904
+1 408-944-0800
FEE
A6..A0 D7 D6 D5 D4 D3 D2 D1 D0
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
0 0 Off
0 1 Counting UP pulses
1 0 Counting DN pulses
1 1 Counting UP and DN pulses. UP increments
the counter, DN decrements it.
FEEC_3 FEEC_2 No. of symbols used for the measurement
0 0 8
0 1 16
1 0 32
1 1 65
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.