MICRF507 Micrel Semiconductor, MICRF507 Datasheet - Page 26

• Manufacturer Part Number: MICRF507
• Description: 470MHz to 510MHz Low-Power FSK Transceiver
• Manufacturer: Micrel Semiconductor

Another use for RSSI is to determine that transmit power
can be reduced in a system. If the RSSI detects a strong
signal, the transmitter could be alerted to reduce its
transmit power and so reduce current consumption.
FEE
The Frequency Error Estimator (FEE) counts pulses from
inside the demodulator to measure the frequency offset
between it’s receive frequency and the transmitter’s
frequency. The maximum offset that FEE can correctly
report a frequency difference is about ±20ppm. The output
of the FEE can be used to tune the XCO frequency, both
for production calibration and to compensate for crystal
temperature drift and aging.
The FEE is enabled when FEEC[1:0] = 11, and is off when
FEEC[1:0] = 00 (do not use other values).
counters. One counter determines the measurement
period by generating a trigger every time it has counted up
the number of bit periods selected by the setting of
FEEC[3:2] as given in Table 15. A second counter
accumulates the net tally of UP and DN pulses from the
demodulator. For each incoming ‘1’ bit, UP carries an
average number of pulses that is twice the modulation
index (β), and likewise for each incoming ‘0’ bit and DN.
The trigger transfers the contents of the second counter to
the FEE register and clears it, after which it begins
accumulating again.
March 2010
Micrel, Inc.
Table 15. FEEC (Frequency Error Estimator Control) Bits
FEEC_1
FEEC_3
0
0
1
1
0
0
1
1
FEEC_0
FEEC_2
0
1
0
1
0
1
0
1
FEE Mode
Off
Do not use
Do not use
Counting UP and DN pulses. UP
increments the counter, DN
decrements it.
No. of bits used for the
measurement
8
16
Do not use
Do not use
It has two
26
The result of the measurement, read from the FEE register
at address 22 (16 hex, 0010110 binary), is actually an
eight-bit two’s complement signed number, which can
have values from -128 to 127. Reading FEE gives the
most recently transferred value. The raw FEE value,
treated as an unsigned number ranging from 0 to 255, can
be converted into a signed integer as follows:
For a straightforward measurement of frequency offset, the
incoming signal has to have an equal number of ones and
zeros, such as a 1010… preamble. The frequency offset
can then be calculated:
where FEE is the value read from the FEE register (treated
as a signed number), P is the number of data bits over
which the count is taken, and r
f
aligned. A positive f
has a higher frequency than the carrier frequency to which
the receiver is tuned. To compensate for this, the
receiver’s XCO frequency should be increased by reducing
XCO_tune bits as detailed in the “Crystal Oscillator (XCO)”
section.
Counting a larger number of symbols (higher P) improves
the accuracy of the measurement. Beware, however, of
overflow, which can cause the FEE value to jump from
+127 to -128 after only one excessive count.
If the frequency offset is too large for the chosen P, then P
must be reduced. P = 8 or P = 16 is safest.
OFFSET
is zero, the transmitter and receiver are perfectly
If FEE < 128 then {FEE
Else {FEE
f
OFFSET
=
4P
r
SIGNED
b
OFFSET
FEE
= (FEE - 256)},
SIGNED
means that the received signal
SIGNED
b
is the bit rate. When
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= FEE}
M9999-032210-B
MICRF507