MICRF507 datasheet(26/46 Pages) MICREL

26 / 46 page

Micrel, Inc.

MICRF507

March 2010

M9999-032210-B

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). It has two

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.

FEEC_1

FEEC_0

FEE Mode

Off

Do not use

Counting UP and DN pulses. UP

increments the counter, DN

decrements it.

FEEC_3

FEEC_2

No. of bits used for the

measurement

Do not use

Table 15. FEEC (Frequency Error Estimator Control) Bits

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:

If FEE < 128 then {FEESIGNED = FEE}

Else {FEESIGNED = (FEE - 256)},

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:

SIGNED

OFFSET

FEE

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 rb is the bit rate. When

fOFFSET is zero, the transmitter and receiver are perfectly

aligned. A positive fOFFSET means that the received signal

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.

MICRF507 Datasheet(PDF) 26 Page - Micrel Semiconductor

MICRF507 Datasheet(HTML) 26 Page - Micrel Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Datasheet Download

Link URL