MICRF506BML datasheet(28/42 Pages) MICREL

28 / 42 page

Micrel

MICRF506BML/YML

The modulator filter will not influence on the

frequency deviation as long as the programmed cut-

off frequency is above the actual bit rate.

The frequency deviation must be programmed so

that the modulation index (2 x single sided frequency

deviation/Baudrate [bps]) always is greater than or

equal to 2 including the total frequency offset

between the receiver and the transmitter:

fDEV = Baudrate + fOFFSET

The calculated fDEV should be used to calculate the

needed receiver bandwidth, see chapter Switched

capacitor filter.

Using the XCO-tune Bits

The RF chip has a built-in mechanism for tuning the

frequency of the crystal oscillator and is often used

in combination with the Frequency Error Estimator

(FEE). The XCO tuning is designed to eliminate or

reduce initial frequency tolerance of the crystal

and/or the frequency stability over temperature. If

the value in XCO_tune is increased (adding

capacitance), the frequency will decrease.

The XCO uses two external capacitors (see figure

5). The value of these will strongly affect the tuning

range. With a 16.0 MHz crystal (TN4-26011 from

Toyocom), and external capacitor values of 1.5 pF,

the tuning range will be approximately symmetrical

around the center frequency. A XCO_tune >16 will

decrease the frequency and vice versa (see figure

6).

A procedure for using the XCO_tune feature in

combination with the FEE is given below. The

MICRF506 measures the frequency offset between

the demodulated signal and the LO and tune the

XCO so the LO frequency is equal to received

carrier frequency.

A procedure like this can be called during production

(storing the calibrated XCO_tune value), at regular

intervals or implemented in the communication

protocol when the frequency has changed.

The FEE can count “UP”-pulses and/or “DOWN”-

pulses (pulses out of the demodulator when a logic

“1” or logic “0”, resp.., is received). The FEE can

count pulses for n bits, where n = 8, 16, 32 or 64.

Example: In FEE, count up+down pulses, counting 8

bits:

July 2006

M9999-092904

+1 408-944-0800

A perfect case ==> FEE = 0

If FEE > 0: LO is too low, increase LO by decreasing

XCO_tune value

v.v. for FEE < 0

FEE field holds a number in the range -128, … ,

127. However, it keeps counting above/below the

range, which is:

If FEE = -128 and still counting dwn-pulses:

1) =>-129 = +127

2) 126

3) 125

…

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