MICRF505BML datasheet(28/42 Pages) MICREL

28 / 42 page

Micrel

MICRF505BML/YML

October 2006

M9999-103106

+1 408-944-0800

The modulator filter will not influence 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

MICRF505 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 will count “UP”-pulses and “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:

A perfect case ==> FEE = 0

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

XCO_tune value and vice versa 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

…

To avoid this situation, always make sure max count

is between limits. Suggestion: Count for 8 (or 16)

bits only.

Procedure description:

In the procedure below, UP+DWN pulses are

counted, and only the sign of the FEE is used. The

value of n is 8 or 16.

Assumption:

A transmitter is sending a 1010… pattern at the

correct frequency and bitrate.

The wanted receiver frequency is the mid-point

between the “0” and “1” frequencies.

Input:

Nothing

Output

The

best

XCO_tune

value

(giving

the

lowest IFEEI)

Local variables:

XCO_Present: (5-bit) holds present value

in XCO_tune bits

XCO_Step:

(4-bit)

holds

increment/decrement of XCO_tune bits

SCO_Sign:

bit)

holds

POS

NEG

(increment/cerement)

increasing

done by reducing the XCO_tune value

XCO TUNE PROCEDURE

INT:

XCO_Present = 0

XCO_Step = 32

XCO_Sign = NEG

Control_Word =

Default RX, clocks match transmitter

LOOP:

XCO_Step = XCO_Step/2

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