MICRF505BML datasheet(25/27 Pages) MICREL

25 / 27 page

May 13, 2004

M9999-051304

MICRF505

Micrel

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:

Baudrate

DEV

OFFSET

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 fre-

quency of the crystal oscillator and is often used in combina-

tion 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.

Figure 20: Crystal oscillator’s external components

If the value in XCO_tune is increased (adding capacitance),

the frequency will decrease.

The XCO uses two external capacitors. 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 (almost) equally divided

between Òincrease frequencyÓ and Òdecrease frequencyÓ.

That is, XCO_tune values greater than approx 16 will de-

crease frequency, and XCO_tune values less than approx 16

will increase frequency.

Figure. 21 XCO Tuning

A procedure for using the XCOtuning 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 imple-

mented 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+dwn pulses, counting 8 bits:

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 a number in the range -128 , ... , 127

However, it keeps counting above/below the range, that 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 |FEE|)

Local variables:

-60,0

-40,0

-20,0

0,0

20,0

40,0

60,0

80,0

100,0

XCO bitvalue

2x1.5pF

2x0pF

PIN 24, XTALOUT

XTALIN, PIN 23

C10

1.5pF

C11

1.5pF

TSX-10A

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