MICRF506BML datasheet(25/28 Pages) MICREL

25 / 28 page

Micrel

MICRF506

September 2004

M9999-092904

(408) 955-1690

where INT() returns the integer part of the argument.

Mod_la

Mod_lb

Mod_la > Mod_lb

Figure 21. Two Different Modulator Current Settings

Modulator Attenuator

A third way to set the deviation is by programming

the modulator attenuator, Mod_A2..Mod_A0, the last

being LSB. The purpose of the attenuator is to allow

small deviations when the bit rate is small and/or the

BT is small (these settings will give a relatively slow

modulator clock, and therefore long rise- and fall

times, which in turn results in large frequency

deviations). In addition, the attenuator will improve

the resolution in the modulator.

Mod_Aa

Mod_Ab

Mod_Ab > Mod_Ab

Figure 22. Two Different Modulator Attenuator

Settings

The effect of the attenuator is given by:

DEVIATION

1+ Mod_A

- Figure 22 shows two waveforms with different

attenuator setting:

Mod_Ab

M od_A a <

. If mod_A

is increased, the frequency deviation is lowered and

vice versa.

Modulator Filter

To reduce the high-frequency components in the

generated waveform, a filter with programmable cut-

off frequency can be enabled. This is done using

Mod_F2..Mod_F0, the least one being LSB. The

Mod_F should be set according to the formula:

Mod_filter on

Mod_filter off

Figure 23. Modulator Waveform with and without

Filtering

Mod_F=0 disables the modulator filter and Mod_F=7

gives most filtering. Figure 22 shows a waveform

with and without the filter.

Calculation of the Frequency Deviation

The parameters influencing the frequency deviation

can be summarized in the following equations:

MOD_CLK =

XCO

Refclk_K 2

Mod_clkS )

DEV =

Mod_I

MOD_CLK

1+ Mod_A

1 + C2

()

Where:

fDEV:

Single sided frequency deviation

[Hz]

fXCO:

Crystal oscillator frequency [Hz]

fRF:

Center frequency [Hz]

Refclk_K:

6 bit divider, values between 1

and 63

Mod_clkS:

Modulator clock setting, values

between 0 and 7

fMOD_CLK:

Modulator clock frequency,

derived

from

the

crystal

frequency,

Refclk_K

and

Mod_clkS

Mod_I:

Modulator

current

setting,

values between 0 and 31

Mod_A:

Modulator attenuator setting,

values between 0 and 15

C1:

-2.17_10

C2:

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