MICRF505BML datasheet(24/27 Pages) MICREL

24 / 27 page

MICRF505

Micrel

M9999-051304

May 13, 2004

Modulator Current

The current used during the rise- and fall times can be

programmed with the Mod_I4..Mod_I0 bit, the last one being

LSB. Figure 15 shows two waveforms generated with two

different currents, where Mod_Ia > Mod_Ib. Higher current

will give a higher frequency deviation and vice versa. The

effect of modulator clock and MOD_1 is illustrated by

To avoid saturation in the modulator it it important not to

exceed maximum Mod_I. Max Mod_I for a given fMOD_clk is

given by:

where min() returns the value of the smallest argument and

int() returns the integer part of the argument.

where int(x): integer and and f_MOD_CLK: Modulater clk

frequency.

Mod_la

Mod_lb

Mod_la > Mod_lb

Figure 17. 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 purposes 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 reso-

lution in the modulator.

Mod_Aa

Mod_Ab

Mod_Ab > Mod_Ab

Figure 18. Two different modulator attenuator settings

The attenuation is given by:

where VMOD2 and VMOD1 is the modulator amplitude after

and before the attenuator, respectively. Figure 16 shows two

waveforms with different attenuator setting; Mod_Aa <

Mod_Ab. 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 last

one being LSB. The Mod_F should be set according to the

formula:

MOD_F

BitRate

150 10

Mod_filter on

Mod_filter off

Figure 19. Modulator Waveform with and Without

Filtering

Mod_F=0 disables the modulator filter and Mod_F=7 gives

most filtering. Figure 18 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:

Refclk_K

MOD_CLK

XCO

¥ ¥

()

Mod

clkS

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

MOD_1

DEVIATION

MOD_CLK

MOD_Imax

INT(f_MOD_CLK 28e - 6) - 1

◊

1 MOD_A

MOD2

MOD_I

1 MOD_A

DEV

MOD_CLK

◊

◊ -

◊◊

()

44 28 10 72

MICRF505BML Datasheet(PDF) 24 Page - Micrel Semiconductor

MICRF505BML Datasheet(HTML) 24 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

Datasheet Download

Link URL