MICRF506 datasheet(27/42 Pages) MICREL

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Micrel

MICRF506BML/YML

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:

fDEVIATION ×

+ Mod_A

Figure 22 shows two waveforms with different

attenuator setting:

. If Mod_A

is increased, the frequency deviation is lowered and

vice versa.

M o d _ A b

M o d _ A a

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:

fMOD_CLK =

fXCO

Refclk_K

× 2(7−Mod_clkS )

fDEV =

Mod_I

fMOD_CLK

+ Mod_A

× C

1 + C2 × fRF

()

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:

July 2006

M9999-092904

+1 408-944-0800

MICRF506 Datasheet(PDF) 27 Page - Micrel Semiconductor

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