MICRF507 datasheet(31/46 Pages) MICREL

31 / 46 page

Micrel, Inc.

MICRF507

March 2010

M9999-032210-B

Frequency Deviation under VCO Modulation

Three factors determine the deviation at which the data

stream is modulated: modulator clock frequency, capacitor

charge/discharge current, and modulator attenuator

setting. After each is presented in turn, the complete

formula for deviation is given.

The modulator clock frequency is determined by the

crystal oscillator frequency and the settings of the

Refclk_K and Mod_clkS fields:

Mod_clkS)

XCO

MOD_CLK

Refclk_K

Set the modulator clock frequency to either 8x or 16x the

frequency deviation rate.

Figure 20. Two Different Modulator Clock Settings

Having fMOD_CLK set at 8 times the bit rate corresponds to a

baseband data signal filtered in a Gaussian filter with a

bandwidth-period product (BT) of 1. When BT is increased,

the waveform will be less filtered. Figure 20 shows two

waveforms with BT=1 (the minimum, with fMOD_CLK at 8

times the bitrate) and BT=2 (with fMOD_CLK at 16 times the

bit rate). Changing BT changes the charge-and discharge

times and therefore the frequency deviation, as seen in

Figure 20.

The capacitor charge/discharge current, set with the Mod_I

field, affects deviation, as shown in Figure 21, where

Mod_Ia>Mod_Ib. Higher current will give a higher

frequency deviation and vice versa. The effect of

modulator clock and Mod_I is as follows:

MOD_CLK

DEV

Mod_I

∝

To avoid saturation in the modulator, it is important not to

exceed maximum Mod_I. Maximum Mod_I for a given

fMOD_CLK is given by:

)

INT(f

Mod_I

-6

MOD_CLK

MAX

⋅

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

Figure 21. Two Different Modulator Current Settings

A third factor determining deviation is the modulator

attenuator

, controlled by the Mod_A field. The attenuator is

used when the bit rate is small and/or the BT is small,

which gives a relatively slow modulator clock and therefore

long rise- and fall times, leading in turn to large frequency

deviations unless the signal is attenuated. Additionally, the

attenuator will improve the resolution in the modulator.

Figure 22. Two Different Modulator Attenuator Settings

The effect of the attenuator is given by:

Mod_A

DEV

∝

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