MICRF505L datasheet(27/43 Pages) MICREL

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Micrel

MICRF505BML/YML

August 2006

M9999-092904

+1 408-944-0800

Modulator

A6..A0

0000100

Mod_F2

Mod_F1

Mod_F0

Mod_I4

Mod_I3

Mod_I2

Mod_I1

Mod_I0

0000101

‘0’

‘1’

Mod_A3

Mod_A2

Mod_A1

Mod_A0

0000110

Mod_clkS2

Mod_clkS1

Mod_clkS0

BitSync_clkS2

BitSync_clkS1

BitSync_clkS0

BitRate_clkS2

0000111

BitRate_clkS1

BitRate_clkS0

RefClk_K5

RefClk_K4

RefClk_K3

RefClk_K2

RefClk_K1

RefClk_K0

The modulator will create a waveform with

programmable amplitude and frequency. This

waveform is fed into a modulation varactor in the

VCO, which will create the desired frequency

modulation. The frequency spectrum can be

narrowed by increasing the rise-and fall times of the

waveform.

The modulator waveform is created by charging and

discharging a capacitor. A modulator clock controls

the timing, as shown in Figure19. For every rise-and

fall edge, 4 clock periods are being used. The

charging current during these 4 clock periods are not

equal, this is to reduce the high frequency

components in the waveform, which in turn will

narrow the frequency spectrum.

The frequency deviation can be set in three different

ways, as will be explained below. A formula for

setting the desired deviation is given at the end of

this chapter.

Modulator Clock

Modulator Waveform

Figure 19. Modulator Waveform and Clock

Modulator Clock

The modulator clock frequency is set by:

)

XCO

MOD_CLK

Refclk_K

Mod_clkS

−

where fMOD_CLK is the modulator clock shown in

Figure 19, fXCO is the crystal oscillator frequency

Refclk_K is a 6 bit number and Mod_clkS is a 3 bit

number. Mod_clkS can be set to a value between 0

and 7. The modulator clock frequency should be set

according to the bit rate and shaping.

Mod_clka

Mod_clkb

Mod_clkb > Mod_clka

Figure 20. Two Different Modulator Clock Setting

A fMOD_CLK of 8 times the bit rate (as in Figure 20)

corresponds to a signal filtered in a Gaussian filter

with a Bandwidth Period product (BT) of 1. When BT

is increased, the waveform will be less filtered.

Minimum BT is 1 (fMOD_CLK is 8 times the bitrate).

Figure 20 shows two waveforms with BT=1 and

BT=2, i.e. the fMOD_CLK is 8 and 16 times higher than

the bit rate. When changing the BT factor, the

charge-and discharge times will also be changed,

and therefore the frequency deviation, as shown in

Figure 19.

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 21 shows two waveforms

generated with two different currents, where

Mod

. Higher current will give a

higher frequency deviation and vice versa. The

effect of modulator clock and MOD_1 is illustrated

by:

MOD_CLK

DEVIATION

MOD_1

∝

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.

MICRF505L Datasheet(PDF) 27 Page - Micrel Semiconductor

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