MICRF405 datasheet(31/46 Pages) MICREL

31 / 46 page

Micrel

MICRF405

April 2006

M9999-041906

(408) 955-1690

The two first blocks are generating a clock for the

modulator. This clock is, together with the user data,

used to control a charge pump. The charge pump

current is controlled by a DAC. Each time the input

data changes state, a charge is then injected into

the capacitor to generate a modulation signal. The

charge magnitude is controlled by the charging

current and by charging time (inversely proportional

with modulator clock). To be able to achieve small

deviations, it is possible to attenuate the modulation

signal. Finally, the signal is filtered to narrow

transmitter output spectrum.

The procedure is first to determine the settings

concerning the data bit rate, then, these values will

be used in the calculation of the frequency deviation.

Finally, the user must see if the desired values

cause the modulator to saturate.

Deviation Setting

Deviation controlled by user parameters FSKClk_K,

MOD_I, and MOD_A, together with physical

parameters fXTAL and KVCO. All user parameters

can be set in software, and fXTAL (crystal oscillator

frequency) is set when designing in the radio chip.

KVCO (VCO gain) is a parameter of the radio chip,

and is not controllable by the user.

The crystal oscillator frequency, fXTAL, is divided by

FSKClk_K to generate the modulator clock. Since

this modulator clock is controlling the rise and fall

times for the modulator, the frequency deviation is

inversely proportional to this clock. The relationship

is shown in equation (3):

XTAL

DEV

FSKClk_K

∝

(3)

It is assumed that FSKClk_K will be constant for

most applications to keep bit-rate and shaping

constant, although this is not a requirement.

The primary two controls of frequency deviation are

MOD_I and MOD_A. Of these two, MOD_I is the

parameter that controls the signal generation, while

MOD_A controls attenuation of this signal. The

reason for using an attenuator is to be able to

generate small deviations at high values of

FSKClk_K. The relationship is shown in equation

(4).

MOD

DEV

MOD

∝

(4)

Finally, the VCO gain is given by equation (5).

()

FreqBand

Const

VCO

−

⋅

(5)

where:

Const1

6324

−

Const2

fC: Carrier frequency of the radio.

FreqBand: Frequency band.

0: 315MHz,

1: 433MHz and

2: 900MHz.

In equation (5), it is evident that the VCO gain is

dependent of carrier frequency. MOD_I is probably

the best parameter to alter if counteracting this effect

if necessary.

Combining equations (3), (4), and (5) gives us an

expression for the frequency deviation:

()

FreqBand

Const

MOD

XTAL

DEV

−

⋅

FSKClk_K

(6)

Observe that equation (6) gives single-sided-

deviation. Peak-to-peak deviation is twice this value.

Shaping

The modulation waveform will be shaped due to the

charging and discharging of a capacitor. The

waveform looks like a Gaussian filtered signal with a

Bandwidth

⋅Period-product, BT, given by:

FSKn

(7)

where:

BT: Shaping factor.

It is evident from this that a low FSKn gives a low

shaping factor, and is thus preferred if it is possible

to choose FSKn freely.

In addition to this, it is possible to smooth the

modulator output in a programmable low-pass filter.

This filter is controlled by the parameter MOD_F.

The parameter should be set according to equation

(8).

MOD

150

≤

(8)

Modulator Saturation

The modulator output voltage is generated by a

capacitor that is being charged. This means that

there is a risk of saturating the modulator if the

charge received by the capacitor is too large. Use

equation (9) to determine the maximum value of

MOD_I that can be used.

6 +

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⋅

≤

−

FSKClk_K

MOD

XTAL

(9)

If it turns out that the MOD_I-range is too small for

your requirements, try increasing FSKn and

decreasing FSKClk_K accordingly.

MICRF405 Datasheet(PDF) 31 Page - Micrel Semiconductor

MICRF405 Datasheet(HTML) 31 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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Datasheet Download

Link URL