SX1223 데이터 시트보기 (PDF) - Semtech Corporation
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SX1223 Datasheet PDF : 25 Pages
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SX1223
4.2.3 Bit Rate Setting for MW1 and MW2
The bit rate is set by first dividing the crystal oscillator frequency by an integer in the range [1..63] in a
programmable divider, then this frequency is divided further by powers of two. The equation describing the bit rate
as a function of RefClk_K and BRn is
BR
=
Re
f XTAL
fClk _ K ⋅ 23+BRn
where:
fXTAL:
RefClk_K:
BRn:
Crystal oscillator frequency.
Integer in the range [1..63] (6 bit).
Integer in the range [0..5] (3 bit).
(4.1)
A procedure to determine the settings for the desired bit rate is described below:
1. Set BRn to 0.
2.
Calculate
RefClk_K
by
using
this
formula:
Re
fClk
_K
=
f XTAL
BR ⋅ 23+BRn
.
3. If RefClk_K is too high, increment n by one, and repeat step 2, above
In some cases several combinations of RefClk_K and BRn will provide the required bit rate. In these cases BRn
should be chosen with the following in mind: A lower BRn offers better waveform shaping or spectral efficiency, but
may cause modulator saturation at some bit rates.
4.2.4 Deviation Setting for MW1 and MW2
Frequency deviation is controlled by user parameters RefClk_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 RefClk_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 (4.2).
fDEV
∝
Re fClk _ K
f XTAL
(4.2)
It is assumed that K will be constant for most applications to keep bit-rate and shaping constant, although this is not
a requirement.
The control parameters of the 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 RefClk_K. The relationship is shown in
equation (4.3).
f DEV
∝
MOD _ I
2 MOD _ A
Finally, the VCO gain is given by equation (4.4).
KVCO
=
Const1
+
(Const2 ⋅ fC ⋅ (2 −
2 − FreqBand
FreqBand ))
(4.3)
(4.4)
where:
Const1:
Const2:
fC:
FreqBand:
− 30.6324 ×109
54.7
Carrier frequency.
Frequency band. 0: 400MHz and 1: 900MHz.
© Semtech 2007
www.semtech.com
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