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From equation (4.4), it can be seen that VCO is proportional to carrier frequency. MOD_I is the best parameter to
alter to counteract this effect if necessary.
Combining equations (4.2), (4.3), and (4.4) gives an expression for the frequency deviation:
RefClk_K MOD _ I Const 1 + ( Const 2 ? f C ? ( 2 ? FreqBand ) )
? f =
? ?
f XTAL 2 MOD _ A 2 ? FreqBand
(4.5)
4.2.5 Shaping for MW1 and MW2
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:
BT = 2 BRn
(4.6)
It can be seen from this equation that a low BRn gives a low shaping factor.
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 (4.7).
MOD _ F ≤
150 × 10 3
BR
(4.7)
4.2.6 Modulator Saturation for MW1 and MW2
The modulator output voltage is generated with 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. The maximum value of MOD_I can be
determined by using equation (4.8).
? 28 × 10 ? 6 ? ? + 1
?
MOD _ I ≤ ? ?
?
f XTAL
Re fClk _ K
?
?
(4.8)
If it turns out that the MOD_I -range is too small for the application, the solution can be found by increasing BRn and
decreasing RefClk_K accordingly.
4.2.7 Summary of Modulator Settings for MW1 and MW2
The necessary equations needed for the use of the modulator are (4.1), (4.5), (4.6), (4.7), and (4.8). The table
below gives a summary of the meaning of the parameters.
Symbol
RefClk_K
BRn
MOD_F
MOD_I
MOD_A
Range
(inclusive)
1..63
0..5
0..3
1..31
0..4
Explanation
The crystal oscillator frequency is divided by this number to produce the modulator
clock, and it is divided further down by 8 to produce bit rate clock.
Number of extra divide-by-two for the bit rate clock.
Programmable smoothing filter after attenuator. This can be programmed in four
steps, and will produce reasonable results for the highest bit rates.
Frequency deviation. The deviation is linearly dependent on this variable.
Frequency deviation attenuator (or range selector). The attenuations are (values 0
through 4, respectively)
1 , 1 , 1 , 1 , and 1
1 2 4 8 16
.
Table 5: Modulator settings
4.2.8 Frequency deviation setting for MW3
In MW3 the modulation is done by switching between two frequency divider ratio sets: M 0 , N 0 , A 0 and M 1 , N 1 , A 1 .
The frequency f 0 will be generated using the first set and frequency f 1 using the second set, corresponding to the
transmission of the two possible values ‘0’ and ‘1’.
? Semtech 2007
13
www.semtech.com
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