73M1903C Datasheet, PDF (44/46 Pages) Teridian Semiconductor Corporation – Modem Analog Front End

73M1903C
Modem Analog Front End
DATA SHEET
After a few rounds of simplification this ratio reduces to:
Fvco / Fxtal = (
4
35
)•(
18
1
)
Nnco1
4
=
Dnco1
Nnco2
=
35
1
Dnco2
18
, where Nnco1 and Nnco2 must be < or equal to 8.
The ratio, Nnco1/Dnco1 = 4/35, is used to form a divide ratio for the NCO in pre-scaler and Nnco2/Dnco2
=1/18 for the NCO in the PLL.
Pre-scaler NCO: From Nnco1/Dnco1 = 4/35,
Pdvsr = Integer [ Dnco1/Nnco1 ] = 8;
Prst[2:0] = Nnco1 – 1 = 3;
Dnco1/Nnco1 = 35/4 = 8.75 suggests a divide sequence of {÷9,÷9,÷9,÷8}, or
Pseq = {x,x,x,x,1,1,1,0} = xDh.
PLL NCO: From Nnco2/Dnco2 = 1/18,
Ndvsr = Integer [ Dnco2/Nnco2 ] = 18;
Nrst[2:0] = Nnco2 – 1 = 0; this means NO fractional divide. It always does ÷18. Thus Pseq becomes
“don’t care”.
Nseq = {x,x,x,x,x,x,x,x} = xxh.
Example3:
Crystal Frequency = 27MHz; Desired Sampling Rate, Fs = 7.2kHz
Step 1. First compute the required VCO frequency, Fvco, corresponding to
Fs = 2.4kHz x 3 = 7.2kHz.
Fvco = 2 x 2304 x Fs = 2 x 2304 x 2.4kHz x 3 = 33.1776MHz.
Step 2. Express the required VCO frequency divided by the Crystal Frequency as a ratio of two integers.
This is initially given by :
Fvco/ Fxtal =
2 • 2304 • 2.4kHz• 3
27 MHz
.
After a few rounds of simplification this reduces to:
Fvco / Fxtal = (
8
125
)•(
96
5
)
.
Nnco1
8
=
Dnco1
Nnco2
=
125
5
Dnco2
96
The two ratios are not unique and many other possibilities exist. But for this particular application, they
are found to be the best set of choices within the constraints of Prst and Nrst allowed. (Nnco1, Nnco2
must be less than or equal to 8.)
Page: 44 of 46
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