DATA SHEET • SKY74038 FREQUENCY SYNTHESIZER
Case 2:
To achieve a desired F
VCO_RF
frequency of 1400 MHz using a crystal frequency of 13 MHz with operation
of the synthesizer in RF mode using the 8/9 prescaler (PS = 0). The crystal frequency does not need to be
divided further, since the maximum comparison frequency is 15 MHz. Divide the operating frequency by the
internal reference frequency to determine the value of N
Total
:
N
Total
= 1400 MHz
13 MHz
=107.6923076
Subtract 3.5 from N
Total
and remove the fractional portion of the result to determine N1:
107.6923076 – 3.5 = 104.1923076
N1 = 104 (decimal)
Fractional portion = 0.1923076 (decimal)
N1 = 000001101000 (binary)
D
21
------------D
10
Register Address 11
2
Multiply the fractional portion of N1 by 256 and remove the fractional portion of the result to determine FN:
0.1923076 × 256 = 49.230746
FN = 49 (decimal)
FN = 00110001 (binary)
D
9
-------D
2
Register Address 11
2
Divide FN by 256 to determine the actual fractional portion:
49 = 0.1914962
256
Subtract this result from the fractional portion of N1:
0.1923076 – 0.1914062 = 0.0009014
Multiply this result by 2097152 (the 21-bit
∆Σ
modulator value 2
21
) and remove the fractional portion to determine
the ME:
0.0009014 × 2097152 = 1890.3728
ME = 1890 (decimal)
ME = 0011101100010 (binary)
D
21
--------------D
9
Register Address 01
2
In this example, N1 is greater than 64, the minimum divide ratio for the 8/9 prescaler.
C1455
Figure 5. Fractional-N Applications: Sample Calculation (2 of 2)
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6
June 4, 2007 • Skyworks Proprietary and Confidential information • Products and Product Information are Subject to Change Without Notice • 101075M
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