SNAS838A October 2022 – November 2022 LMK04832-SEP
PRODUCTION DATA
Frequency planning refers to strategically assigning frequencies to outputs for the purposes of spur minimization. Spurs vary as a function of output frequency, output format, and output assignments. Spurs can be directly coupling from one output to the next or be caused by a mixing product. For instance, if one output is at 3 GHz and another output is at 750 MHz, one can see a 750 MHz-spur coupling through the 3-GHz output. In some situations, it is also possible to have a spur that occurs at the greatest common divisor of the two frequencies (250 MHz in this case). In either case, the choice of which outputs the 3-GHz and 750-MHz frequencies are assigned to can have an impact on spurs.
Factor | General Guidelines and Tips |
---|---|
Output Frequency | To a point, higher frequencies tend to couple stronger to other outputs, but bypassing impacts this. |
Output Format | Stronger signals and single-ended signals tend to couple stronger to other outputs. LVDS tends to couple less than LVPECL as well. For LVCMOS, consider using both sides of the output with one side inverted to the other (Norm/Inv) to minimize crosstalk. |
Frequency Assignment to Output (Frequency Planning) | Outputs that are physically closer and that share the same power supply tend to have stronger crosstalk. Outputs are grouped by supply in the following manner: Clock Group 0: (CLK0,CLK1,CLK12,CLK13), Clock Group 1: (CLK2, CLK3), Clock Group 2 (CLK4, CLK5, CLK6, CLK7), Clock Group 3 (CLK8, CLK9, CLK10, CLK11). Use frequency planning to minimize spur levels to the most critical outputs. |
Frequency planning involves trial and error, but there is some strategy in planning. Try to ensure that the same frequencies are placed on outputs that have the strongest crosstalk and that different frequencies are placed on outputs that have weaker crosstalk
CLK0,CLK1 | CLK2,CLK3 | CLK4,CLK5 | CLK6,CLK7 | CLK8,CLK9 | CLK10,CLK11 | CLK12,CLK13 | |||||||
CLK0,CLK1 | n/a | M | L | L | L | M | H | ||||||
CLK2,CLK3 | M | n/a | M | L | L | M | M | ||||||
CLK4,CLK5 | L | M | n/a | H | L | M | M | ||||||
CLK6,CLK7 | L | L | H | n/a | L | M | M | ||||||
CLK8,CLK9 | L | L | L | L | n/a | H | M | ||||||
CLK10,CLK11 | M | M | M | M | H | n/a | H | ||||||
CLK12,CLK13 | H | M | M | M | M | H | n/a |
L = Low Crosstalk, M = Medium Crosstalk, H = High Crosstalk