Fractional Clock Divider with DSM
Delta Sigma Modulator based fractional clock divider
Mixed-Signal Blockset / PLL / Building Blocks
Using delta sigma (Δ-Σ) modulation technique, a Fractional Clock Divider with DSM reduces the primary fractional spurs by spreading out the range over which the div-by value is varied. This block allows delta sigma modulation of up to 4th order.
clk in — Input clock frequency
Input clock frequency that needs to be divided, specified as a scalar. In a phase-locked loop (PLL) system, the clk in port is connected to the output of a VCO block.
div-by — Ratio of output to input clock frequency
Ratio of output to input clock frequency, specified as a fractional scalar. The value at the div-by port, N.FF, is split into two parts: the integer part (N) and the fractional part (.FF).
For an nth-order delta sigma modulator, the value at the div-by port is achieved by varying N between 2n different integer values.
For an nth order delta sigma modulator, use a value ≥ 2n at the div-by port.
Delta Sigma Modulator order — Order of the Delta Sigma Modulator
3rd order (default) |
1st order |
2nd order |
The order of the delta sigma modulator.
For an nth-order of the delta sigma modulator, the value at the div-by port is achieved by varying the N counter value between 2n different values. Modulator order defines the range of values by which the signal at the clk in port will be divided, providing a division effect similar to N.FF value at the div-by port.
get_param(gcb,'dsm')to view the current Delta Sigma Modulator order.
set_param(gcb,'dsm',value)to set Delta Sigma Modulator order to a specific value.
Enable increased buffer size — Enable increased buffer size
off (default) | on
Select to enable increased buffer size during simulation. This increases the buffer size of the Logic Decision inside the Fractional Clock Divider with DSM block. By default, this option is deselected.
Buffer size — Number of samples of the input buffering available during simulation
1 (default) | positive integer scalar
Number of samples of the input buffering available during simulation, specified as a positive integer scalar. This sets the buffer size of the Logic Decision inside the Fractional Clock Divider with DSM block.
Selecting different simulation solver or sampling strategies can change the number of input samples needed to produce an accurate output sample. Set the Buffer size to a large enough value so that the input buffer contains all the input samples required.
This parameter is only available when Enable increased buffer size option is selected in the Block Parameters dialog box.
get_param(gcb,'NBuffer')to view the current value of Buffer size.
set_param(gcb,'NBuffer',value)to set Buffer size to a specific value.
Inside the Mask
The Fractional Clock Divider with DSM subsystem block consists of four delta sigma modulators of orders one to four encapsulated inside the DSM Selector variant subsystem. The output of the DSM selector drives a Single Modulus Prescaler block. Given the Delta Sigma Modulator order, corresponding delta sigma modulator gets activated.
The modulator order defines the range over which the N counter value is varied. For an nth-order delta sigma modulator, N is varied over 2n different values. This variation is achieved by integrating the changes in the fractional part (.FF) from the previous cycle and quantizing the differential changes.
The general form of the transfer function for an nth order delta sigma modulator is:
Y(z) = Output of the modulator
X(z) = Input the modulator
E(z) = Quantization error
E(z) is calculated by subtracting the value of input X(z) in the present cycle from its value in the previous cycle. In other words, E(z) is a form of a digital highpass filtering.
The higher-order modulators reduce the primary fractional spurs by alternating N over a larger range of integer values. As a result, the fractional spurs are pushed to higher frequencies in the frequency spectrum and can be filtered more effectively by the loop filter in a PLL system.
For example, if the third-order delta sigma modulator is activated, N is varied over 8 different values, which can range from (N-3) to (N+4).
Delta Sigma Modulator Sequence
|Modulator Order||Range||DSM Sequence|
|1st||0, 1||N, N+1|
|2nd||-1, 0, 1, 2||N-1, N, N+1, N+2|
|3rd||-3, -2, -1, 0, 1, 2, 3, 4||N-3, N-2, …, N+4|
|4th||-7, -6, …, 7, 8||N-7, N-6, …, N+8|
 Miller, B. and Conley, R.J., A Multiple Modulator Fractional Divider. IEEE Transactions on Instrumentation and Measurement, vol. 40, no. 3, 1991, pp. 578-583.