Question / Topic

What are integer boundary spurs?

Answer / Content

Integer boundary spurs are spurs created by the feedback divider DSM engine on any Fractional part when the fractional divider ratio is set very close to an integer value.

Of course, DSM engine have a very complex algorithm that intends to minimize these spurs, and the number of spurs and their power is related to the quality of the DSM algorithm and its capability to handle such fractional divider ratios that are close to an integer value. IDT 8V97051 integer boundary spurs can be up to 30dB better than similar devices from other manufacturers.

These spurs are deterministic, so you can calculate their locations in the RF spectrum, but their intensity depends on the DSM algorithm. The Timing Commander GUI helps with calculating the position of the highest integer boundary spur.

To illustrate the concept of integer boundary spurs, we can take the example of a very simple first order DSM engine.
In order to divide  by 5.1, the DSM can divide 9 times by 5 (which is N) and divide one time by 6 (which is N+1).

By doing that, it divides (N+1)x1 and N*9 over a total of 10 cycles, so the results is [(N+1)*1 + N*9]/10 = N + 1/10, so 5.1.
However, with this simple algorithm there is a pattern and the Divide by (N+1) appears every 10 cycles.

So there is a frequency at which Div by (N+1) occurs, and this will create a spur.

In reality much more complex algorithms and a 3rd order DSM are used, but the principle is still the same. Note that in reality, the DSM is defined by an adder and a modulus.

For more detailed information, see the document posted in the table below.

 

Documents

Title Other Languages Type Format File Size Date
Application Notes & White Papers
Integer Boundary Spurs in Fractional-Feedback Phase-Locked Loops (PLLs) - White Paper PDF 537 KB Jul 27, 2016