Finding Integer Boundary Spurs in a Fractional PLL Frequency Plan

A fractional PLL can have clean performance at an exact integer channel and noticeably worse spurs just beside it. A frequency plan qualified only at round values can miss this behavior. Map the distance to integer boundaries across the actual tuning range before concluding that the synthesizer meets its spectral requirement.

Identify the relevant frequency relationship

The TI discussion of integer boundary spur reduction explains that these spurs occur at an offset related to distance from a nearby integer channel. Its examples show how changing the frequency plan can move that relationship.

Record reference frequency, phase detector frequency, divider values, output divider, and fractional settings. Use the synthesizer’s definitions to determine which internal frequency must be compared with the integer boundary.

Create a channel map

For each required channel, calculate the relevant fractional relationship and distance to the nearest integer case. Highlight channels very close to a boundary, not only those exactly on it.

As a simple illustrative relationship, a 2000.2 MHz synthesized frequency near an integer multiple of a 100 MHz comparison frequency is 0.2 MHz from that boundary before any additional output division is considered. Apply the actual device’s mapping when predicting output spur offsets.

Measure the difficult channels

Sweep the nearby channel set with consistent analyzer settings and sufficient close-in resolution. Record spur offset and amplitude relative to the carrier. Keep source power and analyzer input level controlled so instrument overload does not create misleading products.

Compare exact integer operation, nearby fractional channels, and channels farther from the boundary. A moving spur that follows the predicted relationship is stronger evidence than a single unexplained spectral line.

Evaluate supported frequency plan changes

Consider permitted reference division, multiplication, phase detector frequency, or other device features that alter the boundary relationship. Check their effects on phase noise, settling, calibration, and all required channels.

Do not improve one channel by silently moving a problem into another part of the tuning range. Recalculate the complete map and repeat the relevant spectral measurements after a change.

Save register settings, channel list, predicted relationships, analyzer conditions, and measured spur limits. A useful qualification covers the application frequencies most likely to expose the mechanism. Good performance at a convenient integer frequency does not demonstrate equally good behavior at a nearby fractional channel, and changing only the output frequency label cannot explain or eliminate the underlying relationship. Keep the complete frequency plan with each spectral capture rather than only the nominal output frequency.