This application note describes how to use constant negative bleed to optimize the phase noise and spur performance of Analog Devices' ADF4157 and ADF4158 PLLs.
This document describes a method to improve the phase noise (PN) and integer boundary spur (IBS) performance of the ADF4157 and ADF4158 Phase-Locked Loops (PLLs). The improvement is achieved by activating a constant negative bleed current, which adds a constant offset to the charge pump. This offset linearizes the charge pump by moving the operation away from the nonlinear area near the origin, often referred to as the charge pump dead zone. Without this current, sigma-delta quantization noise can fold back in-band, causing excessive noise or spurs, particularly in high-resolution sigma-delta modulators used in these parts.
The note details the effects of bleed current on phase noise and spurs across different charge pump currents and Phase Frequency Detector (PFD) frequencies (12.5 MHz and 25 MHz). It presents measurement results showing that for certain charge pump current values, using negative bleed results in improved integer boundary spurs and phase noise. The document also provides specific loop filter configurations required for different charge pump current settings and PFD frequencies to achieve optimal performance.
| Applicable Models | ADF4157, ADF4158 |
|---|---|
| Activation Register | Register 4, bits DB[24:23] |
| Minimum Charge Pump Current | 0.31 mA |
| Tested PFD Frequencies | 12.5 MHz, 25 MHz |
It is needed for high resolution sigma-delta modulators (high value of modulus) to prevent sigma-delta quantization noise from folding back in-band and causing excessive noise or spurs.
It is activated by setting bits DB[24:23] in Register 4 to the value 0b11.
No, it only improves phase noise and integer boundary spurs for a certain range of charge pump currents; for some values, performance can degrade.