Having the necessary information to improve the decoupling network requires additional measurements. To determine the frequencies where the noise resides, noise power spectrum measurement is necessary. A spectrum analyzer or a high-bandwidth oscilloscope coupled with FFT math functionality can accomplish this.
The FFT math function can be built into the oscilloscope, however, many of these functions do not have resolution sufficient to give a clear picture of the noise spectrum. Alternatively, a long sequence of time-domain data can be captured from an oscilloscope and converted to frequency domain using MATLAB or other post-processing software supporting FFT. This method has the advantage of showing as much resolution as you are willing to process. If neither math capacity is available, the noise frequency content can be approximated by visually examining the time-domain waveform and estimating the individual periodicities present in the noise.
A spectrum analyzer is a frequency-domain instrument, showing the frequency content of a voltage signal at its inputs. Using a spectrum analyzer, you see the exact frequencies where the PDS is inadequate.
Excessive noise at a certain frequency indicates a frequency where the PDS impedance is too high for the device’s transient current demands. Using this information, the designer can modify the PDS to accommodate the transient current at the specific frequency. This is accomplished by either adding capacitors with effective frequencies close to the noise frequency or otherwise lowering the PDS impedance at the critical frequency.
The noise spectrum measurement should be taken in the same manner as the peak-to-peak noise measurement, directly underneath the device, or at a static I/O driven High or Low. A spectrum analyzer takes its measurements using a 50W cable instead of an active probe.
•A good method attaches the measurement cable through a coaxial connector tapped into the power and ground planes close to the device. This is not available in most cases.
•Another method attaches the measurement cable at the lands of a decoupling capacitor in the vicinity of the device that has been removed. The cable’s center conductor and shield are soldered directly to the capacitor lands. Alternatively, a probe station with 50W RF probes can be used to touch the decoupling capacitor lands.
To protect the spectrum analyzer’s sensitive front-end circuitry, add a DC blocking capacitor or attenuator in line. This isolates the spectrum analyzer from the device supply voltage.
This Figure is an example of a noise spectrum measurement of the VCCO power-supply noise, with multiple I/O sending patterns at 100 MHz.
