1. A method for measuring an electrical impedance of a power distribution network of a programmable logic device die containing a plurality of configurable logic blocks powered from the power distribution network said configurable logic blocks having at least a look-up-table an input pin an output pin a load capacitance and a feedback capacitance, and containing a programmable interconnects network for connecting the configurable logic blocks said programmable interconnects network having wire capacitance, comprising the steps of: configuring the programmable logic device die to implement a power supply sinusoidal current load by using the programmable interconnects network to connect input and output pins of a plurality of said configurable logic blocks said power supply sinusoidal current load consuming a sinusoidal current from said power distribution network; configuring the programmable logic device die to implement an activate feature for said power supply sinusoidal current load; configuring the programmable logic device die to implement a programmable frequency generator coupled to the power supply sinusoidal current load so that the frequency of the power supply sinusoidal current load depends on the frequency of the programmable frequency generator; configuring the programmable logic device die to implement a first ring oscillator by using the programmable interconnects network to connect input and output pins of a plurality of the configurable logic blocks said first ring oscillator having a oscillating frequency measurement feature; programming a frequency value of the said programmable frequency generator, activating the said power supply sinusoidal current load and measuring a first average frequency of the said first ring oscillator over a time interval, deactivating the said power supply sinusoidal current load and measuring a second average frequency of the said first ring oscillator over a time interval, and calculating the electrical impedance of the said power distribution network at the said frequency value by processing the values of the said first average frequency, the said second average frequency, and a plurality of design and functionality characteristics of the said programmable logic device.