Circuit and method to measure threshold voltage distributions in SRAM devices

A circuit to measure characteristics of a memory device having a plurality of inverters with transistors of a first transistor type, the circuit comprising:

• an array of ring oscillators, wherein each of said ring oscillators in said array (a) produces a ring oscillator output signal such that said array produces a plurality of ring oscillator output signals and (b) includes a plurality of inverters having transistors of said first transistor type;
• a multiplexer connected to said array of ring oscillators to receive said plurality of ring oscillator output signals and to provide as an output, in response to a select signal, one of said plurality of ring oscillator output signals;
• a control logic unit coupled to said array of ring oscillators and to said multiplexer, said control logic unit enabling each of said ring oscillators in said array so as to cause said array produce said plurality of ring oscillator output signals, further wherein said control logic unit provides said select signal;
• and
• an output circuit receiving said plurality of ring oscillator output signals from said multiplexer;
• wherein said output circuit measures output frequency variability in each of said plurality of ring oscillator output signals.