In-Situ Supply-Noise Measurement in LSIs with Millivolt Accuracy and Nanosecond-Order Time Resolution - Applications of Digital Signal Processing - page 103

VDD noise waveform Local supply difference (LSD)

(VDD - VSS) noise

Local VDD
Power Supply

VMON
output

VMON:
Ring Oscillator

Local VSS
Power Supply

VMON frequency is a function of LSD:
f ( VDD - VSS )

VSS noise waveform

Fig. 5. Concept of the voltage-controlled oscillation. VMON is a ring oscillator whose
frequency is modulated by the voltage fluctuation.

fluctuation, which is based on the fact that the oscillation frequency of a ring oscillator is a
simple monotonic increasing function of the supply voltage(Chen, et al., 1996).

2.1 Time resolution and tracking of LSD
The ring oscillator's oscillation period consists of each inverter's delay, which depends on its
LSD (Chen, et al., 1996). The voltage-measurement mechanism of the ring oscillator and the
definition of our measured voltage are depicted in Fig. 6 in the simple case of a five-stage
ring oscillator. The inverter circuit of each stage of the ring oscillator converts the LSD to

Period: T OSC

V LSD

V LSDm

t

τ τ τ τ τ τ τ τ τ τ

f1 r2 f3 r4 f5

r1 f2 r3 f4 r5

τ τ τ τ τ

f1 r2 f3 r4 f5

T OSC

τ τ τ τ τ r1 f2 r3 f4 r5

Fig. 6. Sampling of a ring oscillator

corresponding delay information. In the ring oscillator, since only one inverter in the ring
is activated, each inverter converts the LSD voltages into delays one after another. This
converted delay

τ is a unique value based on the LSD,

τ fi = f f ( V LSDi ) , τ ri = f r

(

V LSDi ) , (1)