Measuring Ring Oscillator Time Delay for
CMOS, SOI and TFT Technologies

The Validation of AC MOS models using UTMOST was described in November 1996 issue of Simulation Standard. In this issue, the measurement technique for measuring propagation delay of a ring oscillator using UTMOST is explained.

The DC bias connection to a typical ring oscillator circuit for time period measurement is presented in Figure 1. As it can be seen, the VDD bias of the Ring oscillator is connected to HP4145 SMU for V drain and the ring oscillator ground is connected to HP4145 SMU for V source.The Output of the ring oscillator is connected to the HP54501 Digitizing Scope.

Figure 1. Typical measurement setup for measuring propagation delay of a ring
oscillator on a wafer or using a packaged part.

The VDD biasing for the ring oscillator is from the UTMOST "ring_osc" routine measurement setup screen (Figure 2). The ring_osc routine is located in the "Time Domain" analysis section. Press the Routine pointer arrow in the Setup and Result screen until it reads "TIME".Select the "ring_osc" routine and press the "Set Measurement" button to open the Set Measurement screen. VDD_start and VDD_step values can be defined in the setup screen and the number of steps can be specified in the Routine Control screen.

Figure 2. Ring oscillator routine measurement setup screen.

After the proper hardware connection is completed and the VDD biasing is defined in UTMOST, press the measure button in the "Extraction" screen to start the measurement. UTMOST will place the HP4145 in user mode to supply single bias point VDD to the ring oscillator circuit. After the DC biasing is established, UTMOST will control HP54501A Digital Scope to measure the T period of the oscillations. The T period values will be stored in the memory for each VDD bias point. After the last DC bias is measured, UTMOST will plot T period of the ring oscillator vs VDD bias. (Figure 3.) Following the measurement the simulation and the optimization of the ring oscillator circuit can be performed as described in the article titled "AC Model Validation Using Transient Analysis" Part 2 of Simulation Standard November 1996 issue.

Figure 3. Measured versus simulated data of a 21 stage ring oscillator.