Determining the average propagation delay of the inverter using ring oscillator

Next I wanted to determine the average propagation delay of the inverter (using NOR SN7402 connected as NOT gates) using ring oscillator, see the schematic below:

I looked at the signal flowing around the feedback (the signal can be measured at any point, and should give the same result).

From the trace, we see the oscillation of 32.89 MHz; Ha! that will give me the propagation delay! Here is how I calculated:

Computation for the propagation delay

foscillation (MHz)	Time period, t=1/ foscillation	Average propagation delay, tp=t/(2*3)
32.89	30.4 nS	5.07 nS

Now, comparing with the datasheet, it showed the value was well within the datasheet (note the datasheet always indicate the "Max" delay; everyone wants lower propagation delay!!!)

Comparison of the propagation delays

	As Measured	From Datasheet
tp	5. 07 nS	10 -18 nS

The better value (than datasheet) is possibly because with three inverters in a positive feedback, the propagation delay so obtained was average of the three. (A gotcha "note"- this is in a positive feedback loop; the propagation delay is simply not the linear sum of three values in each.) Any worse performance on one was compensated by this averaging effect. Also note, as no external input was used, it meant rise and fall time of the input waveform will have no effect upon measurement.