In this video, the speaker revisits an earlier project involving an RC oscillator circuit using a 74HC14 Schmitt trigger, prompted by a viewer's comment about the limitations of his previous oscilloscope. Leveraging a newly acquired 50 GHz oscilloscope, the primary objective is to accurately measure and compare the 80/20 rise times of a 74AC14 (expected to be faster) and a 74HC14 (expected to be slower) logic gate. The experimental setup involves feeding a square wave into each component and capturing the output on the high-bandwidth scope. The results indicate that while the 74AC14 exhibits a rise time of approximately 1.4-1.5 nanoseconds, the 74HC14 is only slightly slower at around 1.75 nanoseconds, notably with a much cleaner, well-damped waveform compared to the ringing observed in the AC part. The conclusion highlights that the older HC part performs surprisingly well in terms of rise time, challenging its common perception as significantly slower for this specific characteristic.
Revisiting Logic Gate Rise Times with a 50 GHz Oscilloscope
0:07 Revisiting an Old Project: The speaker revisits a past video on an RC oscillator and buffer circuit designed by Allan W2 AEW, which utilizes a 74HC14 Schmitt trigger.
0:59 Upgraded Equipment: A key motivation for the re-evaluation is the acquisition of a new 50 GHz oscilloscope, offering significantly higher resolution (down to 7 picosecond rise time) compared to the previous 100 MHz scope.
1:45 Experiment Objective: The core experiment aims to compare the rise times of two similar logic gates: the supposedly faster 74AC14 and the 74HC14, which is generally considered slower.
2:12 74AC14 Rise Time Measurement: After setting up the 74AC14 with proper grounding, the 80/20 rise time is measured to be approximately 1.4 to 1.5 nanoseconds, although it exhibits some waveform ringing.
4:40 74HC14 Rise Time Measurement: The 74HC14 is then tested, showing a rise time of around 1.75 nanoseconds.
5:03 Surprising Comparison: A direct visual and statistical comparison between the two parts reveals that the 74HC14 is only marginally slower than the 74AC14 in terms of rise time.
5:06 Damping Quality: Notably, the 74HC14 produces a very well-damped waveform with significantly less ringing compared to the 74AC14.
5:49 Key Takeaway: The speaker concludes that the rise times of the older 74HC14 parts are "quite good" and "darn close" to the faster AC series for this particular characteristic, contrary to common assumptions.