This video details an experimental endeavor to accurately measure the rise time (speed) of various oscilloscope probes, a task proven to be more complex than initially expected. An initial attempt using a 74AC14 Schmitt trigger pulser was found to be too slow, yielding a 2.4-nanosecond rise time that couldn't differentiate probe performance. A more sophisticated "avalanche pulser," based on a 3904 NPN transistor driven beyond its nominal specifications at around 120V to achieve sub-nanosecond pulses, was then constructed. Probes were directly connected to this pulser with minimal inductance, and measurements were taken using a 350 MHz Rigol oscilloscope, acknowledging its own bandwidth as a potential limiting factor. Unexpectedly, an inexpensive, unbranded Chinese probe exhibited the fastest measured rise time at 1.22 nanoseconds (though with waveform distortions), while the Rigol-supplied probes also performed remarkably well at 1.25 nanoseconds. The experiment highlighted the challenges of accurate high-speed measurement and concluded that a significantly faster pulse generator (ideally around 100 picoseconds) is essential for truly characterizing probe and oscilloscope bandwidths independently.
Unraveling Oscilloscope Probe Speeds: An Avalanche Pulser Experiment
0:00 Measuring Probe Speed Challenge: The video's creator sets out to measure oscilloscope probe speeds, finding the process surprisingly difficult due to the need for extremely fast signal edges.
0:33 Initial Pulser Limitations: An initial pulse generator, built with a 74AC14 Schmitt trigger, produced a 2.4-nanosecond rise time, which was insufficient to differentiate between the performance of various probes.
2:11 Avalanche Pulser Design: A new "avalanche pulser" was constructed, based on a design from a Jim Williams app note. This circuit uses a 3904 NPN transistor driven at approximately 120V into its avalanche breakdown region to generate very fast, sub-nanosecond pulses, with a 220k resistor and 22pF capacitor acting as a relaxation oscillator.
4:52 Specialized Probe Connection: To ensure accurate measurements, probes were connected directly to a custom output on the pulser designed to minimize inductance, featuring a coil and a ground ring.
5:50 Rigol PVP2350 Probe Test: The 350 MHz probe supplied with the Rigol oscilloscope measured a rise time of 1.25 nanoseconds, demonstrating strong performance.
6:53 HP Probe Test: A 500 MHz HP probe showed a similar rise time of 1.26 nanoseconds.
8:27 Tektronix P2200 Probe Test: A 200 MHz Tektronix P2200 probe measured 1.4 nanoseconds, performing as expected given its bandwidth.
9:36 Tektronix P6106 Probe Test: An older 250-275 MHz Tektronix P6106 probe also recorded a comparable rise time of 1.49 nanoseconds.
11:14 Modern Tektronix Probe Test: A 100 MHz modern Tektronix probe surprisingly achieved a faster rise time of 1.3 nanoseconds in this test.
12:16 Cheap Chinese Probe Test: An unbranded, inexpensive Chinese probe delivered the fastest measured rise time at 1.22 nanoseconds, although the waveform displayed notable distortions ("funny ring" and "weird lump").
14:19 Vintage HP Probe Test: A small, 1980s-era HP probe, rated at 275 MHz, performed very well with a 1.29 nanosecond rise time.
The Rigol-supplied probes demonstrated excellent performance in this test.
The 350 MHz bandwidth of the oscilloscope itself significantly limited the observed rise times, preventing a true assessment of higher-bandwidth probes.
Unexpectedly, some lower-rated probes performed better than higher-rated ones in this specific test setup.
The creator concludes that an even faster pulser (ideally with a ~100 picosecond rise time) is required to accurately characterize the true bandwidths of the probes and the oscilloscope independently.
Oscilloscope probe bandwidth specifications often reflect their performance when paired with a specific oscilloscope, rather than their standalone capability.