Have you confirmed that the scope can show anything above 33MHz?

Probably hit the actual limit of the scope at 33Mhz. What motherboard ? Pentiums run the FSB at the FSB, not like a 486DX2 which doubles it.

It probably only has a couple of samples per cycle at 25Mhz so pretty much guesses, therefore may be incapable of determining if anything is running faster than 50, and may be spurious between 25 and 50.

If it were an analog scope you would be able to measure peaks per number of divisions some way above 25mhz, but there's a possibility that it would have a bad resonance or something and turn into mush at a higher freq, or get bad attenuation at certain freq.

Most likely your 25 MHz scope is aliasing the 66MHz as 33MHz, as BitWrangler says, it only has a sample or 2 at the 25MHz it's rated at, thus presenting wrong frequency.

https://en.wikipedia.org/wiki/Aliasing the paragraph about "Sampling sinusoidal functions" shows pretty good what is probably happening with a small animation ...

DerBaum wrote on 2024-01-10, 03:29:

Have you confirmed that the scope can show anything above 33MHz?

I have not, the manual for it, is only 4 pages long, and it does not mention this feature. However, I have measured 33mhz and also 44mhz, on what I believe should be a 48mhz signal, as I was probing my clock chips two USB clocks, which the chip spec sheet says should be 24/48. So, maybe this probe CAN probe above 25mhz, but it has some kind of tail off in reliability. I'm really now sure, I wonder if there are other OWON probe owners here?

The solution I guess is to buy a higher rated Scope, but the 100mhz version of the same scope costs £300+, so its quite an outlay, when this model (OWON VDS1022(i)) may actually be able to MEASURE signals above 25mhz, but maybe not be able to show their trace accurately.

I guess for retro era PCs, so Pentium and whatnot, I should be good if I can measure up to 66mhz eh?

Cheers

Chris

Horun wrote on 2024-01-10, 03:37:

Probably hit the actual limit of the scope at 33Mhz. What motherboard ? Pentiums run the FSB at the FSB, not like a 486DX2 which doubles it.

Hrm, yeah, its my intuition it has SOME ability above 25mhz, but this tails off as you go higher. Problem is it HAS recorded 44mhz, so its odd it records 66mhz as 33mhz. Others have suggested this is some kind of harmonic aliasing going on, meaning its doing just that.

Kind of leaves me in limbo eh... I guess I have to assume the CPU IS getting 66mhz, as it seems to be at least partially functioning.

I can buy a 100mhz OWON Scope for £255, and I could sell this one on. But Id be gutted if I get that Scope and it still showed 33mhz....

Erg... what a dilema...

in theory OWON VDS1022 does 100Msps so should be able to measure frequency up to ~50mhz in ideal conditions. Then you have advertised 25MHz analog BW.