First post, by dionb
- Rank
- l33t++
Looking at my pile of 'dead, don't know why' PCBs, I've come to the conclusion it's time to up my game when it comes to diagnosis of electronics - it's time to get an oscilloscope.
Now, looking as a beginner the world of oscilloscopes (and logic analyzers) is bewilderingly complex, and expensive enough that you really don't want to purchase a red herring, even second hand. I just might be able to get my hands on some surplus at work, but same applies there - it's unlikely I'd be able to get a second device if it turns out I wangled the wrong one first time. Tektronix has some excellent documentation with advice online, but of course it applies to new scopes and mainly their products:
https://www.tek.com/document/online/primer/xy … g-oscilloscopes
Based on the recommendation there, it appears I should choose an oscilloscope with 5x the bandwidth of the frequencies I want to be able to sample and 1/5 of the lowest signal rise times I want to be able to see. The latter seems to indicate far higher bandwidths than I would need based on frequency alone:
Sample rate should also be at least 2.5x frequency of signal, preferably 5x.
Waveform capture rate and record time also sound relevant, but it's less clear which values I would need.
Now, what I want to be able to do - at a high level - is to diagnose faulty PCBs, i.e. motherboards, ISA, VLB and PCI cards, and to find pinouts for undocumented headers, and to be able to do this for stuff from the 1980s and 1990s. Thinking logically but naively, that sounds like a requirement to be able to capture stuff up to 50MHz (slowest speed of an So7 CPU bus, fastest VLB speed), which would translate to an oscilloscope bandwidth of at least 250MHz. If that ends up being prohibitively expensive, 25MHz (486 CPU bus, slowest VLB and PCI speed) would be plan B, at 125MHz bandwidth, and if even that's not feasible, I might consider starting with ISA bus speeds of 8MHz, which should be possible with 40MHz bandwidth. But... that would also mean TTL, and the table suggests I'd nede 175MHz for that, and 230Mhz for CMOS. It sort of sounds like I sould be aiming for 250MHz bandwidth regardless, unless I need to do 5x that 230MHz as well and it ends up at 1.15GHz 😮
Three questions to the more experienced :
1) does my logic sound sound, or am I missing something fundamental? What specs would you recommend for the applications I mention? How about other specs than just bandwidth/rise time?
2) given those specs, what sort of device should I be looking for? And what would sensible compromises/concessions be if I want something cheaper than the ideal device?
3) are there good online step-by-step guides to actually using one for the purposes I'm looking at?
