Digital Osci and Logic Analyzer

How complicated is it to create a simple pc based oscilloscope and logic
analyzer(excluding the pc software)?

Does it just consist of getting a ADC and interface for sending the data to
the pc? I'm looking at trying to make one in a similar way to what is done
on this site:

http://www.fpga4fun.com/digitalscope.html

I figured that the main the parts are the ADC, the probe, and the method of
sending the information(which I think is the hardest part at high data
rates?)? I've seen some ADC's that have sample rates of over 100Msps which
should give me a 50Mhz bandwidth? Although these aren't cheep I've also
seen some upwards of 1Gsps that would give me a larger bandwidth if I needed
it.

Lets suppose I decide to buy a 1Gsps ADC(which isn't going to happen any
time soon) and I have a probe that works properly with the ADC. How would I
go about storing/streaming all those samples? This would require a memory
chip be able to work down at 1ns or so? I was thinking I could use several
gigs of pc memory in parallel to reduce the latency and increase total
sample size to a few seconds... is this possible? What about encoding for a
digital stream? do something like rle on the bit stream where 01 one would
store 2 bits as one and its number of repetitions? (need to use 2 bits
atleast so things like a clock signal are encoded efficiently)

I'm kinda curious to how high speed logic analyzers overcome some of these
problems?

Another thing I wanted to know is how digital logic signals are "sampled" in
logic analyzers? Surely one doesn't need anything more than a 1-bit ADC to
convert the line signal? (therefor one doens't even need an ADC since it
would act more like a buffer than an real ADC?)


I was thinking about buying a ADC that does about 40Msps and try it out just
to see if it was going to work but and make a homemade probe to play with
it. It doesn't seem like a terribly complex project but there seems to be a
lot of technical issues involved.

Thanks,
Jon


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