[sdiy] DSO recommendation please

[ASSI]

On Saturday 03 April 2010, Tom Wiltshire wrote:
> I'd like a cheap digital storage scope to replace what I have
> already. The most important features to me are the storage facility
> (duh, yeah..) and an ability to do FFT on signals. It'd be really
> handy to see filter responses and such like.  I need at least 2
> channels, and I'd like an increase in bandwidth if possible
> (60MHz?) although I don't need it to go up super-high.

I won't repeat what has already been said in a previous thread about 
USB scopes, it's all in the archives.  Be aware that bandwidth alone 
is a somewhat deceptive parameter for a DSO, especially if you compare 
it to an analog scope.  There are lots of things you can see (I didn't 
say "measure", OK?) on a 60MHz analog scope that will just pass you by 
completely on some (but not all) 60MHz DSO.

> Although I
> play with uPs, I'm still mostly doing analogue stuff, but it'd be
> nice to be able to actually see what the hell is going on whilst
> debugging an SPI connection, for example.

It is much easier to do that with a logic analyser that understands 
the serial protocol and shows you what the signals _mean_ rather than 
just wiggles.  Again, to that end I've bought a USB2.0 based one from 
www.saleae.com (actually through www.elmicro.com since I bought a 
bunch of other stuff, too).  It goes up to 24MHz if used on it's own 
highspeed USB port, which is good enough for what I want to look at.

If you rather need something standalone, are prepared to spend a bit 
more and this logic analyzer functionality appeals to you, look out 
for a mixed signal oscilloscope with two analog and at least four 
(better eight) logic channels.  Be aware that not all of them will 
analyse serial protocols or this functionality is an option you might 
have to pay for.  If you can get something like a HP/Agilent 54621D 
used for a reasonable price you won't be disappointed - but they would 
probably cost around $3.5k used which likely doesn't fit your 
requirement of "cheap".  The most useful function is that you can 
build very complex trigger conditions for the analog traces from the 
signals of the logic analyzer.

Of course there are now cheaper knockoffs that appear to have similar 
specs on paper (like the Rigol DS1062CD).  It is hard to know if they 
allow the same type of seamless workflow without actually trying one 
out (for starters the serial protocol analyzer doesn't seem to be 
there), but the price difference is big enough to make it worth a 
look.


Achim.
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