Re: DSO spec confusion
- From: Joerg <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx>
- Date: Tue, 18 Mar 2008 00:10:45 GMT
Rich Webb wrote:
On Sat, 15 Mar 2008 14:23:08 -0400, Brett Pantalone
<pantalone2001@xxxxxxxxx> wrote:
Hello,
I am a software engineer with some theoretical (but little practical) knowledge of electronics. I'm shopping for an inexpensive (< $1000) digital storage scope to use in my software consulting business and for my personal hobby projects. I'm a little confused by the way these scopes are specified, and I'm hoping someone can clear it up for me.
When a scope is advertised as, for example, 60MHz bandwidth, but has a 100 Msa/s sampling frequency, what does this mean? Doesn't Nyquist dictate that the sampling frequency must be at least 120 MSa/s to properly characterize a 60MHz signal?
Not really. If you have only one sample per half cycle, you'll be only
able to say that there's a signal there.
Also, what about a scope advertised as 60MHz bandwidth, but 1 Gsa/s sampling frequency? What do I get with the additional oversampling?
Say your scope's A/Ds are 8-bits and you want to avoid aliasing
anything into that least significant bit. That means you'll want a
stop band that's down at least 48 dB at the Nyquist limit where
frequencies start "folding back" on you. With a 4th order filter,
you'll get that much attenuation in 0.6 decades or at about 240 MHz.
That's where your sampling Nyquist limit will need to be, so your
sample rate should be at least 480 MHz per channel or 960 MHz for two
channels.
Most of my work will be with audio signals and digital data at frequencies less than 1MHz, but I will occasionally want to verify clock signals of 50MHz or more. What features and specifications should I look for when shopping for a DSO?
For audio freqs, an inexpensive analog scope is fine. You'll probably
mostly be looking at repeating waveforms (or at least would rarely
want/need to use single-shot).
For digital *data* I'd strongly recommend a logic analyzer rather than
a scope. There are some decent, inexpensive PC-based ones. I've been
using the LogicPort from <http://www.pctestinstruments.com/index.htm>
for some time now and like it quite a lot.
All that said, there seems to be a consensus around here that Instek
scopes are pretty close to today's sweet spot. Similar to the
Tektronix models but with a larger data buffer, RS232, & USB.
<http://www.tequipment.net/InstekGDS-2102.html> for one example. I'm
still using my "ancient" TDS220 but if it ever goes up in smoke ...
Yep, I just wanted to suggest Instek. I've got the big one here (GDS-2204) and it runs circles around Tektronix. Also very nice USB connectivity. If you don't have a laptop at hand just pop in a USB stick and it stores the images and raw data on there.
Tektronix has inferior sample memory depth and the TDS210/220 series is prone to 40/80kHz noise, probably from its backlight inverter. IMHO a disgrace to Tektronix. There was also some kind of safety recall because of a sub-par ground connection. You might want to check that.
Brett: Get something with at least 500MSPS, better 1GSPS and 100MHz bandwidth or more. If you do stuff with FPGA or fast micro controllers you'll need it. An old rule is that the sample rate should be at least 10 times the highest spectral component you want to look at. Check out Instek, Newark has them. But don't wait too long since the Dollar is slipping a bit.
--
Regards, Joerg
http://www.analogconsultants.com/
.
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