[Diy_efi] Choice of labscope

[Steve Ravet]

=20

> -----Original Message-----
> From: diy_efi-bounces at diy-efi.org=20
> [mailto:diy_efi-bounces at diy-efi.org] On Behalf Of Marcello A. Belloli
> Sent: Saturday, February 26, 2005 10:00 AM
> To: A list for Do-It-Yourself EFI
> Subject: Re: [Diy_efi] Choice of labscope
>=20
>      Now when you are  talking bandwidth, you are talking the=20
> 100mhz, or
> are you talking the samples per second?  I've trying to understand
> this difference.  Now if I'm reading this right....the samples per
> second is the rate at which you can get a freeze frame (digital
> storage)?  Like the signal you see for but a moment, and want to try
> and capture because looking in real time it goes by too fast.  But a
> repetitive signal you can just basically look at 100mhz, because
> there is no need to capture it.  It is always in front of you.  So
> the bigger the storage buffer, and the faster the storage rate will
> give you the better chance of glich capture.

Bandwidth is the highest frequency that a scope can display, and is
directly related to sampling frequency in a DSO.  The faster the sample
rate the higher the bandwidth.  The max bandwidth in a system that
samples digitally is 1/2 the sampling frequency.  This is called the
Nyquist limit.  You have to sample more than twice during the period of
a waveform in order to be able to reconstruct the waveform.  If you
sample less than twice per period you will reconstruct an incorrect
frequency.  This is called aliasing and it will cause a high frequency
signal to look like a low frequency signal on your scope display.  This
is easy to draw but not so easy to explain.  Google search for Nyquist
and aliasing and you can probably find out all about it.  Practically
the bandwidth of a DSO is usually about 1/5 the sampling rate, allowing
5 samples per period to better reconstruct the waveform.

Now, your scope.  Go to www.tek.com and search for 2232.

This is a 100 MHz bandwidth scope, with a 100MHz sampling rate.  If you
read the above carefully then you should be wondering why the bandwidth
is more than 50 MHz.  That's because sometimes engineers aren't able to
prevent marketers from coming very close to lying in sales material.
This scope has a 100 MHz bandwidth only on signals that are repetitive.
Basically it samples the input over and over and builds up the samples
in memory, giving an effective sample rate of 2GHz.  This only works on
repetitive signals.  If you're trying to find a single event at 100 MHz
you won't see it with this scope.  It's actual bandwidth for non
repetitive signals is probably around 20 MHz.

With that said this scope is probably a dependable performer and since
you will most likely be looking at repetitive signals it's probably
suitable for your purposes, including a 40 MHz PIC project.  It all
comes down to how much you're paying for it.

>      Now if I could get a logic analyzer at a good price would that be
> better.  I see an Hp DSO / logic analyzer for sale on ebay that
> struck my interest.  It has both in one unit.  Just not sure yet if I
> can capture the data to my pc.  The tektronix ones I've been looking
> at work with software I already own for doing captures.

LA and DSO are different tools for different purposes.  I think the
combo LA/DSO boxes are terrific.  I have one at work and while I rarely
cross trigger or otherwise use both functions at the same time, it's
just nice to have both functions in one box.  If you see one that is in
your price range and has the bandwidth (scope) and number of channels
(LA) that you need then get it.

--steve


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