Re: DSO spec confusion

Brett Pantalone 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?

The net rate must be half the frequency but there is a technique
called equivalent time sampling where each a/d sample is delayed
in time on the next sweep. At 100msa/s each sample is at 10nsec
spacing. If you take ten sweeps with the sample point displaced
in time by an additional 1nsec, then you have a 1Gsps equivalent
rate. The tradeoff is a cheaper a/d for a slower data acquisition
rate. Many years ago we had boxcar integrators which effectively
took one data point per sample. By moving the delay, you assemble
a waveform. Clearly the digital scopes are better than this.

Also, what about a scope advertised as 60MHz bandwidth, but 1 Gsa/s sampling frequency? What do I get with the additional oversampling?

At 1Gsps you get the waveform sampled with a 1nsec resolution so you
can actually see some details. If you only sample at just over 2
samples per cycle, the amplitude will be small and the waveform
will look odd.
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 what you want, most of the digital scopes from the Chinese imports
to the (probably chinese import) Tektronix will do fine.
The biggest adjustment you will have is that there are aliasing
effects when you adjust the sweep rate.

For audio and lower frequencies you can also do will with a good
used Tek 2465 family oscilloscope. Analog scopes are a pain when
you are looking at transients but they are good for continuous
signals. However, unless you like screen picturesm, the digital
are good for documenting results.

I have an assortment of scopes but I find I use my Tek TDS3032 almost
exclusively. They are expensive but are great scopes.


Thanks for any advice you can offer.

-- Brett
.