Re: oscilloscope bandwidth question

Thanks for that to you all!
I had newer heard of repetitive sampling mode.
In the mean time i bought the oscilloscope (almost ½ price in the US
cmp. to Oz) and for the price and aplication i am happy with it. One
thing that does puzzle my is why they did not put a little more memory
in. as you say David, the stoage is a few hundred points. It seems
like a cheap feature to build in a little extra mem and greatly amplify
the use of the scope, but i guess Velleman has their resons.

Thanks!

Soren
David L. Jones wrote:
newsgroupie2003@xxxxxxxx wrote:
Hi,
I am looking at buying a cheap scope, thinking og the Velleman hps10
or hps40.
The specs for the hps40 says 40 MS/s sampling rate ans 12 MHz analogue
bandwidth.
What exactly does that mean?
I though a 40 MS rate would give a 20 MHz BW, at least for periodical
signals.

That is only to avoid aliasing on the ADC.
The sample rate really has nothing to do with sampling repetitive
signals on a Digital scope like this.
A DSO could have a 40MS/s sample rate but have a 1GHz bandwidth. It
does this by sampling the signal over many many cycles, and this is
what the Velleman might do at high bandwidths. This is known as
"repetitive sampling" mode.
This mode is useless for "single shot" applications like capturing the
serial data transmission as you want. In this case you want "real-time"
single shot mode. In the case of the Velleman at maximum sample rate it
will take 40MS/s. If you viewed a 10MHz waveform in this mode you would
get *4* sample points on the screen which is obviously fairly useless
to you. As a rule of thumb, in real-time mode you'll want at least 10
samples per cycle to get an idea of what your waveform is like. That
means your 40MS/s 12MHz bandwidth scope will have a useful "real-time"
bandwidth of 4MHz displaying 10 point per cycle. I'd personally round
it down to about 1MHz.

Also, the sample memory in the Velleman is likely to be small (256-512
samples maybe?). This doesn't give you much detail in your single shot
waveform capture.

The 12MHz quoted is the bandwidth of the analog input amplifiers, it
has nothing to do with the sample rate.

I am a PIC novice and wanted the meter to monitor logic levels and
serial data transmission, is that a bad idea?

It should be OK for that, so long as you don't want to see more than
say a dozen bytes of serial data at once. To see more you need a DSO
with a much bigger sample memory.

Dave :)

.

Relevant Pages

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