Maker Pro
Maker Pro

oscilloscope bandwidth question

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

Dan Hollands

Jan 1, 1970
0
Show be fine for your application

20MHz is the theoretical bandwidth but the 12 MHz is more realistic for
seeing something you recognize
 
P

Phil Allison

Jan 1, 1970
0
<[email protected]
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.


** The spec in the owners manual ( Velleman site ) actually says:

" Maximum sampling rate: 40 MS/s "

Means the sampling rate depends on the time base setting.

This is quite unlike analog scopes where the vertical bandwidth is *
independent * of the time base setting.

At low speeds for viewing low frequency waveforms, the sampling rate of the
Velleman scope may be only a few thousand per second - which in effect
reduces the vertical bandwidth to a few kHz.

Then there are all the problems with aliasing.




........ Phil
 
D

David L. Jones

Jan 1, 1970
0
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 :)
 
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.
I am a PIC novice and wanted the meter to monitor logic levels and
serial data transmission, is that a bad idea?
Thanks
Soren

You will do much better looking on ebay, loads of old tek scopes go for
peanuts. You'll also end up wanting a logic analiser at some point or
at least storage of some sort.
 
Y

Yukio YANO

Jan 1, 1970
0
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.
I am a PIC novice and wanted the meter to monitor logic levels and
serial data transmission, is that a bad idea?
Thanks
Soren
It would seem to me that a soundcard oscilloscope would be suitable for
your application. The price would vary from 0/~100 dollars. Google
"Sound Card oscilloscope" to download suitable software and interface
hardware. BW (bandwidth) is not a real problem in your application.

Yukio YANO
 
R

redbelly

Jan 1, 1970
0
David said:
The 12MHz quoted is the bandwidth of the analog input amplifiers, it
has nothing to do with the sample rate.

I'll just add, for the benefit of the OP, that the "analog bandwidth"
is the frequency at which a sine wave is reduced to about 70% of its
actual amplitude. (More precisely the factor is the square root of 1/2)

Mark
 
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!
 
D

David L. Jones

Jan 1, 1970
0
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

The reason for the lack of memory is most likely a combination of
several factors:
1) They are building to a low price point, so a few dollars extra
manufacturing cost here and there can make a big difference to the
final resale price.

2) The screen is only small, so it kinda makes sense just to have
enough memory to fill the resolution of the screen. For the intended
target market (service people who just want to see a basic waveform on
the screen) that is probably good enough. It also means you don't have
to develop the pan & zoom software functionality required, thus getting
them to market quicker etc.

3) The design architecture is probably based on a low end FPGA and they
would have used the internal SRAM to store the data, eliminating the
need for high speed external memory. Most low end FPGA's have very
limited SRAM.

Dave :)
 
Top