oscilloscope bandwidth question

Thanks for that!

Cheers Soren

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.

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 :)

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.

Show be fine for your application

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

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

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

I am looking at buying a cheap scope, thinking og the Velleman hps10

** The spec " 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

Thanks for that to you all! I had newer heard of repetitive sampling mode. In the mean time i bought the oscilloscope (almost =BD 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. J> > Hi,

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 :)