oscilloscope bandwidth question
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Posted by on January 9, 2006, 7:06 pm
Please log in for more thread options 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 | |||||||||||||||||||||||||||||||||||||
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Posted by David L. Jones on January 10, 2006, 12:29 am
Please log in for more thread options newsgroupie2003@yahoo.dk wrote: 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 :) | |||||||||||||||||||||||||||||||||||||
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Posted by redbelly on January 10, 2006, 5:25 am
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David L. Jones wrote: > 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 | |||||||||||||||||||||||||||||||||||||
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Posted by on February 2, 2006, 7:24 pm
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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. Jones wrote: > newsgroupie2003@yahoo.dk 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. >=20 > Dave :) | |||||||||||||||||||||||||||||||||||||
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Posted by David L. Jones on February 2, 2006, 8:46 pm
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newsgroupie2003@yahoo.dk wrote: > 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 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 :) | |||||||||||||||||||||||||||||||||||||
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> 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.