Bandwith / Sample rate

[voho]

hi all,

please let me know the difference beetween Bandwith 100MHZ
and sample rate 1GS/s and how to calculate for example in oscilloscope.

thank s

 

[flatulent]

The sampling rate is just what it is. The bandwidth is that of the analog amplifiers before the sampler. It is good to have a much higher sampling rate so that the trace on the screen is smother.

 

[rwiggins]

May I suggest the following link on the Nyquist Shannon sampling theorom?

https://en.wikipedia.org/wiki/Nyquist–Shannon_sampling_theorem

 

[sujojk]

Bandwidth of an oscilloscope is the maximum frequency it can handle. And the sampling rate (only in digital oscilloscopes) is the number of input samples taken in one second. I know, these two are just 'crude' definitions.

Usually in digital scope, it will be written - 100MHz/1GS/s which means that its bandwidth is 100MHz and Highest sampling rate is 1 Giga samples per second. Bandwidth is specified as 100MHz means it cannot displays signals having more than 100MHz frequency. It is exactly same as bandwidth of other equipments such as amplifiers. Bandwidth limitation is usually happening because of the input filter, signal preamplifier, poor display response etc.

At the same time, Sampling rata specifies the maximum number of samples it can take for the highest input frequency. Whenever we are measuring low frequency signals, the sampling rate will be low. (it will be displayed in some corner of the screen). This is done in accordance with our time base control knob. Smaller the time base, higher the sampling rate. Higher the sampling rate, better the waveform display. At lower frequencies (around 1kHz), a sampling rate of 1MS/s or less is enough for a comfortable reproduction of the signal.

According to Sampling theorem, at least twice the frequency is required to reproduce the signal. Hence higher the number of times, the better wll be the diplay. 100MHz / 1GS/s means, sampling rate 10 times the max. frequency. Suppose if you are feeding a 100MHz sinusoidal signal, with 10 points itself the scope have to draw one full cycle. Then the waveform will not be exactly like a sine wave. But if your scope is having sampling rate of 5GS/s, then you will get 50 points per cycle to draw which will be far better than the previous one.

I hope, these words will clarify your doubts...

-SUjO

 

[FvM]

I previously thought, that flatulent had given an exhaustive answer, now I feel a need for a clarification.

Bandwidth is specified as 100MHz means it cannot displays signals having more than 100MHz frequency.

One reason, why an above-Nyquist sampling rate makes sense, is that the above statement isn't correct, not exactly.

Bandwith is specified as -3dB frequency, which implies, that considerably higher frequencies can be displayed, but with reduced amplitude, depending on the respective frequency characteristics. You're usually able to get some response at twice the nominal bandwidth. If the high frequency cut wouldn't be that soft, you get nasty distortions in time domain. Some early digital oscilloscopes had a nearly rectangular lowpass characteristic and thus showed square waves as as perfect sin(x)/x waveform, making any measurement specialist laughing out loud.

 

[hiral.kotak]

HI,

Bandwidth is the range of frequency which a pericular device i.e. filter will support while the sample rate is the no. of samples of a signal during unit time interval.

Thanks..

HAK..