relation between sampling frequency (Hz) and sampling rate (sample per second)

[Osawa_Odessa]

I want to know the relation between sampling frequency (Hz) and sampling rate (sample per second).
For example, a 1 Hz sin wave sampled at 8000 samples per second. Each cycle of the 1Hz tone will span all 8000 samples (since its period is 1 second). Thus sampling period will be 1/ (8000-1) s
Or sampling frequency will be 8000-1 ≈ 8000Hz.
Is this right?
Thanks.

 

[crutschow]

The sampling frequency and the sampling rate are the same thing.

 

[deepmak]

basically , the sampling rate is the number of samples taken per second which is same as sampling frequency , in your example , the sampling rate is 8000 sample per second so the sampling frequency is 8000 Hz , there is no 8000-1 , over one signal period , the signal is sampled 8000 times , hope this helps
regards

 

[Osawa_Odessa]

Thanks. I thought there are 8000 samples. Then it should have 8000-1 spaces between them. From that I know sampling frequency.
May I ask another question. If I have a signal that has highest frequency 30Mhz. Which sampling frequency should I choose? Based on Sampling Theorem, the sampling frequency should be at least 60Mhz. But I wonder about 80Mhz, 100Mhz, 150Mhz,...
Is it better to use these high sampling frequencies?

 

[deepmak]

the lowest sampling frequency would ve 60 MHz . you can sample at higher rates but then you will have to work with higherfrequencies and need more bandwidth but a i think a bit higher frequency is good , like when sampling 20 Khz voice at 44.1 khz , this is called oversampling and it is good way to prevent aliasing destortion . also as far as i know this help us as well to recover the signal with less steep filter

 

[Osawa_Odessa]

Hi, I often see that in ADC if input signal is 500Mhz at highest frequency, then the sampling frequency is chosen at 4GHz ( eight times larger).
Why we need to sample the signal at such a high sampling frequency while we can sample at a bit higher than 1GHz?

 

[deepmak]

Hi again ,
such higher oversampling is used here to increase SNR ration and give more acurate ADC
you can check https://www.ti.com/lit/an/spra461/spra461.pdf

 

[Osawa_Odessa]

Thanks for the link.
I will read it carefully. By the way, could you have a look at this article and tell me what sampling frequency should I use?
The ADC input signal is about 20Mhz.
I want to use 4 bit ADCs in stead of 8 bit's.

 

[embpic]

what is MS/S this unit is regarding sampling i think what does this mean?
thanx

 

[deepmak]

Hello Embpic , MS/S refers to Mega sample per second

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Hello Osawa again
i had a look at the article , i see that they use the under sampling as the signals are modulated on carrier , also i seen a frequencies plan that range from 6 MHz to 25 MHz , which means you will have variable sampling rate /frequency , however from what i remember , if you sampling rate is 50 MHz and you will use 8 bits , your ADC clock will run at 400 MHz (200 MHz for 4 bits )
i suggest that you read some articles about the use of under sampling and the modulated signals
i would like to help but i have been away from this field for like 13 or 14 years
here is a simple link about it , im sure you will find more
http://www.rfcafe.com/references/electrical/undersampling.htm

 

[Osawa_Odessa]

Thanks a lot, deepmak!
I am a beginer and if you don't mind I will ask you some basic questions.

i had a look at the article , i see that they use the under sampling as the signals are modulated on carrier

Could you tell me how can you know that they use under sampling? I read the article many times but don't recognize this. I thought about oversampling!

 

[deepmak]

Hello Osawa and you are welcome
actually it is written in the article in the second highlighted section .

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also this technique is used in communication systems when you moudlate the information signal with high frequency carrier
the following article also talk about this technique
http://www.edn.com/electronics-blogs/bakers-best/4322744/Turning-Nyquist-upside-down-by-undersampling

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also here
http://sva.ti.com/assets/en/appnotes/Undersampling.pdf

 

[embpic]

thanx
means if oscilloscope having specification tha 20Ms/S. means they are giving 20 000 000 samples per second!!!!!!

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thanx
means if oscilloscope having specification tha 20Ms/S. means they are giving 20 000 000 samples per second!!!!!!

 

[deepmak]

yes , however there are digital scopes having much higher sampling rate thought