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Question on sampling rate?
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rfengg:
My understanding of ADC's can be summarized on the left side of a grain of rice and hence this question.
We have receiver chain designed by an external company in which the input frequency can be between 117-119MHz and the sampling frequency of the ADC after a low noise amplifier is 80MHz.
I know that this system works fine though the signal is under-sampled.

What I am not able to get my head around is, that though the bandwidth of the signal after the LNA is only 2MHz, how can a 80MHz sampling rate, faithfully reproduce a signal which is much higher?
If I assume for the sake of argument, the signal is at a constant frequency of 118Mhz, how can a 80MHz sampling rate, correctly reproduce a 118MHz signal?
rstofer:
Look up equivalent time sampling.  As long as the signal is repetitive, the scheme works.  It has been used on scopes forever, more or less.
radiolistener:
when you capture signal with ADC which working at sampling frequency Fs, it can capture just Fs/2 bandwidth. All frequencies above Fs/2 will be folded into 0...Fs/2 range. The border Fs/2 works like mirror, it mirrors all frequencies back into Fs/2 range.

So, you can receive images from Fs/2...Fs range as Fs/2...0 range signal. In order to receive signals above Fs/2, you're needs to remove 0...Fs/2 range from the input signal with analog filter placed before ADC. Otherwise it will be merged with Fs/2...Fs signal.

In your case, ADC works at 80 MHz, it means that it can capture 40 MHz bandwidth:
- Frequency range 40...80 MHz will be mapped on ADC bandwidth as 40...0 MHz
- Frequency range 80...120 MHz will be mapped on ADC bandwidth as 0...40 MHz
- Frequency range 120...160 MHz will be mapped on ADC bandwidth as 40...0 MHz
- etc...

Your 117-119 MHz range will be captured by ADC as 37...39 MHz. In order to prevent merging with real 37...39 MHz, the real 37...39 frequencies should be removed from input signal with filter. The same you're needs to remove 43...41 MHz range because it also will be mapped to 37...39 MHz.

So, you're needs to put BPF (band pass filter) before ADC. This BPF should pass just 117...119 MHz range and cut off all other frequencies. After that you can get 117...119 MHz range as 37..39 MHz range on 80 MHz ADC output... :)

PS: by the way, 80 MHz is not a good choice for 117...119 MHz, because this frequency range will be mapped to 37...39 MHz, it's too close to the 40 MHz border. It's better to keep frequency of interest at the center of ADC bandwith, it will allows to use bandpass filter with more smooth slopes. 70 MHz will be better choice, because ADC with 70 MHz sample rate will capture 35 MHz bandwidth and 117..119 MHz range will be mapped to 12...14 MHz. So, you can use 0..12 MHz (105...117) and 14...35 MHz (119...140) for slopes of bandpass filter.
rfengg:

--- Quote from: rstofer on July 30, 2019, 12:31:06 am ---Look up equivalent time sampling.  As long as the signal is repetitive, the scheme works.  It has been used on scopes forever, more or less.

--- End quote ---

Thanks rstofer, but the signal into the LNA is quite random and not periodic as in scopes.
oPossum:
I think the correct term is sub-nyquist sampling, not equivalent time sampling.
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