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I am planning to build a software defined radio. For this purpose, I need to sample analog signals using an analog to digital converter. In clasical Nyquist sampling, we need to sample with a rate \$f_s>2f_{min}\$ as we all know. But what I want is not to Nyquist sample but to band-pass sample. For example, I need to sample a signal with 1 GHz center frequency and 10 kHz bandwidth. My question is can I use MCP3002 adc with following analog input model to undersample example case. http://www.imageupload.co.uk/images/2014/03/03/analoginmcp3002.jpg

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  • \$\begingroup\$ I think your sample and hold circuit is going to let you down on this idea because it needs to be as fast as f**** (technical term for anything over 1GHz LOL) - you still need to be able to "snapshot" a fraction of a 1GHz waveform and your RC time constant is looking like 20 nano seconds to me. \$\endgroup\$ – Andy aka Mar 3 '14 at 18:38
  • \$\begingroup\$ sorry if i get it wrong but do we need to care about sample switch side to get analog input bandwidth? \$\endgroup\$ – user38136 Mar 3 '14 at 18:44
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    \$\begingroup\$ The sample and hold circuit needs to be as fast as your 1GHz else it'll act like a low pass filter and kill your signal to noise ratio \$\endgroup\$ – Andy aka Mar 3 '14 at 18:50
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Look at the LTC2285 used for this sort of thing: -

enter image description here

It can run at 125MSps flat out but it has a 640MHz sample and hold - this tells you what the main problem is when undersampling - you need a really fast sample/hold circuit to ensure you "capture" the detail in the much-higher-frequency carrier wave.

Even the lowly LTC2245 with its 10MSps throughput has a sample and hold that is rated close the 2255: -

enter image description here

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  • \$\begingroup\$ thanks for all answers and interest. I really need to learn more about adcs. Is there any good online source for learning all aspects of adcs that you can advise? \$\endgroup\$ – user38136 Mar 3 '14 at 19:33
  • \$\begingroup\$ @user38136 I can't recommend anything other than study, learn and gather experience. Oh, and listen to others and take note of well-supported decent answers on stack exchange! \$\endgroup\$ – Andy aka Mar 3 '14 at 19:34
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I doubt that the MCP3002 can undersample from a 1GHz signal unless you do an external sample-and-hold that is synchronous to the sample clock. If you use a > 1GHz (<1nS sample time) sample-and-hold circuit, you might be able to make this work.

You will need for the sample clock to be very stable since the error will be multiplied several times (1GHz/Sample Rate). Although this is theoretically possible, you may not get this to work and be usable.

If I were you, I would look at a faster ADC if you really want to do undersampling or mix the signal to a more reasonable center frequency (or a complex baseband).

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