# Can I use MCP3002 for undersampling?

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

• 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. – Andy aka Mar 3 '14 at 18:38
• sorry if i get it wrong but do we need to care about sample switch side to get analog input bandwidth? – user38136 Mar 3 '14 at 18:44
• 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 – Andy aka Mar 3 '14 at 18:50

Look at the LTC2285 used for this sort of thing: -

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: -