I want an ADC that samples at 3GS/s.
That means for a Nyquist converter – SAR ADC – I can sample signals at a maximum bandwidth of 1.5 GHz.
I want to sample signals from 0-40 GHz – which determines my input bandwidth.
If I sample a signal at a rate lower than my input bandwidth, am I under samplng ?
Then I want to measure the SNDR/SFDR. I assume though I am going to get 3GS/S. I go to the first nyquist band – 0 to 1.5 GHz. I sweep the sampling clock to see what the maximum sampling is without 3dB degradation is. I try 2.5 GS/S, 2.75 GS/S and 3GS/S. I see that the 3G/S sweep over frequency degrades my SNDR/SFDR by 3dB versus the 2.5GS/S sampling clock.
Therefore, my best SNDR/SFDR is for 2.5GS/S and not 3GS/S.
Next, I want to determine the large signal or input bandwidth of my ADC. I sweep the input frequency of the signal over frequency and with the signal input at odd multiples of the nyquist zone, or here 2.5 GHz, since I determined my best sampling clock is 2.5 GS/S. So I sweep my input frequency over odd multiples of frequency, 0-40GHz – 0, 2.5, 5, 7.5, 10, 12.5, etc until 40 GHz. I also do this for a 3GS/S clock just to compare.
I see that I get SNDR/SFDR degradation of 3dB at a higher input bandwidth for the 2.5GS/S clock versus the 3GS/S clock. So, I use the 2.5GS/S clock – I can get 40 GHz of input bandwidth out of it.
So, my question here is, since I am sampling at 2.5 GS/S and my input bandwidth is up to 40 GHz, according to nyquist, the maximum bandwidth I can sample is 1.25 GHz.
But my input bandwidth is up to 40GHz.
I can have an input signal at 100 MHz, or 37.5 GHz or any others from 0-40 GHz since these fall in my ADC input bandwidth.
So am I under sampling the 37.5 GHz signal but not the 100 MHz signal ?
What is going on here ?
Can someone explain the difference between input bandwidth and sampling frequency and nyquist zones ?
I am confused.
Best Answer
If your sampling is 3 GS/s then, not to cause signal aliasing, the maximum bandwidth is 1.5 GHz (and somewhat less given that you can't have a brick-wall filter that can remove all frequencies above 1.5 GHz).
Yes, you are but, this is something that can be done, providing that the bandwidth of the signal is less than 1.5 GHz.
Your input BW is determined by hardware in the signal chain before the actual ADC input. You have to be able to adequately sample and hold the signal during the ADC sampling period.
No, your maximum bandwidth of signal you can reproduce in the digital domain is 1.25 GHz.
With an input BW of 40 GHz you can't hope to "capture" anything much higher than this unless of course the roll-off of the BW is not too great. I mean, if it's a single order roll-off then a signal (for instance) at 100 GHz will be attenuated roughly 2.5 times. 100 MHz is a stroll in the park.
37.5 GHz will be under sampled but, if that signal is wholly contained in a BW of 1.5 GHz then you can capture the information should it be present.
Image from here. In the picture above, the input BW will extend all the way from DC to the "6th alias" where the desired (but BW limited) RF signal is.
It shows how that RF signal can be bumped down into the baseband by under-sampling. Image from Beyond the first Nyquist zone by Texas Instruments - it looks like a good read to me. This is also relevant if the BW of your signal is too high (same TI document): -
You will get base-band corruption (the magenta bit) due to the BW of the signal extending into the 2nd Nyquist zone.
You can sample at 3 GHz and you'll be able to capture into the digital realm any BW limited signal without corruption even though you may be under-sampling.