Two different thought process that produce two different alias band calculations .... What concepts are these?
If I have two 1 GHz ADC's and they sample with 90 degree offset (A comment said this is Over Sampling)
- Each ADC has an Fs/2 of 0.5 GHz
- The combined system sample streams have an Fs/2 of 1 GHz because the second ADC captures additional sample points between the samples of the first - doubling the highest frequency possible by one alone.
- The system can be thought of as having Fs of 2 GHz, whereas a single equipment Fs is 1 GHz.
- After an FFT, the entire spectrum repeats every 2 GHz, the 'system' Fs not the equipment Fs of 1 GHz.
- After an FFT, The positive frequencies are 0 to 1 GHz and the negative -1 to 0 GHz.
I am still a bit confused by the thought that maybe everything above is false because maybe IQ sampling means ...
- The FFT is being fed a complex number via IQ sample streams.
- The FFT fed a complex number does not have negative frequencies - it can span the entire single equipment Fs of 1 GHz.
- So Fs/2 is still 0.5 and Fs is still 1, each one is not doubled like the first bullet points suggest because the complex notion of FFT allows frequencies up to the single equipment Fs of 1 GHz when complex sampled.
- So also, Fs is 1 GHz and not 2 GHz (3rd bullet point from first set of points) and so the spectrum repeats every 1 GHz and not 2 GHz.
- Also in this case, there is no 'spectral inversion' from negative frequencies.
It makes a big difference to where I calculated the alias band and thus my analog filter whether the first set of statements is true or second set is true
Please can you offer advice. Still require assistance if you can 02/03/2020