Thank you for pointing out the inconsistency of my analysis: the considered Nyquist zone is during sampling, and not during decimation. Setting LO to 56.95 MHz works perfectly, thank you.
JM -- JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l'Epitaphe, 25000 Besancon, France July 20, 2020 5:43 PM, "Brian Padalino" <bpadal...@gmail.com> wrote: > On Mon, Jul 20, 2020 at 11:32 AM jean-michel.fri...@femto-st.fr > <jean-michel.fri...@femto-st.fr> > wrote: > >> Indeed second Nyquist zone before decimation. >> My thought was >> 143.05 MHz -> transpose by 100 MHz using the DDC (NCO at 100 MHz considering >> the >> 200 MHz sampling rate) to reach 43.05, and after transposition, decimating >> to reach >> 8 MS/s (I do have Epcos B3607 SAW filters 140+/-3 MHz frontend to select >> only the >> signal I am interested in). >> It is in the decimation process that I was thinking of being in the third >> Nyquist zone after decimation, which is incorrect because 8 MS/s is -4 to >> +4, so that >> 43.05 is in the 6th Nyquist zone after decimation (\in[36:44] MHz). > > This seems weird. > > Sampling 143.05MHz at 200MHz real will produce the desired signal at 56.95MHz > and conjugated, won't > it? Since it's real, it'll appear at both positive and negative frequencies, > with the negative > component being conjugated. > So if you mix with 56.95MHz, it will take the conjugated negative signal of > the conjugated desired > signal and mix it to 0Hz. Then you can go through the decimation filtering > however you want and > everything is centered at 0Hz. > > Right? > > Brian
