Re: aliasing with X310 BasicRX (higher order Nyquist zone) ?

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" <bpadalino@gmail.com> wrote:

> On Mon, Jul 20, 2020 at 11:32 AM jean-michel.friedt@femto-st.fr <jean-michel.friedt@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