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

From: Brian Padalino
Subject: Re: aliasing with X310 BasicRX (higher order Nyquist zone) ?
Date: Mon, 20 Jul 2020 11:42:51 -0400

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.



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