How do I remove a carrier frequency from a signal to see only the baseband signal using the RF Blockset?

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I have a sinusoid signal that is modulated to 915 MHz. I would like to downcovert the signal through a series of intermediate frequencies (IF) and ultimately to the baseband. If I downconvert the signal to a 50 MHz carrier signal, and then pass the signal through a mixer with a 50 MHz local oscillator (LO), I receive the following error in Simulink:
Error using ==> rfckt.mixer.convertfreq
Mixer output frequency is negative. Check FLO of MIXER and the simulation frequency.

Accepted Answer

MathWorks Support Team
MathWorks Support Team on 7 Jan 2013
The following information has been incorporated into the documentation in Release 2012b (R2012b). For previous releases:
Documentation on how to remove a carrier frequency from a signal in this fashion is missing from the RF Blockset. Here is additional information on this task:
You do not need to explicitly shift down from 50MHz to 0Hz. You can consider the output of the Output Port block to be a baseband IQ pair in the real and imaginary components, centered at 0 Hz, and having the specified bandwidth.
You can reject the adjacent and alternate bands and pass the desired band by applying further filtering and down sampling after the Output Port block. You do this either by using one complex-signal filter or by using a Complex to Real/Imaginary block to split the IQ rail up and then applying two real-signal filters.

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