downconvert IQ data that not centered around zero

[SunJab]

Hi all,

I want to downconvert IQ signal that is not centered around zero. I don't wan't to use the MT downconvert passband VI with its complex input mode selected...One option could be that i upconvert the signal to some higher frequency say 'freq1' and then downconvert freq1 to zero but for this i'll have to resample the signal first at least 2*freq1 that i don't want to do, coz it requires more processing. Is there any way to directly perform downconversion without doing upconversion and resampling? 

thanks

[rpursley8]

If you know the center frequency if the IQ data then just multiply by cosine(2*pi*fc*t) for the I part and sine(2*pi*fc*t) for the Q part.  Lowpass filter both signals to remove the higher frequency component. What is the sampling frequency and the center frequency of the data?

[SunJab]

I have acquired a band of 4MHz IQ data that is centered around zero with an IQ rate of 5MHz. So, the spectrum spans from -2.5MHz to +2.5MHz. This 4MHz band has one signal centered around +1MHz, one centered around 0, and another one centered around -1MHz. All three signals have a bandwidth of 200kHz. I want to down convert the +1MHz and -1MHz signals to zero.

[rpursley8]

I think, and I haven't had a chance to verify, instead of mixing down by multiplying with a cos(2*pi*1MHZ*t) or sin(2*pi*1MHZ*t), you need to multiply by [cos(2*pi*1MHZ*t) + i*sin(2*pi*1MHZ*t)] for the 1 MHz signal and by [cos(2*pi*1MHZ*t) - i*sin(2*pi*1MHZ*t)] for the -1 MHz.  The complex sinusoids should give you only one impulse signal, if you are looking in the frequency domain, to convolve against.

[SunJab]

Thanks. It works!